Preliminary Evaluation of Growth Response of Two Cowpea Accessions to Water Stress

Drought is the most severe abiotic stress factor limiting plant growth and crop production (Rohbakhsh, 2013).With global warming, it is anticipated that water deficit would be escalated by increasing evapotranspiration, increasing the frequency and intensity of drought with an increase from 1% to 30% in extreme drought land area by 2100 (Fischlin et al., 2007). Yield losses in agricultural crops due to drought have been estimated at 17% (Ashraf et al., 2008).
 
Drought is defined as a period of below-average precipitation, when the amounts of available water in the plant rhizosphere drop below the limits required for efficient growth and biomass production (Deikman et al., 2012; Osmolovskaya et al., 2018). It is characterized by a reduction of water content, diminished leaf water potential and turgor loss, closure of stomata and decrease in cell enlargement and growth. Severe water stress may result in the arrest of photosynthesis, disturbance of metabolism and finally the death of plant (Jaleel et al., 2008). Reduced absorption of photosynthetically active radiations, impaired radiation use efficiency and decreased harvest index are the major yield reducing factors under limited supply of soil moisture (Earl and Davis, 2003). According to Duan et al., (2007), plants show certain changes in their growth patterns and physiological process to cope with the drastic effects of drought stress. CO2 assimilation by leaves is reduced mainly by stomatal closure, membrane damage and disturbed activity of various enzymes, especially those of CO2 fixation and adenosine triphosphate synthesis (Farooq et al., 2008). Earl and Davis (2003) reported that three main mechanisms reduce crop yield by soil water deficit: reduced canopy absorption of photosynthetically active radiation, decreased radiation-use efficiency and reduced harvest index. Furthermore, Wahid and Rasul (2005) indicated that a major effect of drought on crops is the reduction in photosynthesis, which arises by a decrease in leaf expansion, impaired photosynthetic machinery, premature leaf senescence and associated reduction in food production.
 
Cowpea [Vigna unguiculata (L.) Walp.] is an important food and forage legume in the semi-arid tropics that includes parts of Asia, Africa, Southern Europe, Southern United States, and Central and South America (Singh, 2005; Timko et al., 2007). It is an important source of protein and one of the most drought-resistant food legumes that is widely grown in the semiarid regions where drought is a major production constraint (Dadson et al., 2005). Cowpea leaves and green pods are consumed as a vegetable and the dried grain is used in many different food preparations. Protein content of cowpea leaves ranges from 27 to 43% and protein concentration of the dry grain ranges from 21 to 33% (Abudulai et al., 2016).
 
In Ghana and other African countries, cowpeas are often cropped in areas with limited rainfall or soil moisture. Cowpea farmers in the dry areas of sub-Saharan Africa obtain low yields, estimated at about 350 kg per hectare (Danso, 2017). Farooq et al., (2017) indicated that yield reduction in cowpea can vary between 34 and 68% depending on the developmental timing of the drought stress. Studies by Anita and Lakshmi (2015) revealed that plant height and number of branches were maximum when cowpea plants were irrigated. Nkoana et al., (2019) reported that cowpea genotypes responded differently to drought stress. In soybean, a significant reduction was observed in plant height at pod development stage under water stress condition (Shadakshari et al., 2014). Although cowpea is regarded as drought tolerant, much variation has been reported to occur within genotypes (de Ronde and Spreeth, 2007). The objectives of the present study were to assess the effect of water stress on growth of two cowpea accessions and ascertain whether differences exists between genotypes.

The study was conducted at the experimental site of CSIR-Plant Genetic Resources Research Institute, Bunso, Eastern Region-Ghana in the minor season from 11 January to 25 February, 2018. Seeds of two cowpea accessions (i.e. UCC 321 the white type and GH 4769, the red type) obtained from the same institute were sown in the field on 11 January, 2018 at a rate of three seeds per hill and later thinned to one. A spacing of 60cm x 30cm was used. The experiment was a 2 x 2 factorial replicated three times in a randomised complete block design.
       
Agronomic practices which were undertaken during the experimental period include fertilizer application (NPK-15-15-15) at a rate of 5g per plant at two weeks after planting. Watering and weeding were carried out as and when necessary. Insect pests were controlled using K-optimal insecticide (Landa-cyhalothrin 15g/l +Acetamiprid 20g/l: EC) at a recommended rate of 40ml to 15l of water at two weeks interval.
       
The water stress treatment commenced at two weeks after sowing on 25 January, 2018. These comprised of well-watered plants daily and water-stressed plants for thirty-two days during the growing period. The highest daily maximum air temperature was observed in February (36.0°C) and the minimum in January (21.0°C) (Table 1). The average daily relative air humidity was in the range of 73-84% during the experimental period. No rainfall was recorded during the experimental period.
 

       
Plant height in centimetres and number of leaves per plant were measured at one week interval after treatment in both cowpea accessions. Five cowpea plants were sampled from each treatment in each replication from both accessions. Sampled plants were washed to remove debris and separated into roots, stems, and leaves for the determination of plant dry mass. Samples were oven dried at 80°C for 3 days and weighed using an electronic balance. The total plant dry mass was calculated as the sum of the root, stem, and leaf masses. The root to shoot ratio was calculated as a ratio of below ground to above ground dry masse.
       
Statistical analyses were conducted using SPSS Statistics 21 (IBM, Chicago, IL, USA). Two-way ANOVA was used to test the effects of treatment and accession. When a significant interaction between treatment and accession was detected, Tukey’s HSD test was be performed to identify significant differences between drought treatments.

Table 2 shows the effect of water stress on plant height of UCC 321 and GH 4769 cowpea accessions. Significant differences were observed in plant height at 2, 3 and 4 weeks after treatment (WAT) with no significant difference at 1 WAT. The well-watered treatment of GH 4769 obtained the highest plant height at 3 and 4 WAT. The well-watered treatment of UCC 321 was not significantly different from the non-watered treatment at 3 WAT. Both accessions differed significantly at 3 and 4 WAT. Growth occurs through cell division, cell enlargement and differentiation and involves genetic, physiological, ecological and morphological events and their complex interactions (Farooq et al., 2009). Although water stress affects most of the functions of plant growth, this effect depends on the level of water stress, the length of time to which the plant is subjected to water stress and the genotype of plant species (Boutraa et al., 2010). The decrease in plant height under water stress in the present study could be attributed to reduced turgor, which affected cell division and expansion since growth involves both cell growth and development (Okon, 2013). Furthermore, under severe water deficiency, cell elongation of higher plants can be inhibited by interruption of water flow from the xylem to the surrounding elongating cells (Nonami, 1998).  In pea, drought stress impaired early seedling growth of five cultivars tested (Okcu et al., 2005).  Khan et al., (2001) reported that plant height decreased significantly with increasing water deficit in maize.
 

       
Table 3 shows the effect of water stress on number of leaves of two cowpea accessions. Water stress had no significant effect on leaf number at 1 and 2 WAT. Significant (p<0.001) difference was observed at 3 and 4 WAT. The well-watered plants had the highest number of leaves in both cowpea accessions. The production of leaves represents an increase in the photosynthetic surface area for plants. The reduction in number of leaves observed in the present study may lead to lower photosynthetic activity. However, reduced turgor pressure and slow rate of photosynthesis under drought conditions mainly limit leaf expansion (Rucker et al., 1995). Tisné et al., (2010) and Shahinnia et al., (2016) reported that leaves which develop under water deficit generally have smaller cells and higher stomatal density. Studies on soybean by Zhang et al., (2004) and others crops by Farooq et al., (2009) showed that water stress reduced leaf growth and consequently reduced leaf area. Perhaps, the reduction in leaf number under water stress in the present study might be a drought avoidance mechanism.
 

       
The effect of water stress on leaf, stem, root and total plant dry masses and R/S ratio of UCC 321 and GH 4769 are shown in Table 4. Water stress had significant (p<0.05) effect on leaf, stem, root, total plant dry mass and root to shoot ratio in both cowpea accessions. The well-watered plants had the highest leaf, stem, root and total plant dry masses with the non-watered plants obtaining the lowest. The highest root to shoot ratio was observed in the non-watered plants. No significant difference was observed between both accessions. Plant productivity is strongly related to biomass partitioning under water stress (Li et al., 2008). Yin et al., (2005) observed a significant reduction in shoot height, dry mass accumulation, leaf number, total leaf area, and root mass under water stress. According to Leport et al., (2006), drought disturbs the balance of assimilates as most of them are translocated to the roots in order to improve water uptake. The export of assimilates from source to sink generally depends upon the rate of photosynthesis and the sucrose concentration in leaves (Komor, 2000). Nam et al., (1998) indicated that water stress inhibits dry matter production through its inhibitory effect on leaf expansion and consequently reduced light interception. Drought stress caused low seed yield due to deficiency of irrigation at critical stage (Boukecha et al., 2018). The high root to shoot ratio observed under water stress in the present study could be interpreted as a strategy to maximize absorptive root surfaces and increase the water and nutrient uptake rate (Cornelissen et al., 2003).
 

Water stress significantly reduced growth performance in both cowpea accessions used in the study. Significant reductions were observed in plant height, number of leaves, stem dry mass, root dry mass and total dry mass under water stress in both cowpea accessions. Root to shoot ratio was significantly higher under water stress than in the well-watered treatment in both cowpea accessions. Both cowpea accessions differed significantly in plant height. It is therefore recommended that cowpea growers should ensure that their crop does not suffer from water stress during growth stages.

The authors express their gratitude to CSIR-Plant Genetic Resources Research Institute, Bunso, Ghana for the facilities. We are also grateful to Dr. Daniel Ashie Kotey for climatic data acquisition.