Comparative physiological responses of english lavender cultivars to drought stress and abscisic acid treatments with implications for agricultural water management

Abstract

Global warming is one of the important environmental issues that has led to climate change and affected agriculture and water resources. Considering the negative impact of drought in limiting the yield of agricultural products, it is necessary to introduce management methods such as the use of suitable cultivars and growth regulators like abscisic acid (ABA). This research, by simultaneously studying the physiological parameters related to photosynthesis, water relations, and osmotic regulation in two varieties of English lavender (Hidcote, Munstead) under different drought and ABA treatments, fills the existing gap in understanding the mechanisms of adaptation, particularly the relationship between changes in photosynthetic gas exchange and osmotic regulation. This valuable medicinal plant addresses drought stress. In this research, the effect of different levels of drought stress (90–100%, 70–80%, 50–60%, and 30–40% field capacity) and ABA (0, 15, and 30 µM) on the physiological parameters of two lavender plants was investigated. Drought stress significantly reduced RWC in both cultivars, while the exogenous application of ABA helped reduce water loss. Hidcote showed better water retention (maximum RWC 85.95%) compared to Munstead (maximum RWC 73.49%). The photosynthesis rate of Hidcote decreased from 11.46 to 8.33 µmol m⁻² s⁻¹ with 30 µM ABA, and Munstead’s rate decreased from 9.1 to 6.05 µmol m⁻² s⁻¹ with 30 µM ABA. The rate of transpiration also showed a similar decreasing trend. Water use efficiency (WUE) increased with decreasing field capacity (FC) and increasing ABA concentration. Munstead showed superior WUE compared to Hidcote across treatments. this study showed that proline content accumulation increases under stress, with Hidcote reaching a maximum of 5.07 µmol/g FW at D4A3 and Munstead a maximum of 4.3 µmol/g FW at D4A2. The conductivity of the mesophyll decreased under stress but showed numerical differences. In general, the evaluation of the two cultivars showed that the Hidcote cultivar has better water -holding capacity, a higher photosynthesis rate in optimal conditions, and a more sensitive response to stress. In contrast, the Munstead cultivar showed better water use efficiency, more stable performance under stress, and better adaptation to drought conditions. These findings help to understand the physiological mechanisms and plant water relations that affect water stress tolerance in lavender and, by analyzing the differences in cultivar responses to stress, provide new knowledge in the field of selection and improvement of drought -tolerant cultivars.

Introduction

Lack of access to water is a pressing problem that affects farming output on a large scale. The rise in temperatures and subsequent shifts in climate have led to an increase in drought -related challenges in recent times¹. The dry and swelteringly hot summers in parts of Iran have caused a shortage of water resources—a challenge for sustaining crop yields^2,3. Current agricultural practices are increasingly focusing on water management techniques to highlight the importance of adopting approaches to enhance water use efficiency. Using plant hormones and species that can withstand drought conditions to improve water efficiency is a strategy to support plant health and reduce the effects of drought, as highlighted in studies^4,5.

Drought stress affects plant biology in ways that involve plant hormones modulating responses to stress situations. Under initial or moderate drought conditions, the availability of water in the environment first leads to the closure of stomata, becoming an obstacle for photosynthesis. The limitation of CO₂ intake causes reduced efficiency of photosynthetic electron transport and photochemical processes. Under these circumstances, abscisic acid (ABA) is a plant hormone that plays a significant role in the plant’s response to drought stress. It initiates stomatal closure and maintains balance within the plant through interaction with the ubiquitin -proteasome pathway^6–8. Application of exogenous abscisic acid modulates the activities of antioxidant enzymes and the accumulation of proline under drought stress in wheat varieties. ABA at 100 μM enhanced drought-mediated SOD activity by 45% and CAT activity by 35%. Application of ABA caused a 2.5-fold increase in the proline content of treated plants compared to untreated controls. A study on drought stress-induced cross-talk between ABA signaling and the antioxidant defense system showed that, under severe drought conditions, ABA-mediated regulation enhances the proline content by 180%⁹. Martinez-Garcia et al¹⁰. studied the differential regulation of antioxidant enzymes by abscisic acid and osmolyte accumulation in drought-sensitive and drought-resistant tomato cultivars. They demonstrated that ABA accumulation under stress conditions increased 2.3 times, and the activity of antioxidant enzymes increased by 75% in resistant cultivars. Research on exogenous ABA in increasing antioxidant enzyme activities and proline accumulation under drought stress conditions showed that exogenous ABA at 75 μM resulted in a 55% increase in SOD activity, a 48% increase in CAT (catalase) activity, a 92% increase in proline content, and improved photosynthetic efficiency by up to 42%¹¹.

Evaluation of the effect of abscisic acid on the activity of antioxidant enzymes and proline accumulation under drought conditions in wheat cultivars showed that ABA treatment (100 μM) increased SOD activity by 45% and CAT activity by 35% under drought stress. Additionally, the proline content in plants treated with ABA increased 2.5 times compared to the control¹². A cross-talk study between ABA signaling and the antioxidant defense system during drought stress showed that ABA-mediated regulation increased proline content by 180% under severe drought conditions⁹. Martinez-Garcia and colleagues¹⁰ investigated the differential regulation of antioxidant enzymes and osmolyte accumulation by abscisic acid in drought-sensitive and drought-resistant tomato cultivars and showed that ABA accumulation is 2.3 times higher under stress conditions. Additionally, the activity of antioxidant enzymes increases by 75% in resistant cultivars. Research on the role of exogenous ABA in increasing the activity of antioxidant enzymes and proline accumulation under drought stress conditions showed that the use of exogenous ABA (75 μM) led to a 55% increase in SOD activity, a 48% increase in CAT activity, a 92% increase in proline content, and an improvement in photosynthetic efficiency by up to 42%¹¹.

The present work investigates the complex interaction between drought stress and ABA treatments in English lavender. While there have been a few studies on isolated physiological responses, limited information is available about the temporal progression and interconnected nature of these responses under combined drought and ABA treatments. Despite the large volume of research carried out on drought stress responses in medicinal plants, critical knowledge gaps persist in understanding the progressive development and interplay of these responses under combined drought and ABA treatments. Particularly, very little is known about the dynamic relationship between photosynthetic efficiency and water relations, together with mechanisms of osmotic regulation during the stress period. Additionally, although reports on cultivar-specific variations in drought tolerance are documented in the literature, systematic documentation of how different lavender cultivars modulate their physiological and biochemical responses under simultaneous drought stress and ABA application has not been forthcoming. Therefore, continuous monitoring at well-defined time intervals of physiological responses, the elaboration of correlation matrices between different physiological parameters during the stress period, and the construction of a reaction profile specific to each cultivar, showing the temporal evolution of the mechanisms of adaptation to stress, can be of great importance.

It is predicted that the rapid progress of climate change will cause frequent and severe droughts in the near future. These droughts will undoubtedly affect the photosynthetic capacity and water relations of plants and significantly impact plant productivity. Considering the role of ABA in reducing the effects of drought, this research was conducted to elucidate the mutual effects of physiological and biochemical responses to combined drought stress and exogenous ABA treatments in English lavender cultivars (Hidcote and Munstead) and to provide drought -resistant cultivation strategies. The findings of this research provide valuable physiological insights into how lavender cultivars cope with extreme drought conditions.

Materials and methods

Experimental setup

The research was conducted at the Department of Plant Production, University of Agricultural Sciences and Natural Resources of Gorgan, Iran (36°30’N, 53°57’E, 155 m above sea level), during the 2016–2017 agricultural years. This study examined the effects of irrigation regimes and abscisic acid (ABA) on the physiological traits and gas exchange of two lavender cultivars. The experiment was designed as a factorial study within a completely randomized block design, with three replications, each replication consisting of three experimental units. The lavender cultivars studied were Lavandula angustifolia cv. Hidcote and L. angustifolia cv. Munstead. The irrigation regimes included four levels: 100–90%, 80–70%, 60–50%, and 40–30% field capacity. Additionally, ABA foliar spray was applied at three concentrations: 0, 15, and 30 μM.

Plant material

This experiment was conducted at the Faculty of Plant Production, University of Agricultural Sciences and Natural Resources of Gorgan, Iran, at 36°30’N, 53°57’E, and 155 m above sea level for two successive growing seasons, namely 2018–2019 and 2019–2020. The present study investigated the effects of various irrigation regimes and ABA applications on physiological traits and gas exchange characteristics of two lavender cultivars. The factorial study in a completely randomized block design had three replications, each replication consisting of three experimental units. The two lavender cultivars used were L. angustifolia cv. Hidcote and L. angustifolia cv. Munstead. Irrigation was imposed at four levels: 100–90%, 80–70%, 60–50%, and 40–30% field capacity. ABA foliar spray was performed at three concentrations: 0, 15, and 30 μM.

Application of drought stress treatments

Drought stress treatments were administered based on weight. Initially, an equal amount of gravel was placed at the bottom of each pot for drainage, and the pots were filled with soil to a uniform weight of 9 kg using a scale. The soil in each pot was then saturated with water and left on a mesh surface for 48 h to reach field capacity after excess water drained. At this point, the pots were weighed, and their soil was completely dried at 105 °C for 48 h. After determining the soil moisture percentage at field capacity, the moisture content for different treatments was calculated. Daily weighing of a sample pot in each block allowed for the calculation of water deficit, and the required amount of water was added to the pots. To ensure accurate water allocation, each moisture treatment included an extra pot to account for the dry weight of the plants.

From then on, all the main pots transferred to the open space were subjected to the same irrigation regimes for two continuous months. From this stage forward, daily measurement of soil moisture was conducted, and irrigation was continued whenever the percentage of soil moisture by weight was reached. In this respect, Khorasaninejad et al. (2018) was referred to¹³. The ABA for treatment was provided by Sigma -Aldrich^®. ABA was applied through foliar spray at three different concentrations: 0, 15, and 30 μM during three growth stages: the end of the vegetative growth period, the beginning of flowering, and full flowering. The ABA solutions were prepared by first dissolving in 0.5 ml of 1 N sodium hydroxide solution before dilution to the required volume with distilled water prior to spraying. Approximately 8 weeks after starting the irrigation treatments (when 50% of the plants were in full bloom), physiological characteristics (carotenoids, chlorophyll a, b, and a+b) and gas exchange were measured. Three replications were selected from each treatment, with five young, mature leaves taken from each replication for measurement.

Gas exchange parameters and photosynthetic efficiency

After applying the treatments, plant gas exchange was measured using a portable gas exchange measurement device at the University of Agricultural Sciences and Natural Resources of Sari (model GFS-3000-FL Walz, Germany). All measurements were taken between 10 and 12 AM on the most developed upper (sun-exposed) leaf. Gas exchange was performed on a day with a photosynthetically active radiation (PAR) of 1000 micromoles of photons per square meter per second.

After removing the roots, a methanolic extract of the plant was prepared, and several parameters were measured, such as chlorophyll a, b, and a+b, photosynthetic rate, stomatal conductance, vapor pressure deficit between leaf and air, transpiration rate, and intercellular CO₂ concentration for each pot. Additionally, some parameters like mesophyll conductance (mmol m⁻² s⁻¹) (Fischer et al., 1998) based on Eq. 1, mesophyll efficiency (μmol m⁻² s⁻¹ mmol⁻¹ water) based on Eq. 2 (Sisakhtnejad and Zolfeghari, 2014), WUE (μmol CO₂/mol water) based on Eq. 3 (Ahmadi and Siosemardeh, 2005), and intrinsic water use efficiency (IWUE) of the leaf (photosynthetic WUE) (μmol CO₂ m⁻² s⁻¹) based on Eq. 4 (Ahmadi and Siosemardeh, 2005) were indirectly determined^14–16.

1

2

3

4

A: Net photosynthetic rate (μM CO₂ m⁻² s⁻¹).

Ci: Intercellular CO₂ concentration (mM⁻¹).

gs: Stomatal conductance (M H₂O m⁻² s⁻¹).

E: Transpiration rate (M H₂O m⁻² s⁻¹).

Relative water content

For the relative water content, three fully expanded leaves were taken from each experimental unit from the tip of the stem. Segments of 1 cm were cut and their fresh weight was measured using a digital balance. The saturated weight was determined after keeping the segments in covered Petri dishes containing distilled water in complete darkness at 4 °C for 24 h. After removing the sections from the water, excess surface moisture was blotted off using filter paper and their turgid weight measured. The dry weight was then determined by oven-drying the same plant samples at 70 °C for 24 h. Finally, relative water content was determined by the following formula given by Yamasaki and Dillenburg¹⁷:

5

Leaf proline content

The Bates et al. method was used for the measurement of proline concentration (Bates et al., 1973). According to this method, 0.2 g of leaves were placed into 10 ml of a 3% sulfosalicylic acid aqueous solution and left to stand for 24–48 h. after filtering the resultant solution, 2 ml of this solution was mixed with 2 ml of ninhydrin reagent, and then 2 ml of acetic acid was added to each tube. The samples were subsequently placed at 100 °C for 1 h in a water bath and then cooled on ice. In the next step, 4 ml of toluene was added to each tube, and the samples were homogenized using a mixer. The absorbance of the resulting upper colored phase of the samples was measured by a spectrophotometer at a 520 nm wavelength. Using a proline standard curve, the proline concentration in each treatment was determined¹⁸.

Soil properties and climatic conditions

A soil sample was collected and analyzed both chemically and physically to evaluate its quantitative and qualitative properties. The results of these analyses are summarized in Table 1. Fig. 1 illustrates the climatic conditions during the period from September 2017 to August 2018.

Table 1.

The soil physical and chemical analysis¹⁹.

Lab. No1	pH2	EC*103	SP4	TNV5	N 6 %	OC7 %	P(ava)8 ppm	K(ava)9 ppm10	%Clay	%Silt	%Sand	Texture
947	7.4	4.076	141.86	5.59	0.09	0.9	24.8	256	12	42	46	Loam

(1) Laboratory number, (2) Power of hydrogen, (3) Electrical conductivity, (4) Saturation percentage, (5) Total neutralizing Value, (6) Nitrogen, (7) Organic carbon, (8) Phosphorus, (9) Potassium, (10) Part per million.

Fig. 1.

Meteorological data from September 2017 to August 2018 for the experimental site of the study (National Meteorological organization, Iran) (Gorgini Shabankareh et al., 2021).

Data analysis

Data analysis was performed using SAS 9.1 software. Comparison of means was carried out by the LSD tests at p < 0.05. The heat map showing biochemical attributes and essential oil profiles under different irrigation regimes supplemented with ABA was drawn by applying CIM Miner-One Matrix software. It describes the methodology of PCA for biochemical traits, checking their response after the application of drought stress and ABA. The packages of XlSTAT were used for this analysis, while a heat map for these traits was plotted using Clustvis.

Results

Relative water content

The present study shows that both drought stress and exogenous application of ABA cause changes in RWC within the leaves of lavender cultivar plants. The impact of drought conditions significantly reduced RWC across the cultivars, indicating the intensity of water shortage on the hydration status of the plants. However, under the same conditions, exogenous foliar sprays of ABA significantly increased water retention within leaves. The treatments with 30 μM ABA resulted in maximum retention, underlining its efficiency at this concentration in mitigating drought-induced RWC loss. More interestingly, with severe levels of drought, RWC increased by 55.67% with 15 μM ABA and 89.98% with 30 μM ABA compared with the controls at the same level of drought. The results also suggest the use of ABA as a drought-mitigating strategy for maintaining water content in plants under water shortage. Variance analysis, depicted in Table 2, provides the statistical explanation for the effects of drought stress and ABA application on RWC and enzymatic activities. The main enzymatic responses recorded in plants experiencing drought stress accounted for substantially increased antioxidant enzyme activity, such as superoxide dismutase, peroxidase, ascorbate peroxidase, and catalase. This induction may play an important role in protection against oxidative stress triggered by water deficit and, therefore, in plant resistance under severe conditions. The RWC variance was highly significant, confirming that drought and ABA have a strong impact on the water content across different treatments (P < 0.01). This response aligns with the literature that ABA promotes drought tolerance through facilitating osmotic adjustment and reducing water loss, thus preserving RWC (Fig. 2).

Table 2.

Variance analysis of the effects of drought stress and abscisic acid application on physiological responses of english lavender Cultivars.

Mean of squares
Traits	R (repeat)	C (cultivar)	D (Drought stress)	A (Abscisic acid)	C×D	C×A	D×A	C×D×A	Error	CV (%)
df	2	1	3	2	3	2	6	6	46
RWC	23.82	535.13**	4803.81**	1461.29 **	0.975 ns	9.02 ns	110.34 **	20.92 ns	14.68	6.47
A	0.276	18.27**	41.46**	14.42**	1.75**	0.968*	2.378**	0.687*	0.231	7.41
E	0.0041	0.990**	43.54**	68.97**	0.163 **	1.033**	9.84**	0.188**	0.0029	1.42
gs	0.023	0.375**	6.40**	2.45**	0.181**	0.052 ns	0.379**	0.0366 ns	0.018	15.05
Ci	1630.95	5816.71**	103205.04**	93955.93**	939.15 ns	3.77 ns	1578.91 ns	908.50 ns	699.87	12.53
A/Ci	0.0001	0.0040**	0.0241**	0.0130**	0.0018**	0.0004**	0.0047**	0.00048**	0.000079	19.93
Ci/gs	71622.85	14808.65 ns	39331469.60**	16199953.11**	74125.32*	63647.82*	336025.52**	54123.52*	22835.14	31.33
A/E	0.16	18.13**	42.21**	25.36**	9.17 ns	3.37**	12.11**	3.70**	0.089	12.06
A/gs	9.66	57.07**	552.43**	113.58**	28.62**	47.89**	12.70**	17.36**	0.99	9.67
Pro	0.150	1.017	15.92	9.78	0.575	0.526	0.388	0.474	0.022	4.78

^ns no significant difference. ^*Significant difference at P < 0.5. ^** Significant difference at P < 0.1. R Repeat, C Cultivar, D Drought stress, A Abscisic acid, C×D Interaction between cultivar and drought stress, C×A Interaction between cultivar and Abscisic acid, D×A Interaction between drought stress and Abscisic acid, C×D×A Interaction between cultivar and Drought stress and Abscisic acid. RWC, Relative water content, A Net photosynthetic rate, E Transpiration rate, gs Stomatal conductance, Ci Intercellular CO2 concentration, A/Ci Mesophyll conductance, Ci/gs Mesophyll efficiency, A/E (WUE): Water use efficiency, A/gs (IWUE) Intrinsic water use efficiency, Pro Proline.

Fig. 2.

Interaction between drought stress and ABA on RWC.

Photosynthetic rate, transpiration rate, and intercellular CO₂ concentration

According to the data provided in Fig. 3 to explain the relationship between drought stress and exogenous application of ABA on photosynthesis rate for the two varieties, Hidcote and Munstead, several trends can be observed. The results reflect a clear relationship between the various levels of FC and the concentrations of ABA with their respective effects on photosynthetic rates. Specifically, it is observed that for Hidcote at a high irrigation level of 90–100% FC, the absence of ABA results in a maximum photosynthesis rate of 11.46 µmol m⁻² s⁻¹; however, as ABA concentrations increase to 15 µM and subsequently to 30 µM, a marked decline in photosynthetic efficacy is noted, with rates decreasing to 8.37 µmol m⁻² s⁻¹ and 8.33 µmol m⁻² s⁻¹, respectively. Conversely, for the Munstead variety under the same irrigation conditions, a reduction in photosynthesis is also observed, albeit to a lesser extent, with rates recorded at 9.1 µmol m⁻² s⁻¹ under no ABA treatment and declining to 6.05 µmol m⁻² s⁻¹ at the highest ABA concentration. The trend holds across the different moisture levels represented by 70–80% and 50–60% FC, showing that the more ABA is applied, although useful under drought conditions, it depresses photosynthetic activities across the board when moisture is readily available. Interestingly, when analyzing data for the severe drought-stressed conditions (30–40% FC), lower photosynthetic rates are observed for both varieties, further accentuated by high levels of ABA. This points to a threshold beyond which the generally protective role played by ABA may become counterproductive (Fig. 3).

Fig. 3.

Interaction between Drought stress and Abcisic acid on Photosynthesis rate.

Hidcote and munstead, lavander cultivars; D1A1, 100-90% of field capacity (control)-Drought stress without Abcisic acid; D1A2, 100-90% of field capacity (control)-Drought stress with 15 µM/L Abcisic acid; D1A3, 100-90% of field capacity (control)-Drought stress with 30 µM/L Abcisic acid; D2A1, 80-70% of field capacity Drought stress without Abcisic acid; D1A2, 80-70% of field capacity Drought stress with 15 µM/L Abcisic acid; D2A3, 80-70% of field capacity Drought stress with 30 µM/L Abcisic acid; D3A1, 60-50% of field capacity Drought stress without Abcisic acid; D3A2, 60-50% of field capacity Drought stress with 15 µM/L Abcisic acid; D3A3, 60-50% of field capacity Drought stress with 30 µM/L Abcisic acid; D4A1, 30–40% of field capacity Drought stress without Abcisic acid; D4A2, 30–40% of field capacity Drought stress with 15 µM/L Abcisic acid; D4A3, 30–40% of field capacity Drought stress with 30 µM/L Abcisic acid.

Drought stress and ABA application independently cause severe reductions in the transpiration rates for the two cultivars. The transpiration rates are considerably reduced with increased drought stress from D1 to D4 in both Hidcote and Munstead cultivars (Fig. 4). For Hidcote, the transpiration rate decreases by 46% from D1A1 to D4A1. However, for Munstead, the transpiration rate decreases by 38% from D1A1 to D4A1. Also, with the ABA application level increasing from A1 to A3, the transpiration rates for the two cultivars decrease. For Hidcote, the transpiration rate decreased by 42% from D4A1 to D4A3. For Munstead, for example, the transpiration rate reduced by 35% from D4A1 to D4A3. The maximum recorded transpiration rate among the treatments stands at approximately 8.2 mmol m⁻² s⁻¹ for the Munstead variety under the D3A2 condition. This represents an approximate 66% increase over the Munstead cultivar’s transpiration rate compared to the control condition of D1A1. The lowest rate is observed in the Hidcote variety under the D4A3 condition, at about 0.5 mmol m⁻² s⁻¹. compared to the control (D1A1), a significant reduction of about 92% in the transpiration rate was observed for the Hidcote variety.Looking at the trends in general, the Munstead cultivar shows a less significant decrease in transpiration rate with increasing drought stress levels and ABA application compared to the Hidcote cultivar. At the maximum stress, D4A3, Munstead recorded a 60% reduction in transpiration compared to Hidcote, which recorded a 92% reduction. In general, the highest transpiration rate in Munstead under D3A2 represents a 66% increase compared to the control. The lowest transpiration rate in Hidcote under D4A3 shows a 92% reduction compared to the control. Munstead exhibits a less pronounced reduction in transpiration rate under severe stress compared to Hidcote. generally, the Munstead cultivar showed a higher transpiration rate, but transpiration in both cultivars decreased with increasing drought stress and ABA levels (Fig. 4).

Fig. 4.

Interaction between Drought stress and Abcisic acid on transpiration rate.

Stomatal conductance was significantly affected (p ≤ 0.01) by the main effects of cultivar, drought stress, and abscisic acid (ABA) foliar spray (Table 2). Results showed that the intercellular CO2 concentration for Munstead was significantly higher (approximately 14.5%) than that of Hidcote (Fig. 5A).

Fig. 5.

Main effects of cultivar (A), Drought (B) and Abscisic acid (C) on Intercellular CO₂ concentration.

As drought stress increases from 90 to 100% FC to 30–40% FC, the intercellular CO2 concentration decreases by 44.4% at 90–100% FC, 25.3% at 70–80% FC, 11.1% at 50–60% FC, and 9.1% at 30–40% FC (Fig. 5B). The greatest reduction in intercellular CO2 concentration is observed under the most severe drought stress (30–40% FC). Also, the results show that as the ABA concentration increases, the intercellular CO2 concentration decreases by 25% from 0 μM to 15 μM ABA and by 19.4% from 15 μM to 30 μM ABA. The greatest reduction in intercellular CO2 concentration is observed at the highest ABA concentration (30 μM) (Fig. 5C).

Stomatal conductance, mesophyll conductance, and mesophyll efficiency

By considering the interaction analysis of drought stress and ABA on stomatal conductivity, optimal and suboptimal conditions are clearly recognizable. In well-watered conditions, that is, 90–100% FC without ABA application (0 µM), the highest stomatal conductance was 2.12 mmol H2O M⁻² s⁻¹, considered here as the control or optimal growing condition. However, the greatest impact on stomatal conductance was recorded in the interaction treatment of severe drought stress and high ABA concentration, 30–40% FC/30 µM, which resulted in the minimum conductance of 0.21 mmol H2O M⁻² s⁻¹, representing a 90.1% decrease compared to the optimal condition.The intermediate treatments showed a gradual decline in effectiveness, with moderate conditions (70–80% FC with 15 µM ABA) resulting in a conductance of 0.88 mmol m⁻² s⁻¹, representing a 58.5% reduction from optimal conditions. This analysis highlights that the best stomatal conductance is obtained under well-watered conditions without ABA application, while severe drought stress combined with high concentrations of ABA should be avoided, as it strongly reduces stomatal conductance. The data clearly demonstrate that both drought stress and ABA concentration have significant negative impacts on stomatal conductance, with their combined effect being most pronounced under extreme conditions (Fig. 6).

Fig. 6.

Interaction between Drought stress and Abcisic acid on stomatal conduction.

In the present study, the interaction between drought stress and abscisic acid (ABA) on Mesophyll conductance in both Hidcote and Munstead cultivars showed that The best performance was observed in Hidcote under well-watered conditions (90–100% FC) with no ABA application (0 µM), exhibiting the highest Mesophyll conductance of 0.213 mmol H₂O m⁻² s⁻¹. This was much higher compared to Munstead’s best performance (0.13 mmol H₂O m⁻² s⁻¹) under the same conditions and indicated that Hidcote had stronger stomatal regulation under optimal conditions. On the other hand, the poorest performance was observed for both cultivars under severe drought stress (30–40% FC) with a high ABA concentration of 30 µM (D4A3), both of which had the lowest conductance of 0.01 mmol H₂O m⁻² s⁻¹. compared to the optimal conditions (control), this decrease was 95.3% for Hidcote and 92.3% for Munstead. The intermediate treatments (30–40% and 50–60% FC) showed a gradual decline, with Hidcote maintaining slightly higher conductance values compared to Munstead across most treatment combinations.While both cultivars showed the same trend in response to increased drought stress and increasing ABA concentration, Hidcote was noticeably more resistant under moderate stress conditions to maintaining Mesophyll conductance. It can be concluded from these data that, although both varieties are considerably affected by drought stress and ABA application, Hidcote is more adaptable under stress conditions in maintaining higher Mesophyll conductance values than Munstead, particularly in optimal conditions (Fig. 7).

Fig. 7.

Interaction between Drought stress and Abcisic acid on mesophyll conductance.

The analysis indicates that there are significant differences among treatments in mesophyll efficiency and among cultivars as well. Mesophyll efficiency, contrary to stomatal conductance and mesophyll conductance, was found to increase with increased drought and ABA application (Fig. 8). The lowest mesophyll efficiency for Hidcote (23.67 µmol m⁻² s⁻¹ mmol⁻¹ water) was observed under well-watered conditions (90–100% FC) with no ABA application (D1A1). In contrast, the highest mesophyll efficiency was recorded in Hidcote under severe drought stress (30–40% FC) with 30 µM ABA (D4A3) (1742.75 µmol m⁻² s⁻¹ mmol⁻¹ water), showing a dramatic increase of approximately 7360% from the optimal condition.Quantitatively, the Munstead cultivar generally had higher baseline values but less extreme responses; its lowest value was 37.84 µmol m⁻² s⁻¹ mmol⁻¹ water at 90–100% FC and 0 µM ABA (D1A1), while its highest was 1468.24 µmol m⁻² s⁻¹ mmol⁻¹ water at 30–40% FC with 30 µM ABA (D3A3), a 3879% increase. According to the results presented in Fig. 8, drought stress and ABA application have a significant effect on mesophyll efficiency, but their interaction had the most pronounced effect. These results indicate that Hidcote was more sensitive than Munstead under stress conditions, probably due to different mechanisms of stress adaptation. These findings suggest that maintaining well-watered conditions and managing ABA levels is crucial for optimal plant performance, particularly for the Hidcote cultivar. The results imply that Munstead might be more suitable for cultivation in drought-prone areas due to its more moderate response to stress conditions.

Fig. 8.

Interaction between Drought stress and Abcisic acid on mesophyll efficiency.

Water use efficiency (WUE) and leaf internal water use efficiency (LIWUE)

In the present study, the provided graph shows the interactive effects of drought stress (indicated by field capacity, FC) and abscisic acid (ABA) concentration on the water use efficiency of two lavender varieties, Hidcote and Munstead (Fig. 9). The results show that Water use efficiency increases as FC decreases from 90 to 100% to 30–40% across all ABA levels for both varieties. Higher ABA concentrations (15 and 30 µM) lead to significantly higher water use efficiency compared with no ABA application (0 µM), especially at low FCs. Munstead consistently exhibited higher water use efficiency than Hidcote across the treatment combinations. By decreasing the FC from 90 to 100% to 30–40% with increasing ABA concentrations, the trend of the graph shows a positive slope, indicating that water use efficiency increases. The highest values of water use efficiency occur at 30–40% FC in conjunction with 30 µM ABA. This might therefore be considered the optimal combination of drought stress and ABA signaling for enhancing the water conservation capability of the plants. In contrast, the lowest water use efficiency is observed at 90–100% FC with 0 µM ABA, indicating that well-watered conditions (90–100%FC) without ABA do not provide the necessary stimuli for the plants to upregulate their drought adaptation mechanisms. based on the results, Munstead (1742.75 µmol CO2 mol⁻¹ water) and Hidcote (1468.24 µmol CO2 mol⁻¹ water) achieved the highest water use efficiency at 30–40% FC in combination with 30 µM ABA (D4A3). The lowest water use efficiency was observed in both cultivars at 90–100% FC with 0 µM ABA (D1A1). These findings suggest that Munstead is a cultivar better adapted to tolerate drought stress and maximize wateruse efficiency through ABA-mediated mechanisms.

Fig. 9.

Interaction between Drought stress and Abcisic acid on leaf internal water consumption efficiency.

The present study explains the interaction of drought stress and different concentrations of abscisic acid on leaf internal water use efficiency in two varieties of lavender (Fig. 10). Detailed analysis of Fig. 10 shows that there is a significant difference in the internal water use efficiency of leaves between the treatments explaining the positive as well as negative effects of drought stress and ABA.

Fig. 10.

Interaction between Drought stress and Abcisic acid on plant water efficiency.

Upon examining the treatment levels, it is apparent that increasing drought stress (from 90 to 100% field capacity (FC) to 30–40% FC) generally results in an increase in leaf intrinsic water use efficiency, with the lowest efficiency observed at the highest moisture level and zero ABA (4.84 µmol CO2 m⁻² s⁻¹). However, with the intensification of drought stress, especially with the use of ABA, a significant increase in efficiency is observed, particularly at 30–40% FC. At this level, the intrinsic water use efficiency of the leaf reaches 25.66 µmol CO2 m⁻² s⁻¹ when treated with 30 µM ABA and 23.73 µmol CO2 m⁻² s⁻¹ with 15 µM ABA foliar application. It appears that treatments such as 30–40% FC in combination with both 15 µM and 30 µM ABA produce maximum intrinsic water use efficiencies. These results contrast sharply with the findings at higher moisture contents (90–100% FC), which exhibited substantially lower water use efficiency readings, suggesting that moisture saturation may hinder optimal water utilization in these cultivars. the results also show that at the best -performing condition of 30–40% FC and 15 µM ABA, Munstead reached a maximum efficiency of 16.2 µmol CO2 m⁻² s⁻¹, while Hidcote peaked at 25.66 µmol CO2 m⁻² s ^-¹ under the same conditions. This represents a 58.4% lower efficiency for Munstead compared to Hidcote. The lowest efficiency for both varieties was at 90–100% FC with 0 µM ABA (D1A1), with values of 4.84 µmol CO2 m⁻² s⁻¹ for Munstead and 5.14 µmol CO2 m⁻² s⁻¹ for Hidcote. Across the range of treatments, the difference in maximum efficiency between the two varieties is 9.46 µmol CO2 m⁻² s⁻¹, with Hidcote being the superior cultivar. These differences demonstrate that Hidcote is better adapted to optimize water use under drought stress via ABA-mediated mechanisms (Fig. 10).

Proline contant

Drought stress and exogenous application of ABA cause changes in Proline content within the leaves of lavender cultivar plants (Fig. 11). The evaluation of Hidcote cultivar data shows a clear trend of increasing proline content as field capacity (FC) decreases. Proline levels range from 1.69 to 2.87 µmol/g FW at 90–100% FC, with the highest value observed at 30 µM ABA application. As FC decreased to 70–80%, proline content increased further and reached a maximum of 3.76 µmol/g FW under 30 µM ABA treatment. the highest amount of proline, at 5.07 µmol/g FW, was observed in the Hidcote variety treated with 30–40% FC and 30 µM ABA (D4A3). These findings suggest that Hidcote plants actively accumulate proline, a well-known osmolyte, in response to increasing drought stress, and this response is further enhanced by the application of exogenous ABA. In contrast, proline accumulation in the Munstead cultivar presented a different pattern. Proline content at 90–100% FC was relatively low, ranging between 0.71 and 1.89 µmol/g FW, with the highest value at 30 µM ABA. The results showed that, by increasing drought stress to 70–80% FC, the proline content increased significantly and reached a maximum of 3.7 µmol/g FW at 30 µM ABA (D2A3). The highest amount of proline, 4.3 µmol/g FW, was observed in the Munstead cultivar at 30–40% FC with 15 µM ABA treatment (D4A2).These results indicate that the Munstead cultivar also responds to drought stress by accumulating proline, but the magnitude of this response is not as pronounced as in the Hidcote cultivar, especially at lower FC levels (Fig. 11).

Fig. 11.

Interaction between Drought stress and Abcisic acid on proline content.

These differences in proline accumulation between the Hidcote and Munstead varieties indicate that various physiological mechanisms and adaptations have taken place in response to drought stress. Apparently, the Hidcote cultivar is more sensitive to water deficit conditions, as its highest proline content occurred at a high level of drought stress (30–40% FC). In contrast, the Munstead cultivar seems to maintain a more moderate proline response, which might indicate differences in its strategy for drought tolerance. The application of exogenous ABA at lower FC levels enhances proline accumulation in both cultivars. The obtained results suggest that abscisic acid is an important regulator in the biosynthesis and accumulation of proline, thus representing an adaptive mechanism against drought stress. The differential responses of the two cultivars to ABA treatment could also indicate possible differences in ABA sensitivity or underlying regulatory mechanisms.

Heat map analysis

To better understand the physiological responses of two cultivars of Lavandula angustifolia, specifically cv. Hidcote and cv. Munstead, subjected to varying treatments of drought stress and the application of abscisic acid, a heat map was plotted. Figure 12 shows the heat map of the impacts of these treatments on photosynthetic gas exchange, plant water relations, and osmotic regulation, which could reveal important information about the adaptive mechanisms of these cultivars under stress conditions. Upon examining the results encapsulated in the heat map, a discernible trend emerges in the relationship between the severity of drought stress and the physiological attributes measured, such as relative water content (RWC), assimilation rate (A), transpiration rate (E), stomatal conductance (gs), intercellular CO2 concentration (Ci), the ratio of assimilation to intercellular CO2 concentration (A/Ci), and osmotic regulation parameters such as proline (Pro). In the analysis, the best and worst levels of treatments for each parameter across both cultivars are pointed out, delineating the variation in performance under imposed stress conditions. The most striking evidence for enhanced drought tolerance under imposed stress conditions was in cv. Hidcote, which showed better resistance compared to cv. Munstead due to drought stress and the application of abscisic acid, indicative of a more robust physiological response. The maximum RWC for cv. Hidcote was 85.95% in the C1D1A2 treatment, showing an increased capacity to retain water compared with the maximum of 73.49% in cv. Munstead (C2D1A2 treatment). Similarly, for cv. Hidcote, the highest assimilation rate was 11.46 µmol m⁻² s⁻¹ for the treatment C1D1A1, while for cv. Munstead, this value was considerably lower, amounting to only 9.10 µmol m⁻² s⁻¹ for the treatment C2D1A1. In contrast, under severe drought stress, harmful effects were manifested in both cultivars. Thus, for the treatment C1D4A1 of cv. Hidcote and C2D4A1 of cv. Munstead, RWC drastically reduced to 27.30% and 21.53%, respectively. These values were accompanied by sharp declines in assimilation rates, which once again underlined the role of drought stress and hormonal regulation as factors in reducing plant performance.

Fig. 12.

Heat Map of the effect of drought stress and abscisic acid on changes in photosynthetic gas exchange, plant water relations and osmotic regulation of two cultivars Lavndula angustifolia cv Hidcote and Lavndula angustifolia cv Munstead. C1D1A1 Hidcote cultivar× 100-90% of field capacity (control)-Drought stress× without Abcisic acid, C1D1A2 Hidcote cultivar× 100-90% of field capacity (control)-Drought stress× 15 µM/L Abcisic acid, C1D1A3 Hidcote cultivar× 100-90% of field capacity (control)-Drought stress× 30 µM/L Abcisic acid, C1D2A1 Hidcote cultivar× 80-70% of field capacity Drought stress× without Abcisic acid, C1D2A2 Hidcote cultivar× 80-70% of field capacity Drought stress× 15 µM/L Abcisic acid, C1D2A3 Hidcote cultivar× 80-70% of field capacity Drought stress× 30 µM/L Abcisic acid, C1D3A1 Hidcote cultivar× 60-50% of field capacity Drought stress× without Abcisic acid, C1D3A2 Hidcote cultivar× 60-50% of field capacity Drought stress× 15 µM/L Abcisic acid, C1D3A3 Hidcote cultivar× 60-50% of field capacity Drought stress× 30 µM/L Abcisic acid, C1D4A1 Hidcote cultivar× 40-30% of field capacity Drought stress× without Abcisic acid, C1D4A2 Hidcote cultivar× 40-30% of field capacity Drought stress × 15 µM/L Abcisic acid, C1D4A3 Hidcote cultivar× 40-30% of field capacity Drought stress × 30 µM/L Abcisic acid, C2D1A1 Munstead cultivar× 100-90% of field capacity (control)-Drought stress× without Abcisic acid, C2D1A2 Munstead cultivar× 100-90% of field capacity (control)-Drought stress× 15 µM/L Abcisic acid, C2D1A3 Munstead cultivar× 100-90% of field capacity (control)-Drought stress× 30 µM/L Abcisic acid, C2D2A1 Munstead cultivar× 80-70% of field capacity Drought stress× without Abcisic acid,C2D2A2 Munstead cultivar× 80-70% of field capacity Drought stress× 15 µM/L Abcisic acid,C2D2A3 Munstead cultivar× 80-70% of field capacity Drought stress× 30 µM/L Abcisic acid, C2D3A1 Munstead cultivar× 60-50% of field capacity Drought stress× without Abcisic acid, C2D3A2 Munstead cultivar× 60-50% of field capacity Drought stress× 15 µM/L Abcisic acid, C2D3A3 Munstead cultivar× 60-50% of field capacity Drought stress× 30 µM/L Abcisic acid, C2D4A1 Munstead cultivar× 40-30% of field capacity Drought stress× without Abcisic acid, C2D4A2 Munstead cultivar× 40-30% of field capacity Drought stress × 15 µM/L Abcisic acid, C2D4A3 Munstead cultivar× 40-30% of field capacity Drought stress × 30 µM/L Abcisic acid.

A/Ci is an indicator of photosynthetic efficiency. The data show that, with increased drought stress, A/Ci is reduced in both cultivars, indicating that photosynthetic efficiency decreases when water limits photosynthesis. This decline is expected from the observed decrease in gs and the subsequent limitation to CO2 supply for photosynthesis. However, the response of A/Ci to ABA is less consistent across different levels of drought and cultivars, pointing to the complex interaction between ABA signaling, stomatal regulation, and photosynthetic capacity under stress. The same trend is observed in the relationship between Ci and gs (Ci/gs), indicating reduced efficiency of CO2 diffusion under drought stress.

Notably, the high concentration of proline in both cultivars subjected to extreme treatments reflects an osmotic adjustment mechanism; the proline concentration for cv. Munstead reaches 5.5 µmol/g FW in treatment C2D4A2, demonstrating the plant’s effort to mitigate the harsh effects of drought. Furthermore, the relationship between stomatal conductance (gs) and intercellular CO2 concentration (Ci) reveals distinct adaptation strategies employed by each cultivar. For instance, cv. Hidcote maintained a relatively higher stomatal conductance (up to 2.37 mmol H₂O m⁻² s⁻¹ in C1D1A1), which was correlated with more effective CO2 uptake under less severe stress. In contrast, under high stress levels, gs of cv. Munstead consistently declined to values as low as 0.37 mmol H₂O m⁻² s⁻¹ in the C2D4A1 treatment, adopting a conservative water-use strategy that nonetheless limited gas exchange and thereby reduced photosynthetic efficiency (Fig. 12).

Principal component analysis (PCA)

To determine the relative contribution of each trait under drought conditions and to assess the effect of ABA application on these traits, principal component analysis (PCA) was performed (Fig. 13). The results of the PCA showed that the first and second components explained 63.57% and 40.23% of the total variance, respectively, accounting for 97.87% of the total variance. The eigenvector values in the first component indicated that plant water use efficiency (A/E), leaf intrinsic water use efficiency (A/gs), mesophyll efficiency (Ci/gs), intercellular CO2 concentration (Ci), and proline (Pro) had the greatest influence on the formation of this component. In the second component, with 40.23% of the variance, mesophyll conductance (A/Ci), photosynthetic rate (A), stomatal conductance (gs), relative water content (RWC), and transpiration rate (E) were identified as the best traits in explaining the variations. Therefore, using these two components and ignoring the other components would result in a loss of only about 13.03% of the variations (Fig. 12). Additionally, the PCA results showed that the changes in the first component were related to the D4A3 condition for both genotypes. in the second component, the greatest changes observed were for the traits of photosynthetic rate (A), stomatal conductance (gs), and mesophyll conductance (A/Ci) related to D1A1, and for the trait of RWC related to D1A2 (Fig. 13). The figure below shows the results of principal component analysis (PCA) for the Hidcote and Munstead cultivars under drought stress and ABA application. Principal component analysis was used to understand the physiological behavior of these cultivars under imposed environmental conditions. The Results show that RWC values demonstrate a clear distinction between the two cultivars. The Hidcote cultivar exhibited a more pronounced decrease in RWC under increasing drought stress; the lowest RWC was observed under the C1D4A3 treatment. Conversely, Munstead presented relatively higher RWC across different levels of drought stress, reflecting that this cultivar is more tolerant of water deficit stress. In addition, The Hidcote cultivar showed a more dynamic response to the treatments, with the highest photosynthesis rate (A)observed under moderate drought stress (C1D2A2) and a subsequent decline under severe stress. The Munstead cultivar, on the other hand, maintained a more consistent photosynthesis rate (A) across the drought stress levels. The Hidcote cultivar was more sensitive to drought stress, showing greater reductions in gs and Ci with increasing water deficit. In the Munstead cultivar, better maintenance of these parameters was observed, especially under severe drought conditions at C2D3 and C2D4. The Hidcote cultivar demonstrated higher wateruse efficiency (A/Ci, A/E, and A/gs) under moderate drought stress (C1D1A2), indicated by increased A/Ci and A/E ratios. In contrast, the Munstead cultivar maintained a more stable wateruse efficiency across the treatments. Also, the results showed that the Hidcote cultivar exhibited higher proline accumulation under moderate drought stress (C1D3A2 and C1D3A3), suggesting a greater reliance on this compatible solute to alleviate the effects of water deficit. The Munstead cultivar, on the other hand, showed a less pronounced proline response, indicating a potential alternative mechanism for drought tolerance. Overall, the PCA results highlight the contrasting responses of the Hidcote and Munstead cultivars to drought stress and ABA application. The Munstead cultivar appears to exhibit a more robust and adaptive physiological response, maintaining better water status, photosynthetic performance, and wateruse efficiency under imposed environmental conditions. These findings suggest that the Munstead cultivar may possess inherent drought-tolerant characteristics that could be valuable for future agricultural applications or breeding programs aimed at improving crop Resilience to water scarcity.

Fig. 13.

Principal component analysis (PCA) Effects of Drought Stress and Abscisic acid on photosynthetic gas exchange, plant water eelations, and osmotic adjustment in two Lavandula angustifolia Cultivars.

Discussion

The present work was undertaken to study the physiological responses and water relations of two cultivars of English lavender subjected to drought stress and exogenous abscisic acid (ABA) application. Observed changes in photosynthesis and water relations suggested that the plants may overcome the adverse effects of drought by leveraging some of the ABA effects. Drought stress affected gas exchange for both cultivars and led to proline accumulation. The highest values of leaf relative water content, photosynthetic rate, stomatal conductance, mesophyll conductance, and transpiration rate were observed under non-drought conditions, while mesophyll efficiency, plant water use efficiency (WUE), intrinsic water use efficiency (IWUE), and proline accumulation increased under drought stress and exogenous application of ABA.

Drought impairs the water status of a plant and significantly influences physiological parameters such as photosynthesis, respiration, and the uptake of mineral nutrients. Drought impairs gas exchange and the water relations of plants. Sun et al²⁰. reported that the current results indicated that plants grown under drought conditions had lower relative water content (RWC) compared with those grown under non-stress conditions. This, considering the effect of drought on water uptake and metabolic leaf water loss from the plant, leads to a decrease in leaf relative water content. Among the various methods of measuring leaf water status, relative water content is a critical indicator of drought stress in leaves, impacting the plant’s ability to tolerate stress severity and ultimately affecting its performance and sustainability. drought stress populates intercellular spaces and reduces water content within the plant body by increasing osmotic substances inside the tissues to facilitate water uptake from the soil; hence, the relative water content decreases due to drought stress. Drought hampers growth through a reduction in RWC and total water potential. The mechanism of osmotic adjustment helps drought-tolerant plants maintain and enhance RWC. Reduced RWC was also documented in lavender and plant species from the Mediterranean region¹⁰. Similar to other reports under water deficit conditions, the leaf water potential of these varieties declined significantly. Due to low water uptake, metabolic production is reduced to sustain normal cell functions compared to plants receiving adequate water^1,21. It therefore appears that, under drought conditions, lavender plants mitigate stress by sustaining high RWC values through increased water accessibility to cells mediated by non-stomatal factors. However, under severe drought conditions, RWC was reduced owing to the limited access of lavender plants to water and its lower allocation to aerial organs, while the use of exogenous ABA maintained RWC in the leaf. ABA serves as an endogenous signal in the perception of ambient conditions and in adaptive responses to biotic stresses. As a major regulator of plant water status, this hormone specifically targets guard cells, inducing stomatal closure. It provides systemic signaling under severe water scarcity, thus preventing a decrease in leaf relative water content^7,22. This effect of exogenous ABA in maintaining stomatal closure was more evident in the Hidcote cultivar than in Munstead.

Photosynthetic rate (A), transpiration rate (E), and stomatal conductance (gs) are responsive to changes in environmental water status⁴. Accordingly, stress effects have appeared as reduced photosynthesis and growth of the whole plant. Drought stress is suspected of reducing photosynthesis by triggering stomatal closure, thus limiting CO₂ availability to mesophyll cells rather than directly affecting the apparent photosynthetic rate. stomatal closure is a predominant symptom of drought in the soil. There is a parallel decline in photosynthesis and stomatal conductance under drought stress²³. A well-established fact is that drought stress impairs mesophyll metabolism, leading to a decline in photosynthetic capacity by impairing RUBP synthesis and/or the activity of Rubisco²⁴. Results showed that, for the most severe drought condition in this work (30–40% FC with a foliar spray of 30 µM ABA), the stomatal and mesophyll conductance measured were 0.19 and 0.012 mol H₂O m⁻² s⁻¹ for Hidcote and 0.24 and 0.010 mol H₂O m⁻² s⁻¹ for Munstead, respectively. A comparison of both cultivars shows that the Hidcote cultivar had a more rapid mechanism for stomatal conductance limitation. The stomatal and mesophyll conductance reported in both cultivars seem sufficient to support reduced transpiration in this study, as they maintain an adequate internal amount of CO₂ for a reasonable photosynthetic rate.

According to researchers, the optimal value of stomatal conductance is around 0.1 mol m⁻² s⁻¹. Higher values do not significantly increase net photosynthesis and become less effective²⁵. Conversely, if stomatal conductance is consistently below 0.1 mol m⁻² s⁻¹, non-stomatal factors may also contribute to reduced net photosynthesis. Consequently, the declines in net photosynthesis observed in the leaves of lavender may result from stomatal limitation. Stomatal closure is the immediate plant response to drought stress and decreases the rate of photosynthesis accordingly²⁶.

Drought stress, affecting both light-dependent and light-independent reactions, causes significant damage to photosystems and blocks the electron transport chain²⁷. This stomatal closure leads to a deficiency in carbon dioxide in plants and induces photorespiration. This not only lowers efficiency in carbon fixation but also reduces total photosynthetic output. While this study has shown that, with increased severity of drought, photosynthesis is constrained, the efficiency of water use increases, with the Hidcote cultivar showing better efficiency in water use. Drought stress has been reported to decrease net photosynthesis under drought stress in several plant species²⁸. Under stress conditions, plants reduce their stomatal conductance to prevent excessive water loss through transpiration, which in turn reduces net photosynthesis. This response can, nevertheless, improve leaf intrinsic and whole-plant water use efficiency²⁹. The ability of cultivars to keep their stomata closed can be screened to identify drought-resistant cultivars. The resinous cultivars of lavender studied for sensitivity and transpiration rate showed that Munstead demonstrated a higher transpiration rate under stress conditions compared to Hidcote, which may indicate an inability to keep its stomata closed. the high transpiration rate and subsequent water loss ultimately lead to a decrease in photosynthesis and water use efficiency in the plant. In the present study, the higher transpiration rate in the Munstead cultivar led to lower rates of photosynthesis and water use efficiency.

Other effects of drought include disturbance in plant homeostasis. Plants respond to water scarcity via increased endogenous production of growth regulators like ABA, which increases the expression of multiple genes and activates signaling pathways, leading to a reduction in activities such as stomatal conductance and internal CO₂ availability for photosynthesis³⁰. Following a decline in CO₂ availability, a decline in the carboxylation activity of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) follows, resulting in depressed photosynthesis^31,32. During drought, ABA acts as a signaling molecule in stomatal closure by inducing the opening and closing of ion channels via protein kinase and phosphatase activity. Hormones, particularly ABA, regulate stomatal conductance, chlorophyll content, and osmotic regulation, increasing drought tolerance²⁷. ABA also plays the role of a primary chemical signal by inducing secondary messengers, such as reactive oxygen species (ROS), nitric oxide, and Ca²⁺, which ultimately result in stomatal closure, reduced transpiration rate, and increased water use efficiency³³.

In the present study, reduced irrigation led to a decrease in net photosynthesis, transpiration, and stomatal and mesophyll conductance at all levels of abscisic acid application. Studies have shown that during abiotic stresses, ABA-treated plants often have lower gas exchange rates compared to plants without ABA treatment³⁴. Based on this principle, it can be stated that the external application of ABA allows the plant to access ABA sooner than in the natural state, initiating stress resistance reactions faster. Hormones, particularly ABA, modulate stomatal conductance and osmotic regulation, increasing drought tolerance²⁷. In this study, with the external application of ABA, lavender plants showed greater resistance to drought stress by limiting gas exchange and increasing mesophyll efficiency, subsequently increasing water use efficiency, in which the Hidcote cultivar had a significant advantage in all mentioned traits.

We also examined, in the present study, the most important change in non-enzymatic systems, namely changes in proline accumulation in the leaves of lavender plants. It was found that in both cultivars tested, proline accumulation increased under drought conditions. Comparing the two lavender cultivars for leaf proline accumulation under drought conditions, the Hidcote cultivar appeared more responsive to ABA. However, the trend of proline accumulation in this cultivar showed that, under 30–40% field capacity conditions, the application of ABA played an extremely effective role in proline accumulation. According to the results of the present study, besides altering stomatal behavior, lavender varieties benefit from proline accumulation to maintain cell homeostasis. Changes in proline levels have been demonstrated in some plants to correlate with the potential for drought tolerance, and this change can be used as an indicator for selecting drought-resistant plants³⁵. under drought stress, plants increase their osmolyte concentration to enable continued water uptake under stress conditions. Proline is one of the most prevalent compatible osmolytes among organic osmolytes³⁶. lavender cultivars likely enhance proline accumulation for the reasons stated above. The increase in proline under drought stress has also been reported in wild soybean³⁷ and fennel³⁸. This has led to speculation that ABA might induce proline accumulation, as both accumulate in response to stress conditions. ABA likely plays a pivotal role in the production of compatible osmolytes, such as proline, in response to drought by initiating signals that produce a wide range of regulatory proteins, including transcription factors and signaling factors, and functional proteins, including enzymes that regulate reactive oxygen species. Studies on drought stress in wild varieties of eggplant showed that proline concentration increased under water stress in all accessions, with the rise being higher in the more tolerant ones, showing an average increase of over 30-fold compared to controls³⁹. Other works also reported that exogenous ABA played a vital role in increasing proline content under drought conditions^20,40. Examples include studies by Sun et al²⁰. and Yang et al.⁴⁰.

Conclusion

The present study investigates, for the first time, how two English lavender cultivars, Hidcote and Munstead, tolerate drought stress by simultaneously examining physiological parameters related to photosynthesis, water relations, and osmotic adjustment under various drought and ABA treatments. The Application of exogenous ABA with drought stress significantly mitigated water loss. Hidcote, through better utilization of exogenous ABA under drought conditions, maintained higher relative water content in leaves by initiating mechanisms such as stomatal closure. Photosynthetic rate and transpiration rate were reduced by drought stress and ABA. Water use efficiency (WUE) increased with decreasing field capacity (FC) and increasing ABA concentration. Munstead exhibited higher WUE than Hidcote across all treatments. The Hidcote cultivar was more drought -sensitive compared to the Munstead cultivar. However, it mitigated the negative effects of drought stress by controlling osmolytes through the accumulation of proline. The comparative study of both varieties showed that the Hidcote cultivar, under optimal conditions, exhibited a higher photosynthetic rate and greater responsiveness to stress. The Munstead cultivar displayed higher water use efficiency, a more stable physiological state, and better adaptation to drought conditions. The Use of ABA under drought conditions shows strong potential in breeding strategies to enhance resistance to environmental stresses in plants. This is particularly significant in drought-prone regions, where optimizing water use efficiency can reliably contribute to mitigating agricultural setbacks and ensuring economic viability for lavender growers. The results of the present study provide a better understanding of how to select a suitable cultivar for dry conditions. The munstead cultivar can thus be recommended for long-term dry conditions.

Abbreviations

LRWC

Leaf relative water content

A

Net photosynthetic rate

Ci

Intercellular CO2 concentration

gs

Stomatal conductance

E

Transpiration rate

A/Ci

Mesophyll conductance

Ci/gs

Mesophyll efficiency

A/E, WUE

Water use efficiency

A/gs, IWUE

Intrinsic water use efficiency

Pro

Proline

D₁

100-90% of field capacity (control)-Drought stress 1

D₂

80-70% of field capacity- Drought stress 2

D₃

60-50% of field capacity- Drought stress 3

D₄

30-40% of field capacity- Drought stress 4

A1

Control

A2

15 µM/L Abcisic acid

A3

30 µM/L Abcisic acid

D1A1

100-90% of field capacity (control)-Drought stress without Abcisic acid

D1A2

100-90% of field capacity (control)-Drought stress with 15 µM/L Abcisic acid

D1A3

100-90% of field capacity (control)-Drought stress with 30 µM/L Abcisic acid

D2A1

80-70% of field capacity Drought stress without Abcisic acid

D1A2

80-70% of field capacity Drought stress with 15 µM/L Abcisic acid

D2A3

80-70% of field capacity Drought stress with 30 µM/L Abcisic acid

D3A1

60-50% of field capacity Drought stress without Abcisic acid

D3A2

60-50% of field capacity Drought stress with 15 µM/L Abcisic acid

D3A3

60-50% of field capacity Drought stress with 30 µM/L Abcisic acid

D4A1

30-40% of field capacity Drought stress without Abcisic acid

D4A2

30-40% of field capacity Drought stress with 15 µM/L Abcisic acid

D4A3

30-40% of field capacity Drought stress with 30 µM/L Abcisic acid

Author contributions

A. wrote the main text of the manuscript and B performed the data analysis and C.D. prepared the manuscript figures.All authors reviewed this manuscript.

Funding

This research was funded by Gorgan University of Agricultural Sciences and Natural Resources (Grant No. 0171851) as part of a doctoral dissertation project.

Data availability

All data generated or analysed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.