Selection of cowpea cultivars for high temperature tolerance: physiological, biochemical and yield aspects

Abstract

High temperature stress can hinder the development of cowpea resulting in several damages including vegetative and reproductive phases of the crop. In this context, the objective of this study was to select cowpea cultivars tolerant to high temperature stress using various parameters related to physiological, biochemical, and yield aspects. For this, the cultivars Carijó, Itaim, Pujante, Rouxinol, and Tapahium were used, maintained in two temperature regimes: 20–26–33 °C and 24.8–30.8–37.8 °C. The experiment was carried out in growth chambers, in a 5 × 2 factorial arrangement (cultivars × temperature regimes). Responses differentiated among the cultivars Carijó, Itaim, Pujante, Rouxinol, and Tapahium with the increase of 4.8 °C in air temperature. The high temperature promoted a greater quantity of aborted flowers, leading to a reduction in the yield of the cultivars Carijó, Pujante, Rouxinol, and Tapahium. The photosynthesis, stomatal conductance, leaf transpiration and enzymatic activities were significantly influenced by high temperature. From the combination of the responses of biometric, physiological and productive variables, the cultivar Itaim can be considered as tolerant to an increase of 4.8 °C in air temperature.

Introduction

The cowpea [Vigna unguiculata (L.) Walp.] is a legume from the African continent, widely used as a food source, generating employment and income for small growers (Rocha et al. 2016). In Brazil, according to data from the National Supply Company CONAB (2019), in the 2018/2019 harvest, cowpea occupied an area of 1327.5 thousand hectares, with an estimated production of 651.8 thousand tons. This production is concentrated in the North and Northeast, whereas in the latter, cultivation is predominantly in the semi-arid region.

However, in recent years, due to the increase in technologies and their wide adaptation to edapho-climatic conditions, cowpea started to be cultivated not only by small growers but also in medium and large properties, gaining cultivation areas in the Brazilian Midwest region, mainly in the Mato Grosso State, with a production of 229.6 thousand tons, occupying more than 200 thousand hectares in area (CONAB 2019).

Cowpea is a rustic legume, with good conditions for adaptation in places with high temperatures and low water availability (Araújo et al. 2018), but even with these characteristics, the cowpea yield is still low in the Northeast region of Brazil, because in this region the temperature can exceed the optimum for the crop, which will contribute to the increase of the water deficit, affecting the final yield of the crops. According to Djanaguiraman et al. (2018), the rise in temperature and the water deficit is the abiotic stresses that most limit agricultural productivity.

Studies carried out by Hatfield and Prueger (2015), show that the growth and development of plants depend on temperature and that each species has a specific range represented by a minimum, maximum and optimum. According to Vale et al. (2017), cowpea grows in a wide temperature range, between 18 and 37 °C, however, the optimum temperature point varies with the plant’s phenological stage. Thus, temperatures outside the optimum point can impact seed germination to final production (Sehgal et al. 2018), with the reproductive phase being the most affected. Studies have shown that temperature above 35 °C, causes the abortive flowers, stimulates the senescence of the leaves, decreasing the photosynthetic capacity, affecting the productivity of cowpea pods and seeds, as this increase interferes with physiological and biochemical aspects of plants (Zandalinas et al. 2018).

In response, plants decrease the chlorophyll content, photosynthesis rate and transpiration (Taiz et al. 2017) avoiding water loss. However, plants have developed defense mechanisms consisting of ROS scavenging enzyme activities (Reactive oxygen species), such as Superoxide Dismutase (SOD), Catalase (CAT), Glutathione Peroxidase (GPX) and Ascorbate Peroxidase (APX), to adapt and survive abiotic stresses (Merwad et al. 2018). Thus, the characterization of defense mechanisms at the cellular level is of great importance for breeding, genetic engineering and the creation of cultivars tolerant to environmental stresses.

According to the Intergovernmental Panel on Climate Change (IPCC 2013), climate forecasts indicate an increase of up to 4.8 °C in air temperature by the end of the century. Where the northeast semi-arid region of Brazil will be one of the most affected, as they are the most vulnerable regions (Djanaguiraman et al. 2018). This increase in temperature may affect plants at the molecular, physiological and biochemical level (Gray and Brady 2016), compromising their growth, metabolism and development (Eftekhari et al. 2017).

Therefore, a better understanding of plant responses to high temperatures is essential to improve crop yield and quality, in addition to enabling the farmers to select thermotolerant genotypes with high production potential. In addition to generating jobs and income for family farming, cowpea is considered a key crop in the context of global climate change and food security (Rocha et al. 2016, Gomes et al. 2019). In this context, the objective of this study was to select cowpea cultivars tolerant to high temperature stress using various parameters related to physiological, biochemical and yield aspects.

Materials and methods

The experiment was carried out in Fitotron type growth chambers with temperature, humidity, and photoperiod controls. Commercial seeds of five cowpea cultivars were used BRS Carijó, BRS Itaim, BRS Pujante, BRS Rouxinol and BRS Tapahium, supplied by Embrapa Semiarid. Ten seeds were seeded per pot with a capacity of 7 L and after 15 days of sowing thinning was performed leaving only one plant per pot. Fertigation was carried out 2 days before planting based to the results of the soil chemical analysis and as recommondated for the crop (de Cavalcanti 2008).

The experiment was carried out in a 5 × 2 factorial arrangement (cultivars x temperature), using four replications. The temperature regimes were: T° 1: 20–26–33° C (20 °C: between 8 pm and 6 am; 26 °C: between 6 am and 10 am; 33 °C: between 10 am and 3 pm; 26 °C: from 3 pm to 8 pm) for the chamber 1. For chamber 2 the regime T° 2: 24.8–30.8–37.8 °C (24.8 °C: from 8 pm to 6 am; 30.8 °C: from 6 am to 10 am; 37.8 °C: from 10 am to 3 pm; 30.8 °C: from 3 pm to 8 pm) was adopted. Temperature values were determined from minimum, average and maximum temperatures, ranging from 18–22, 25–27 and 32–34 °C, respectively, in the São Francisco Sub-Middle Valley, Brazil. In this study, an increase of 4.8 °C was used, based on the temperature scenario indicated by IPCC (2013).

Determination of the phenological cycle

The plants were daily evaluated after sowing (DAS) to determine the phenological cycle. For evaluation, the scale of Oliveira et al. (2018) was used, where the vegetative phase included the stages, V0—Germination; V1—Emergence; V2—Primary leaves; V3—First open composite leave; V4—Third open trifoliate leave. The reproductive phase was divided into R5—Pre-flowering; R6—Flowering; R7—Pod formation; R8—Pod filling; and R9—Maturation.

Productive and biometric parameters

The evaluation of shoot dry mass (SDM) and root dry mass (RDM) was carried out after harvest, by cutting the stem close to the soil to separate the shoot from the roots. The materials were packed in paper bags and kept in an oven at 65 °C for ± 72 h.

To evaluate the flower abortion data (starting at stage R6), the flowers of each plant were daily counted to obtain the number of aborted flowers for each cultivar. Seed yield, average pod weight (APW), pod length (PL), pod diameter (PD), number of pods per plant (NPP), number of seeds per plant (NSP), seed production (PS) were recorded at the time the plants reached the pod maturation stage   specific to each cultivar cycle.

Physiological parameters   were recorded 30 days after planting, when the plants were in phase V4. Portable Infrared Gas Analyzer (IRGA), model Li-6400, was used to record the gas exchange parameters, photosynthesis rate (A), stomatal conductance (gs), transpiration (E) and leaf temperature (Tf)  using artificial light fixed at 1500 µmol m⁻² s⁻¹. The chlorophyll content of the leaves and relative chlorophyll index was determined using a portable chlorophyll meter, model CFL 1030 FALKER. For the readings, it was chosen leaves located in the middle third of the plant, without injuries, with green color, and fully expanded.

ROS scavenging enzyme activities

Healthy, green, and fully expanded  leaves located in the middle third of the plant, were collected after 30 days of planting for biochemical analyses. The samples were immediately stored in aluminum foil envelopes and immersed in liquid nitrogen (N₂). The plant extracts were prepared using 1 g of vegetative tissue  material grinded in liquid nitrogen and suspended in 3 ml of extraction buffer (pH 7.5) at a concentration of 100 mM potassium phosphate supplimented with 0.01 g of polyvinylpolypyrrolidone. Then, the extract was centrifuged at 15,000 g for 15 min, at 4 °C, and the supernatant was used as a crude enzymatic extract.

Total soluble protein content was determined according to the method of Bradford (1976) at 595 nm. The catalase activity (CAT) was determined following the decomposition of H₂O₂ for 60 s through spectrophotometric readings at 240 nm at 25 °C according to the method described by Havir and Mchale (1987). The activity of ascorbate peroxidase (APX) was determined as described by Nakano and Asada (1981), by measuring the oxidation rate of ascorbate at a wavelength of 290 nm, at 25 °C for 60 s. The activity of the enzyme guaiacol peroxidase (GPX) was determined by monitoring the reduction of guaiacol using a spectrophotometer with a wavelength at 470 nm, at 25 °C, for 60 s following the protocol developed by Cakmak and Horst 1991). The activity of superoxide dismutase (SOD) was determined according to the methodology of Giannopolitis and Ries (1977), with spectrophotometer readings at 560 nm wavelength, defining the SOD unit as the amount of enzyme needed to inhibit in 50% of NBT photoreduction.

Statistical analysis

The data were subjected to Analysis of Variance (ANOVA) to compare the treatments followed by  Tukey’s test at 5% probability using the statistical software Sisvar® version 5.6.

Results

Phenological cycle

The results of the phenological characterization of the five cultivars evaluated  is given in Table 1. In general, the cultivars showed similar behavior in phases V0 and V1, for 1–2 days,  at both temperature regimes.

Table 1.

Number of days for each phenological stage and cycle length of five cowpea cultivars maintained under two temperature regimes

Phenological cycle (days)
Cultivar	Temperature	V0	V1	V2	V3	V4	R5	R6	R7	R8	R9	Cycle
Carijó	20–26–33 °C	2	2	7	9	13	2	2	15	4	5	61
	24.8–30.8–37.8 °C	2	2	6	6	16	1	4	10	2	6	55
Itaim	20–26–33 °C	1	2	8	6	15	2	1	16	3	7	61
	24.8–30.8–37.8 °C	2	1	5	6	17	1	2	9	5	4	52
Pujante	20–26–33 °C	1	2	7	7	27	1	2	18	3	5	73
	24.8–30.8–37.8 °C	2	2	5	9	42	2	35	10	3	4	114
Rouxinol	20–26–33 °C	1	2	8	6	37	1	1	12	5	12	85
	24.8–30.8–37.8 °C	2	2	6	8	44	2	1	29	5	14	113
Tapahium	20–26–33 °C	1	1	9	9	12	1	6	19	3	4	65
	24.8–30.8–37.8 °C	2	2	8	8	14	2	16	16	2	17	87

V0, Germination; V1, Emergence; V2, Primary leaves; V3, First open composite leave; V4, Third open trifoliate leave. The reproductive phase was divided into: R5, Pre-flowering; R6, Flowering; R7, Pod formation; R8, Pod filling; and R9, Maturation

It was found that the phenological cycle of the cultivars Carijó and Itaim was shorter for plants exposed to high temperature, with a reduction of 6 and 9 days respectively, when compared to plants exposed to a temperature of 20–26–33 °C. However, it was noted that for the cultivars Pujante, Rouxinol and Tapahium, the high temperature prolonged the phenological cycle by 41, 28 and 22 days, respectively.

During the reproductive phase, the cultivar Pujante maintained flowering phase (R6)  for an additional 33 days under the high temperature regime. The prolonged flowering phase was also observed in cultivar Tapahium. A increase in temperature perturbed the flowering induction. Under the temperature regime of 24.8–30.8–37.8 °C a high percentage of aborted flowers (AF) was observed (Fig. 1a), and delayed time to reach the R7 stage (pod formation).

Fig. 1.

Percentage of aborted flowers (FA) (a) and Average seed production (g) (b) of cowpea cultivars maintained under two temperature regimes *Lowercase letters for temperature and uppercase letters for cultivars

An increase in 4.8 °C of air temperature promoted a significant increase in the percentage of aborted flowers (FA) for the cultivars Carijó, Pujante, Rouxinol and Tapahium (Fig. 1a). The cultivar Itaim did not showd a significant increase in the flowers abortion at the higher temperatures.

Productive and biometric parameters

The interaction between cultivar and temperature was significant for seed yield and its related traits including, shoot dry mass (SDM), average pod weight (APW), pod length (PL), pod diameter (PD), number of pods per plant (NPP), number of seeds per plant (NSP) (Table 2).

Table 2.

Average value of shoot dry mass (SDM), pod weight (APW), pod length (PL), pod diameter (PD), number of pods per plant (NPP), number of seeds per plant (NSP) of cowpea plants maintained under two temperature regimes

Temperature	Carijó	Itaim	Pujante	Rouxinol	Tapahium
SDM (g)
20–26–33 °C	16.7 ± 2.85 aA	11.77 ± 3.94 aA	16.4 ± 5.33 bA	17.91 ± 6.26 bA	9.47 ± 3.22 aA
24.8–30.8–37.8 °C	16.95 ± 6.85 aB	9.12 ± 2.85 aB	28.35 ± 2.86 aA	29.75 ± 4.41 aA	15.47 ± 5.61 aB
APW (g)
20–26–33 °C	2.00 ± 0.19 aA	1.96 ± 0.80 aA	3.41 ± 0.52 aA	3.37 ± 1.14 aA	2.67 ± 1.13 aA
24.8–30.8–37.8 °C	2.01 ± 0.60 aA	1.26 ± 0.30 aA	0.48 ± 0.17 bA	1.55 ± 0.89 bA	1.64 ± 0.83 aA
PL (cm)
20–26–33 °C	16.03 ± 0.94 aA	13.17 ± 1.58 aA	19.19 ± 2.34 aA	20.31 ± 3.68 aA	16.87 ± 2.18 aA
24.8–30.8–37.8 °C	14.87 ± 1.56 aA	11.32 ± 1.65 aA	3.40 ± 1.8 bB	8.02 ± 2.8 bB	13.58 ± 2.75 aA
PD (mm)
20–26–33 °C	7.82 ± 0.18 aA	8.56 ± 0.33 aA	11.34 ± 0.66 aA	10.56 ± 0.75 aA	9.17 ± 0.98 aA
24.8–30.8–37.8 °C	8.07 ± 0.50 aA	8.61 ± 0.12 aA	2.62 ± 0.25 bB	5.31 ± 1.13 bB	9.32 ± 0.13 aA
NPP
20–26–33 °C	15.25 ± 3.09 aA	14.50 ± 1.91 aA	9.50 ± 2.38 aB	5.25 ± 2.5 aC	8.25 ± 1.70 aB
24.8–30.8–37.8 °C	4.25 ± 2.06 bB	14.00 ± 4.24 aA	0.25 ± 0.15 bB	1.50 ± 0.38 bB	3.25 ± 1.06 bB
NSP
20–26–33 °C	107.0 ± 6.53 aA	83.0 ± 15.81 aA	85.0 ± 23.90 aA	53.0 ± 17.45 aB	77.0 ± 11.95 aA
24.8–30.8–37.8 °C	54.0 ± 17.82 bA	61.0 ± 6.07 aA	2.0 ± 0.5 bB	10.0 ± 3.26 bB	29.0 ± 10.54 bB

Means followed by the same lowercase letter in the column and uppercase letter in line are not different according to Tukey’s test at 5% of probability

The cultivars Pujante and Rouxinol showed high shoot dry mass (SDM) when kept in a temperature regime of 24.8–30.8–37.8 °C with 28.35 and 29.75 g, respectively because they were in the vegetative stage for a longer duration under high temperature regime (Table 1). For the other cultivars, temperature did not show any affect on SDM. 

For the average pod weight (APW), no difference between cultivars maintained at the temperature regime of 20–26–33 °C was observed. However, the high temperature caused a significant reduction in APW for the cultivars Pujante and Rouxinol. This result was due to the result of the reduction in pod length (PL) of these cultivars, as observed in Table 2.

High temperature influenced the number of pods per plant (NPP) and the number of seeds per plant (NSP) in the cultivars Carijó, Pujante, Rouxinol and Tapahium. The cultivar Itaim showed prominence in relation to the other cultivars, with no reduction in NPP and NSP due to the high temperature (Table 2). This result is because of the greater number of flowers aborted for the cultivars Carijó, Pujante, Rouxinol and Tapahium, excwpt for the cultivar Itaim (Fig. 1a).

The evaluated cultivars showed significant differences in seed production (PS) when submitted to different temperature regimes. In the regime of 20–26–33 °C the cultivars Carijó, Pujante, Rouxinol and Tapahium had high seed production rate. No reduction in seed production with the 4.8 °C increase in temperature was recorded in cultivar Itaim. The high temperature reduced 96%, 81%, 55% and 40% the seed production of the cultivars Pujante, Rouxinol, Tapahium and Carijó, respectively (Fig. 1b).

Analysis of physiological parameters

The cultivar and temperature interaction also perturbed the photosynthesis (A), stomatal conductance (gs) and leaf transpiration (E). The photosynthetic activity of the cultivars Carijó, Itaim and Tapahium was not affected by the high temperature. However, the cultivar Pujante showed higher photosynthetic activity in plants maintained under the 24.8–30.8–37.8 °C regime, because the increase in stomatal conductance (gs) consequently resulted eidening the opening of stomata and further increased transpiration (E) Contrasting results was observed in the cultivar Rouxinol maintained same temperature regime (Fig. 2a).

Fig. 2.

Photosynthetic parameters including Photosynthetic rate (a), Transpiration rate (b), Stomatal conductance (c), Total chlorophyll (d) and Leaf temperature (e, f) recorded in different cowpea cultivars grown under to two temperature regimes. *Lowercase letters for temperature and uppercase letters for cultivars

A higher transpiration rate (E) and high stomatal conductance (Fig. 2b, c) was observed for cv. Itaim and Tapahium when submitted to a temperature of 20–26–33 °C.

A significant increased was observed for the chlorophyll content and temperature, with the highest rates observed in the lowest temperature chamber (Fig. 2d). The varieties Itaim, Tapahium and Carijó had higher levels of total chlorophyll, with an index of 59.95 for cv. Carijó, 63.72 for cv. Itaim and 62.66 for cv. Tapahium.

The leaf temperature (Tf) had no influence on the cultivar and temperature interaction. However, the isolated effect of temperature can be observed, verifying that the high air temperature provided a higher Tf (Fig. 2e). The tf was also affected according to the cultivar (Fig. 2f). The cultivars Carijó and Rouxinol showed lower stomatal opening in the two applied temperature regimes, resulting in lower values of gas exchange rates (A and E) when compared to the other cultivars. This behavior was reflected in the Tf of these varieties, which registered the highest values.

ROS scavenging enzyme activities

For the enzymes CAT, APX, GPX and SOD, there was a significant interaction between cultivars and temperature. The enzyme activity observed between cultivars in both temperature regimes (Fig. 3).

Fig. 3.

Specific activity of Catalase (CAT) (a); Ascorbate Peroxidase (APX) (b); Guaiacol Peroxidase (GPX) (c) and superoxide dismutase (SOD) (d) in cowpea cultivars submitted to different temperature regimes. *Lowercase letters for temperatures and uppercase letters for cultivars

The cultivars Rouxinol and Tapahium showed high activities of the CAT enzyme in the two submitted temperature regimes (Fig. 3a). The cultivar Pujante showed higher enzymatic activity when submitted to the higher temperature regime, with an increase of 133% in CAT activity. The other cultivars showed low activity of this enzyme, with no significant difference with the high temperature.

Significant reductions of 58.53%, 63.72% and 49.72% in the specific activity of APX were observed only in the cultivars Itaim, Pujante and Rouxinol, respectively, for plants exposed to high temperature (Fig. 3b).

When the GPX enzyme was evaluated, it was observed a significant difference according to the temperature regimes that the cultivars were submitted for the Pujante and Rouxinol materials, with an increase of 105% and a reduction of 64.42%, respectively, with an increase of 4.8 °C in temperature (Fig. 3c).

The specific activity of SOD was differently affected according to the cultivar, with the cultivars Carijó, Itaim, Pujante and Tapahium no significant difference in the specific activity with an increase of temperature of 4.8 °C in air temperature, and the cultivar Rouxinol presented a significant reduction of this enzyme activity with an increase in the temperature (Fig. 3d).

Discussion

The reduction in the phenological cycle of the cultivars Carijó and Itaim can be explained by the fact that high temperatures cause changes in phenology and shortening the cycle of plants, due to the greater accumulation of degree-days (Bergamashi and Bergonci 2017). A delayed flowering phase (R6) in cultivars Pujante and Tapahium at high temperature, resulted the increase in the number of abortion flowers (Fig. 1a). Similarly with cv. Rouxinol in phase (R7), the increase of 4.8 °C in air temperature caused the pods information to fall, resulting in the prolongation of this phase. The vegetative development increases with an increase in air temperature, since the optimum temperature for vegetative development is higher than the optimum temperature for reproductive development (Hatfield and Prueger 2015). The response of the different genotypes may vary due to the high temperature. Studies carried out under controlled conditions confirm the sensitivity of the cultivars BRS Pujante and BRS Tapahium to the high temperature, with a percentage of aborted flowers of 33% and 66%, respectively (Angelotti et al. 2020). For cowpea, the reproductive stage is more sensitive to the temperature increase, resulting in the loss of flower buds, pods and seed production (Singh et al. 2010). Thus, the selection of a tolerant cultivar in this case will be extremely important to reduce losses due to climate change.

The exposure of plants to high temperatures during the grain filling phase, even for a short period, can accelerate leaves senescence, decrease seeds number and weight and reducing the crop yield. The plants tend to divert resources to deal with thermal stress, limiting the photosynthesis essential for reproductive development (Hoffmann Junior et al. 2007; Sita et al. 2018). The reduction in seed production is also related to the impact of the high temperature during the flowering period. In cowpea, plants submitted to night temperatures of 30 °C showed low viability of pollen grains and indehiscent anthers (Freire Filho et al. 2005), which has a direct effect on the framework and final pod retention, further affecting the number of seeds per pod.

Thermal stress can have a negative impact on physiological parameters, drastically reducing the growth rate and yield. This is due to the leaf sensitivity to high temperature. Photosynthesis can be inhibited as a result of the chlorophyll loss and reduced carbon fixation and assimilation (Yuan et al. 2017). Consequently, this reduction hinders the formation of floral components and the development of new flowers, resulting in fewer pods and seeds (Sharma et al. 2016). The results obtained in this study demonstrating the high temperature having an impact on the metabolism of cowpea plants, causing changes in the growth pattern. However, the cultivars presented different physiological responses due to the different thermal requirement of each genotype.

The cv. Pujante was little sensitive to stress in the vegetative phase without any decrease in  stomatal conductance, thus resulting in a high rate of photosynthetic activity, which did not observed in cv Rouxinol. Although photosynthesis is affected at high temperature, causing changes in the development and growth of plants, these cultivars showed unreduced growth. This fact is explained since, when the cowpea reaches the maturation stage (R9), the plants enter senescence, however, as can be seen in Fig. 1a, the cultivars Pujante and Rouxinol presented a high percentage of flower abortion, which prolonged the vegetative development, reducing the senescence of the leaves, and consequently, resulting in an increase in the shoot dry mass (SDM).

The increase in air temperature resulted a higher leaf temperature. It is known that about 90% of the water that the plants absorb is used to regulate the temperature through transpiration (Taiz et al. 2017), therefore, with the decrease in gs, there is a decrease in transpiration and, consequently, an increase in Tf.

The changes in ROS scavenging enzyme activities under high temperature conditions, as observed in this study, can be associated with the adaptability of the evaluated genetic materials when submitted to thermal stress. Thus, it is recognized that high temperatures cause changes in the metabolism of a wide range of enzymes (Mansoor and Naqvi 2013). Dantas et al. (2015) observed significant changes in the ROS scavenging enzyme activities of CAT and APX in watermelon seedlings submitted to thermal stress, and concluded that these changes provided a better adjustment of cellular functions during applied stress. Associated with the maintenance of ROS scavenging enzyme activities, the synchrony of the behavior pattern of the SOD, APX, CAT and GPX enzymes is fundamental in regulating the level of ROS produced in the plant cell. As the O₂·⁻ are generated, they are dismutated to H₂O₂ by SOD and then H₂O₂ is eliminated by the action of APX, CAT and GPX, which converts it into water and oxygen (Barbosa et al. 2014). Thus, the efficiency of this process reduces the level of oxidative stress, is considered toxic for the balance between the de O₂·⁻ and H₂O₂ levels (Guimarães et al. 2018). The cultivars Pujante, Rouxinol and Tapahium can be classified as sensitive to the applied thermal stress, since they showed significant reductions in seed production associated with lack of synchrony in the activities of the evaluated enzymes. The cv. Carijó can be considered semi-tolerant because it does not present, in general, significant changes in ROS scavenging enzyme activities, however, showed reduced seed production with increasing temperature. The cultivar Itaim, on the other hand, is tolerant of the applied thermal stress, with no significant changes in ROS scavenging enzyme activities and seed production (Figs. 1, 3).

Cowpea cultivars can respond differently to the increase in air temperatures. This first phase of selecting high temperature tolerant cultivars is an important step. However, the impact of temperature coupled with water deficit can be more detrimental, so it is necessary to understand the temperature and water interaction in different cowpea cultivars  to develop effective adaptation strategies.

Conclusions

This study provides the information to select thermotolerant cowpea cultivars, from the impacts on production, physiological and enzymatic activities. The findings revealed responses differentiated among the cultivars Carijó, Itaim, Pujante, Rouxinol and Tapahium with the increase of 4.8 °C in air temperature. The high temperature promoted a greater quantity of aborted flowers, leading to a reduction in the yield of the cultivars Carijó, Pujante, Rouxinol and Tapahium. The photosynthesis, stomatal conductance, leaf transpiration and enzymatic activities were significantly altered by high temperature. From the combination of the responses of biometric, physiological and productive variables, the cultivar Itaim can be considered as tolerant to an increase of 4.8 °C in air temperature. The indication the thermotolerant cowpea cultivar will be extremely important for cultivation areas in warmer regions due to the high demand for this legume for human consumption. Further analyses are required including the interaction with the water element and field validation.