Physiological and morphological drivers of early-spring vigor in warm-climate crops under suboptimal temperatures

Abstract

Shifting the sowing date of summer crops such as melon (Cucumis melo L.) to early spring presents both agronomic opportunities and challenges. Under suboptimal temperatures, slowed physiological development allows weeds to gain a competitive advantage. Enhancing early seedling vigor under cool conditions is therefore essential for successful crop establishment and effective weed suppression. In this study, ten melon accessions representing a broader collection (~ 180) were evaluated for early vigor and performance under suboptimal temperatures. Pulling resistance, root system scanning, and physiological measurements were used to identify key traits associated with early root establishment. Accessions were compared under optimal (25 °C) and suboptimal (16 °C) temperature regimes. Pulling strength showed a strong correlation with root biomass and volume across accessions. The AY accession exhibited clear tolerance to low temperatures, maintaining high physiological activity, whereas PI414723 displayed reduced carbon assimilation due to diffusional limitations. These findings demonstrate that tolerance to suboptimal temperatures exists even within warm-climate crops such as melon and highlight the potential of AY as a valuable genetic resource for developing early-season, stress-tolerant melon varieties.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-50691-z.

Introduction

In response to climate change, many melon (Cucumis melo L.) growing regions have shifted sowing from the traditional summer season to late winter or early spring to reduce the adverse effects of elevated temperatures and the associated increase in evapotranspiration and pest pressure^1,2. However, early-season planting under suboptimal temperatures slows crop development, and therefore promotes weed dominance³. These challenges highlight the need to identify melon accessions with rapid early-stage growth and adaptation to suboptimal temperatures.

Melon is one of the most popular fruits globally (http://faostat3.fao.org/). Its origin traces back to warm regions in the Indian peninsula, with related species found in other warm areas, including East Asia, and Australia^4,5. Consequently, melon domestication occurred independently in multiple regions⁵, leading to a diverse range of plant traits^6,7. This variety provides a valuable resource for improving melon adaptation to different agro-technical and climatic growth conditions.

Weed management represents a primary challenge in open-field melon cultivation, as the crop’s prostrate growth habit and trailing vines often fail to achieve full canopy closure^8,9. This leaves the soil surface exposed to light, promoting successive flushes of aggressive summer weeds, such as Purple nutsedge (Cyperus rotundus), Puncturevine (Tribulus terrestris L.), Johnsongrass (Sorghum halepense), and pigweeds (Amaranthus spp.), which significantly outcompete the crop for resources. The vulnerability of melon is further exacerbated by a lack of registered herbicides, necessitating a reliance on mechanical weeding^8,10. However, the efficacy of mechanical intervention depends on the crop possessing greater physical resistance to displacement than the surrounding weeds. These management difficulties are compounded by environmental stressors; while weeds maintain high competitiveness across various temperatures, melon development is significantly impaired under suboptimal conditions¹¹. Furthermore, early-stage weed interference alters the red: far-red (R: Fr) light ratio, triggering physiological suppression of both above- and below-ground development that can result in permanent yield reductions¹². Consequently, achieving timely and effective weed control, particularly under thermal stress, is critical for ensuring crop productivity and stability.

Crop plants with early vigor have a competitive advantage against weeds¹³. Early and rapid canopy establishment suppresses weed growth by limiting their access to light¹⁴. In addition to the above-ground suppression, a well-developed root system can hinder weed emergence and establishment¹⁵. Strong root development also enhances plant resilience to environmental fluctuations, resource limitations, and various biotic and abiotic stresses^16–19.

Most early vigor studies determining differences among various varieties and/or accessions focused on wheat (Triticum sp.) and other cereal crops¹³, while research on dicot crops remains limited²⁰. To our knowledge, no previous study has examined the diversity of melon accessions for their early vigor under varying temperatures. Early vigor is strongly affected by environmental factors, particularly temperature, which drives genotype-by-environment (G × E) interactions. Low and suboptimal temperatures are well-documented to inhibit physiological, enzymatic, and molecular processes in plants^21–23. Thus, identifying melon accessions with consistent root development under moderate and suboptimal temperatures has multiple advantages. It can enhance early vigor and weed competitiveness across varying climates while also ensuring safe, sustainable, and effective mechanical weed control.

Suboptimal temperatures significantly inhibit plant physiological activity. Previous studies have identified the minimum, optimum, and maximum temperatures for melon growth as 10 °C, 34 °C, and 45 °C, respectively²⁴. These conditions reduce chlorophyll production, decrease membrane fluidity, and restrict nutrient translocation, ultimately limiting plant function^22,23,25. Warm-region crops, such as melons, are sensitive to moderate temperature reductions and brief frost events²⁶. During abiotic stress, such as unfavorable temperatures, the translocation of carbohydrates and minerals from roots to shoots can be significantly inhibited^27–30. Therefore, physiological measurements can provide insights into differential responses among accessions, aiding in the identification of those with adaptive traits.

Root system phenotyping is challenging due to its belowground development and the destructive nature of most measurement methods^31,32. Therefore, field-based root assessments and screening breeding populations for root traits require intensive labor. In this context, root pulling using a digital force gauge provides a simple and effective first line method for assessing root system pulling strength under both field and laboratory conditions¹⁰. Given the link between pulling strength and root system structure, this method serves as a useful proxy for root system dynamics under varying conditions^33–35. The force required to uproot a plant is strongly correlated with key root traits such as length, diameter, biomass and other parameters^36–38. However, this method should be used cautiously to minimize biases introduced by soil properties (e.g., texture, soil water content) and pulling direction³⁸.

Current knowledge of melon root system diversity and function remains limited, and the relationship between root system architecture and mechanical pulling strength is still poorly understood. Only a few studies have examined variation among melon accessions in these traits. To develop sustainable strategies for mechanical weed management during early melon growth, a deeper understanding of root system characteristics and their mechanical properties is therefore essential.

This study aims to screen and characterize a diverse melon population for early development, with a particular focus on performance under suboptimal temperature conditions. By integrating the pulling method, root system screening, and leaf-level gas exchange and chlorophyll fluorescence measurements, the study seeks to identify melon accessions combining tolerance to suboptimal temperatures with strong and resilient root systems.

Materials and methods

Plant material and growing conditions

From the Newe Ya’ar collection of 177 melon (Cucumis melo L.) accessions⁶, 10 accessions representing the main variety types were selected (Table 1). Plants were grown in 2-liter pots (Pot dimensions: top diameter = 16 cm, base diameter = 14 cm, height = 14 cm; volume ≈ 2.46 l) filled with local soil (chromic Haploxerert, fine-clayey, montmorillonitic, thermic) comprising 55% clay, 25% silt, and 20% sand. Soil porosity was found to be ~ 60%, the bulk density of the soil was 1.02 g cm^− 3, where field capacity and wilting point is 37% and 22% respectively (see Figure S1). The final mixture contained 2% organic matter and a pH of 7.2. Each accession was grown in five biological replicates.

Table 1.

The ten representative melon accessions used in this work from the Newe Ya’ar collection. The accessions are not subject to patent protection.

Line Name	Variety name	Sub-Species	Variety type	Marketing type	Origin (Company)
AY	Ananas Yoqne’am	melo	Reticulatus	Ananas	Israel (H’azera)
DUL	Dulce	melo	Reticulatus	American cantaloupe	USA (Bonanza)
NA	Noy Amid	melo	Inodorus	Yellow Canary	Israel (Newe Ya’ar)
NY	Noy Yizre’el	melo	Cantalupensis	Ha’Ogen	Israel (Newe Ya’ar)
PI414723	PI414723	agrestis	Momordica	N.D	India (Pitrat M. - INRA)
PSR	Piel De Sapo Redon	melo	Inodorus	Piel De Sapo	Spain (DEFO)
QME	Qishu Meshulash	C. callosus	N.D	N.D	Israel (Wild accession)
SAS	Sakata Sweet (17–725)	agrestis	Makuwa	N.D	Japan (Skata)
TAD	TAM DEW	melo	Inodorus	Honey Dew	USA (Texas A&M)
VEP	Vedrantais	melo	Cantalupensis	Charentais	France (Pitrat M. - INRA)

N.D. Not Defined.

Plant development under varying temperatures

In the first experiment, the 10 accessions from the Newe Ya’ar collection were grown under greenhouse conditions during the winter of 2022 (planting date—January 3rd) under natural light conditions (Fig. 1). Temperature was set to a minimum of 18 °C (heating activated) and a maximum of 30 °C (fan activated) with an average temperature of ~ 20 °C. Plants were pulled at three phenological stages: two-three true leaves, three-four true leaves, and four-five true leaves. Each measurement point (stage by accession) had five replicates.

Fig. 1.

Experimental workflow overview.

Source: AI-generated via Gemini.

Pulling strength was measured using a digital force gauge (AXIS, FB500N, range 0.1–50 N). The plant shoots were attached to the device with a plastic clamp and pulled horizontally, recording the maximal pulling strength for each plant. To ensure uniform soil properties, all plants were irrigated to field capacity on the morning of the pulling day, and pulled after two hours when water level is still under field capacity (figure S2). Soil water content was measured using a true time-domain reflectometer (TDR; Acclima Inc., Meridian, ID, USA) following a two-point calibration (0 and 100%) conducted weekly. After the third pulling event, the soil was washed gently with water, and the root system was scanned and analyzed using WinRhizo 2019a (Regent Instruments Inc., Ottawa, ON, Canada)³⁹. Roots were scan in high resolution of 600 DPI, to ensure the detection of small diameter roots. Roots and the shoots were then oven-dried for 48 h at 72 °C before weighing. Then, the specific root length (SRL, g m^− 1) and specific root surface area (SRA, g cm^− 2) were calculated from the WinRhizo scanning and dry weight measurement.

In the second experiment four accessions different in their pulling strengths were selected to represent the variety in the collection. These accessions were grown in controlled environment rooms under two temperature regimes: low (16 °C) and ambient (25 °C). Plants were grown under artificial photosynthetic active radiation (PAR, 400–700 nm) lights (~ 1000 µmol m^− 2 s^− 1, 12/12 hours day/night period), simulating early spring daylight conditions. Pulling was performed at three developmental stages, as outlined in the first experiment, with five replicates for each accession at each stage, totaling 120 plants. Roots and shoots were then dried and weighed as described above.

From the four selected accessions, two with distinct temperature responses were chosen for further study. These accessions were grown under identical controlled environmental conditions as described previously. At the four-five true leaf stage, physiological measurements, including CO₂ and H₂O fluxes, along with chlorophyll fluorescence, were conducted using the LI-COR 6800F system (Licor, Lincoln, NE, US). Pulling strength measurement, root scanning, and below and above biomass evaluation were conducted as described above, with five replicates per treatment.

GAS exchange and fluorescence parameters

In the second controlled environment experiment, gas exchange and fluorescence were measured using the LI-6800F system. Measurement conditions were set to 415 ppm CO₂, 40% relative humidity, a light intensity of 1000 µmol m⁻² s⁻¹, and a temperature corresponding to the ambient room temperature. The youngest fully matured leaf was measured in all plants. Photosynthetic efficiency was assessed using the Pulse Amplitude Modulation (PAM) method, which was conducted simultaneously with flux measurements. To evaluate dark-adapted PAM parameters, measurements were taken in the early morning before the lights were turned on. The functionality of Photosystem II (PSⅡ)⁴⁰, named was evaluated using the following equation:

1

where and are the maximal and minimal fluorescence measured under dark conditions, and F_v is the delta between them. The light acclimated quantum yield of PSⅡ (Y(Ⅱ)) was calculated as:

2

where and are the maximal and minimal fluorescence measured under light conditions.

Statistical analysis

Data normality was assessed using the Shapiro-Wilk test, and homogeneity of variance was tested using Levene’s test. Differences between accessions were analyzed using one-way analysis of variance (ANOVA), followed by Tukey-HSD with Bonferroni correction for multiple comparisons (α < 0.05). Pearson correlation analysis was conducted to evaluate the strength and direction of the linear relationships between mechanical pulling resistance and root morphological traits. These correlations were calculated using Excel (Microsoft Corp., Redmond, WA, USA), with significance defined at α = 0.05.

Results

Variation in pulling strength indicates differences in root morphology

In the initial screening of 10 representative melon accessions, we assessed early vigor and variation in the plant pulling force. No significant differences in pulling strength were observed among accessions at the first two phenological stages (two-three and three-four true leaves; Figure S3). However, at the third stage (four-five true leaves), distinct responses emerged (Table 2). The AY, TAD, PI414723, and SAS accessions exhibited the highest anchoring strength, 11.52, 9.92, 8.3, and 6.7 N, respectively. In contrast, both the wild (QME) and commercial accessions (DUL) had the lowest values, 1.72 and 1.8 N, respectively. No significant effect of subspecies, or variety was found on pulling strength. Complementary to mechanical pulling resistance, we characterized the root architectural traits of each accession. The AY accession exhibited the most robust root system, significantly surpassing all other genotypes in total root volume, length, and diameter.

Table 2.

Summary of measured traits across the ten melon accessions. Data represent the mean ± SD. Different letters indicate significant statistical differences between accessions for each trait according to Tukey’s HSD test (α < 0.05).

Accession	Length (mm)	Surface Area (cm 2 )	Root Diameter (mm)	Root Volume (cm 3 )	Pulling (N)	Shoot_Wt (g)	Root_Wt (g)	S/R ratio	SRL (g m − 1 )	SRA (g cm − 2 )	SRV (g cm − 3 )
AY	1535.04 ± 240.14a	312.28 ± 86.15a	0.64 ± 0.08a	5.12 ± 2.06a	11.52 ± 2.22a	1.64 ± 0.03a	0.28 ± 0.04a	5.81 ± 0.89bc	5.05 ± 0.68d	0.95 ± 0.15b	14.53 ± 1.21bc
DUL	1041.26 ± 299.29ab	159.55 ± 55.51b	0.48 ± 0.03b	1.95 ± 0.8b	1.80 ± 0.92c	0.56 ± 0.10c	0.10 ± 0.03c	5.28 ± 0.14bc	9.06 ± 0.49a	1.3 ± 0.09a	14.98 ± 2.20bc
NA	891.54 ± 494.89ab	140.85 ± 64.25b	0.52 ± 0.07ab	1.8 ± 0.75b	4.03 ± 2.14bc	0.86 ± 0.18bc	0.14 ± 0.01bc	6.16 ± 1.22bc	6.38 ± 0.48c	0.99 ± 0.03b	12.49 ± 1.41c
NY	1516.46 ± 389.45a	240.81 ± 64.44ab	0.51 ± 0.04ab	3.06 ± 0.89ab	5.80 ± 2.60bc	1.16 ± 0.29ab	0.18 ± 0.07ab	6.36 ± 0.62b	7.59 ± 0.52b	1.22 ± 0.05a	15.66 ± 2.74bc
PI414723	700.78 ± 167.29b	166.04 ± 76.41ab	0.74 ± 0.29a	3.48 ± 2.76ab	8.25 ± 2.49ab	1.61 ± 0.31a	0.21 ± 0.04ab	7.68 ± 0.68a	3.64 ± 0.17e	0.78 ± 0.04c	14.74 ± 2.03bc
PSR	1333.99 ± 367.51ab	222.71 ± 81.56ab	0.52 ± 0.08ab	3.01 ± 1.47ab	5.48 ± 2.44bc	1.23 ± 0.21ab	0.17 ± 0.05b	7.86 ± 1.31a	7.45 ± 1.12abc	1.19 ± 0.02ab	15.28 ± 2.71bc
QME	1014.69 ± 218.27ab	173.36 ± 53.06ab	0.54 ± 0.06ab	2.38 ± 0.97ab	1.72 ± 0.96c	0.62 ± 0.05	0.14 ± 0.02bc	4.39 ± 0.80c	7.66 ± 1.02b	1.28 ± 0.08a	17.13 ± 2.56ab
SAS	1139.69 ± 295.13ab	189.87 ± 78.66ab	0.51 ± 0.11ab	2.58 ± 1.45ab	5.60 ± 1.55abc	0.73 ± 0.15c	0.11 ± 0.02c	5.58 ± 0.57bc	8.29 ± 0.34ab	1.26 ± 0.17ab	15.75 ± 2.10b
TAD	1184.71 ± 510.43ab	238.42 ± 116.81ab	0.62 ± 0.07ab	3.84 ± 2.1ab	9.93 ± 3.79ab	0.72 ± 0.25c	0.20 ± 0.06ab	3.23 ± 0.25d	6.42 ± 0.71c	1.24 ± 0.24ab	19.15 ± 0.49a
VEP	941.06 ± 590.78ab	167.65 ± 77.81ab	0.64 ± 0.20ab	2.48 ± 0.71ab	4.16 ± 2.56bc	0.57 ± 0.03c	0.14 ± 0.02bc	4.00 ± 0.74 cd	5.29 ± 0.19d	0.95 ± 0.15bc	14.06 ± 0.28 cd

To evaluate the relationship between mechanical performance and root morphology, a Pearson correlation analysis was conducted (Fig. 2). Pulling resistance across all accessions was primarily driven by root biomass (R²= 0.77) and root volume (R² = 0.83), whereas specific root volume showed only a weak association (R²= 0.21). For instance, the superior pulling strength of the AY accession reflected its robust root volume of 4.25 cm³, nearly double that of the low-strength QME accession (2.38 cm³). Furthermore, all ten accessions maintained a consistent root-to-shoot ratio (R² = 0.69), suggesting that while absolute root size varied significantly between genotypes, the relative biomass allocation remained stable under the tested conditions.

Fig. 2.

Heatmap of root traits, biomass, and pulling strength. Heatmap illustrating the Pearson determination coefficients (R²) between various root traits, root and shoot dry weights, and pulling strength in melon accessions. SRV specific root volume, SRA specific root area, SRL specific root length, S/R ratio shoot-to-root ratio. Asterisks indicate the statistical significance of the correlations: *** p < 0.05, **** p < 0.01, and ***** p < 0.001.

Temperature-driven variation in pulling strength and biomass across accessions

In order to confirm the results of the pulling experiment, four accessions with different pulling strengths: the highest (AY), the lowest (Dulce), and two lines with moderate pulling strength (PI414723, and NY). The accessions were grown under two temperature regimes 16º and 25º. The pulling (Fig. 3) and weight (Fig. 4) results revealed significant variation in accession responses to temperature.

Fig. 3.

The effect of growing temperature on pulling strength in four melon accessions. Pulling strength of four melon accessions measured at three developmental stages: two-three true leaves, three-four true leaves, and four-five true leaves. Measurements were conducted at 25 °C (a, c), and 16 °C (b, d). Each dot represents five replicates with standard error. Linear correlation parameters are provided in Table 3. DAP- days after planting. Different letters indicate statistically differences between accessions at the same DAP, according to one-way ANOVA with Tukey-HSD test (α < 0.05).

Fig. 4.

The effect of growing temperature on foliage dry weight in five melon accessions. Foliage dry weight of five melon accessions measured at three developmental stages: two-three true leaves, three-four true leaves, and four-five true leaves. Measurements were conducted at 16 °C (a, c), and 25 °C (b, d). Each dot or bar represents five replicates with standard error. Linear correlation parameters for subplot (a) and (b) are provided in Table 4. DAP- days after planting. In panels (c) and (d), different letters indicate statistically significant differences based on one-way ANOVA with Tukey-HSD (α < 0.05).

The AY accession demonstrated the highest tolerance to suboptimal temperatures, reaching the two-three true leaves stage 10 days earlier than the other accessions (Fig. 3A). Biomass accumulation rates remained stable at both 16 °C and 25 °C, with 0.1 and 0.09 g day⁻¹ (Table 3). In comparison, the NY and Dulce accessions emerged later than AY and exhibited only moderate stability under sub-optimal conditions. Their pulling strength rates were 0.43 and 0.44 N day⁻¹ at 16 °C, increasing to 0.71 and 0.68 N day⁻¹ at 25 °C for NY and Dulce, respectively (Table 4). While their biomass accumulation rates at 16 °C were 0.13 g day⁻¹ for both accessions, they reached 0.14 and 0.23 g day⁻¹ at 25 °C, with Dulce even surpassing the development rates of AY under optimal conditions.

Table 3.

Linear correlation coefficients between days after planting and plant pulling strength under two temperature regimes presented in Fig. 5.

Accession	25			16
	Slope	Intercept	r 2	Slope	Intercept	r 2
AY	0.5	2.82-	0.96	0.53	4.4-	0.95
NY	0.72	7.70-	0.96	0.47	15.66-	0.99
Dulce	0.68	6.72-	0.86	0.44	8.4-	0.92
PI414723	0.79	11.08-	1.00	0.12	1.12-	0.98

Table 4.

Linear correlation coefficients between days after planting and plant shoot biomass under two temperature regimes presented in Fig. 6.

Accession	25			16
	Slope	Intercept	r 2	Slope	Intercept	r 2
AY	0.09	−0.80	0.97	0.1	1.31-	0.98
NY	0.13	1.81-	0.99	0.14	3.36-	0.87
Dulce	0.23	4.17-	0.90	0.13	3.26-	0.83
PI414723	0.24	4.59-	0.92	0.0011	−0.24	0.90

In contrast, the PI414723 accession was highly susceptible to sub-optimal temperatures. Although it demonstrated the highest biomass accumulation rate under optimal conditions (0.24 g day⁻¹ at 25 °C), its performance declined sharply under sub-optimal temperatures, where biomass accumulation was nearly eliminated (an approximate 100% reduction).

The root system response is related to the physiological response

To further investigate temperature response, we grow two accessions with contrasting tolerance levels: AY (stable) and PI414723 (susceptible to sub-optimal temperatures) under both optimal and sub-optimal temperatures (25 °C and 16 °C, respectively). Due to seedling collapse at 16 °C, the PI414723 accession was evaluated with four replicates instead of five. In the AY accession, only root volume differed significantly between the two temperature regimes (Fig. 5). In contrast, PI414723 showed a significant difference only in pulling strength.

Fig. 5.

The effect of melon accession and growing temperature on root traits and Pulling strength. Influence of melon accession (AY and PI414723) and growing temperature on pulling strength (a), root biomass (b), root length (c), and root volume (d) 27 days after planting. Each AY treatment includes five replicates, while each PI414723 treatment includes four replicates. Different letters indicate statistically significant differences according to the Tukey-HSD test (α < 0.05).

The difference in pulling strength between the tolerant and susceptible accessions was minor at 25 °C (7.6 N for AY and 11.2 N for PI414723) but became pronounced at 16 °C (6.1 N for AY and 2.8 N for PI414723; Fig. 5a). Root biomass did not significantly differ between the accessions across temperatures (Fig. 5b). However, at 25 °C, AY exhibited significantly greater root length (1210 mm) and root volume (5.62 cm³) compared to PI414723, which had lower values at both 25 °C (267 mm and 0.62 cm³) and 16 °C (214 mm and 0.59 cm³).

Physiological measurements further supported these findings (Fig. 6). Carbon assimilation rates declined in both accessions under suboptimal temperatures (under 16 °C). However, while AY showed a 40% reduction, PI414723 experienced a 70% reduction (Fig. 6a). Stomatal conductance remained constant in AY, whereas PI414723 showed a 90% reduction at 16 °C (Fig. 6b).

Fig. 6.

The effect of growing conditions on Photosynthetic and Physiological Parameters in Melon Accessions. Impact of growing conditions on carbon assimilation rate (a), stomatal conductance (b), PSⅡ functionality () (c), and PSⅡ quantum yield (d) in temperature-tolerant (AY) and temperature-susceptible (PI) melon accessions. Each treatment includes five replicates, and bars represent the mean with standard deviation. Different letters indicate statistically significant differences based on Tukey-HSD (α < 0.05).

Lastly, PSⅡ functionality () decreased in both accessions, 25% in AY and 50% in PI414723 (Fig. 6c). The PSⅡ quantum yield (Y(Ⅱ)) was reduced by 50% in AY and 65% in PI414723 under suboptimal conditions (Fig. 6d).

Discussion

Early crop vigor is a critical trait for early-season sowing, as it enhances weed suppression and promotes successful crop establishment¹³. Furthermore, genotype × environment (G × E) stability is essential to ensure consistent crop performance under the variable environmental conditions that occur throughout the growing season^26,28. The importance of maintaining such stability is becoming increasingly evident in the context of climate change, where crops must perform reliably under fluctuating and often stressful conditions. This challenge is further intensified by the global shift toward reducing chemical inputs in agricultural systems, which demands varieties with inherently robust and adaptive traits⁴¹. Although the AY accession did not exhibit the fastest development under optimal conditions, it maintained a stable developmental rate across both temperature regimes. This stability, combined with its 10-day developmental advantage under suboptimal temperatures, likely contributes to its suitability as an early-season cultivar with both agronomic and economic value. Early emergence enhances crop competitiveness and management efficiency, while also providing farmers with clear economic benefits through improved resource utilization and earlier market access⁴².

Precise and highly accurate root system phenotyping methods are often complex, costly, and require specialized equipment, which limits their accessibility and scalability for large surveys. For instance, several studies have employed X-ray computed tomography (CT)^43,44, or custom-designed growth boxes⁴⁵ to investigate belowground dynamics. While these approaches provide valuable insights into root architecture and function, their major limitation lies in their unsuitability for large-scale screening, which is essential for evaluating extensive plant populations such as those examined in the present study. Despite its simplicity, the pulling method employed in this study showed a strong and consistent correlation with key root traits, particularly root volume and biomass, across all melon accessions and temperature regimes. Similar relationships between pulling strength and root system characteristics, such as total root length and volume, have also been reported in previous studies^37,38. Therefore, this method represents a practical and effective tool for large-scale, first-step screening of plant populations or for field validation of more advanced and resource-demanding root phenotyping approaches. The effectiveness of this method has also been demonstrated in screening other crops, such as maize, rice, and alfalfa^38,42,46.

It is important to recognize that the relevance of the measured root traits extends beyond mechanical weed management, as they also play a central role in nutrient and water uptake^17,47. Consequently, these traits can strongly influence plant responses to nutrient limitation and water scarcity. Future studies on root system behavior could benefit from integrating this simple pulling method with more precise phenotyping approaches, thereby enabling a more comprehensive understanding of the functional and structural characteristics of root systems under varying environmental conditions.

The historical journey of melon (Cucumis melo) from its origins in the Indian subcontinent, through East Asia and eventually the Western world has greatly enhanced its internal diversity, ranging from physiological adaptations to the wide variability in fruit traits^5,7,48. Both natural selection and human-mediated cultivation across diverse latitudes have contributed to this variability, increasing the likelihood of identifying accessions within the species that possess tolerance to both heat and cold stress⁴⁹. The results of the present study highlight that even in a warm-season crop such as melon, accessions exhibiting tolerance to suboptimal temperatures can be found, likely as a result of indirect selection. Annas Yoqneam (AY) accession, for instance, was originally cultivated on specially prepared fallow fields under dryland conditions without irrigation, sown in early spring, and relied entirely on soil moisture conserved from winter precipitation⁵⁰. This early sowing during early spring suggests that this accession possesses physiological mechanisms that enable tolerance to such unfavorable environmental conditions. Moreover, its ability to rely on residual soil moisture from winter precipitation implies the presence of a strong and efficient root system, capable of sustaining growth under limited water availability. Consequently, this accession serves as the genetic foundation for many contemporary early-season commercial melon lines. However, it has not yet been directly utilized in breeding programs specifically targeting tolerance to sub-optimal temperatures.

Stomata closure is one of the first line process in response to temperature decrease^23,51. This in turn, led to a reduction on the photosynthetic activity and decrease in the carbon assimilation rate^25,52. Under optimal conditions, both accessions exhibited similar physiological behavior. However, under suboptimal conditions, their responses were distinct. While AY stomata remain open under suboptimal temperatures, the PI414723 stomatal conductance decrease sharply under same conditions. This stomatal closure induced additional feedback inhibition of photosynthesis and therefore on the carbon assimilation rate⁵³. In addition, it was demonstrated that fluorescence parameters exhibit a gradual response to stress, with reduction serving as a late-stage indicator, occurring at higher stress levels⁵⁴. At this stage, the decrease in the xanthophyll pool may mask other parameters, such as Y(II). This can explain the similar Y(II) values observed in both accessions under suboptimal temperatures, although PI414723 was under greater stress under these conditions.

Several pathways can lead to stomata closure under suboptimal conditions as abscisic acid (ABA) and calcium movement in the plant^25,51. In the current work, the AY stomata seems to be inert to the temperature reduction. This accession, cultivated for more than 100 years, challenge our understanding if it was inbreed as suboptimal temperatures accession for early sowing, or it was selected for this trait over the last decades^7,50. The limited response to ABA can also explain the early emerge of the AY seedlings under sub optimal temperatures.

Tolerance to suboptimal temperatures can be expressed in both root and shoot tissues, with physiological and molecular adjustments in one organ potentially influencing the other through systemic signaling and resource allocation^55,56. Such cross-organ interactions may involve hormonal and molecular regulation, metabolic adjustments, and shifts in source-sink dynamics that collectively contribute to whole-plant acclimation under suboptimal temperature conditions^57,58. The current study demonstrated a direct link between aboveground physiological responses and belowground development under suboptimal temperature. In the tolerant AY accession, both root and shoot growth rates remained stable across low and ambient temperature conditions. In contrast, the susceptible PI414723 accession exhibited a concurrent reduction in root and shoot development, consistent with a marked decline in key physiological parameters.

In the present study, we successfully characterized root system responses in a large melon population using the pulling method, which was validated through root system scanning. The measurement device, a simple force gauge, is affordable, widely accessible, and easy to operate without the need for specialized training. Moreover, the method is time-efficient and provides a reliable first-line assessment of key root traits. Applying this approach as an initial screening tool prior to more labor- and time-intensive analyses could substantially improve the efficiency of studies on root dynamics and physiology.

Future research on these traits is supported by the availability of fully de novo assembled and annotated genomes for the melon accessions evaluated in this study, including AY, along with readily available segregating populations⁷. These genomic resources provide a strong foundation for investigating the genetic basis of root pulling resistance, early seedling vigor, and temperature tolerance.

Integrating these physiological and genomic insights could ultimately enable the development of melon varieties or specialized rootstocks optimized for sustainable, non-chemical weed management. Such innovations would enhance melon adaptability to both current and future environmental conditions, contributing to more resilient and resource-efficient agricultural systems.

Conclusions

The current study demonstrated that the considerable genetic diversity within this warm-climate crop results in a wide range of responses to temperature. The limited negative response to suboptimal temperatures observed in the tolerant AY accession led to earlier emergence and higher physiological activity compared to other melon accessions. This likely explains its longstanding use as an early-season cultivar. The combination of rapid emergence and vigorous root system development enables its growth without supplemental irrigation, making it well-suited for conditions that favor early-season weed competition and mechanical weeding. Although the fruits of this accession are not compatible with current market demands, mainly due to their short shelf life, the integration of genetic and molecular tools offers an opportunity to introgress these valuable adaptive traits into modern melon cultivars. Such efforts could lead to the development of early-season varieties with improved tolerance to environmental variability and enhanced suitability for sustainable cultivation systems.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

AC- design the experiment and wrote the manuscript; EO –statistical analysis; FB – vegetative material growth and handling; EA – conduct the plant pulling, ES - vegetative material growth and handling; EV- soil characterization; RNL – experimental design.

Funding

This research received no external funding.

Data availability

Data supporting the findings of this study are available from the corresponding author (amnonc@volcani.agri.gov.il) upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.