Evaluation of irrigation cut off effect at flowering stage on yield and yield components of cowpea genotypes

Water availability is an important factor affecting plant growth and yield, mainly in arid and semi-arid regions, where plants are often subjected to periods of drought. The occurrence of morphological and physiological responses, which can lead to some adaptation to drought stress, may vary considerably among species. In arid and semi-arid areas that are subjected to more irregular rainfall distribution and occurrence of long summer, the use of more rustic cultivars, tolerant to water stress and with greater ability to recover from drought, should be recommended. The plant tolerance to water deficit is an important defense to keep the production process in conditions of low water availability. Cowpea [Vigna unguiculata (L.) Walp.] is an important crop largely grown in warm and hot regions of Africa, Asia and the Americas and is often regarded as being well-adapted to high temperatures and drought compared with other crop species (Ehlers and Hall,1997). Drought adaptation in cowpea has been related to the minimization of water losses by the control of stomatal aperture. Turk et al. (1980) showed that cowpea is highly sensitive to water stress during the flowering and pod-filling stages. Ahmed & Suliman (1980) showed that the reproductive stage of development is the most sensitive to water deficit in cowpea, causing a reduction in water-use efficiencies and seed yields of at least 50%. In contrast, the genotypes showed a better ability to recover from stress at vegetative stage. This research aims to study the effect of drought stress in the end of the growing season on grain yield and its components of cowpea genotypes and was conducted to identify correlations between grain yield, different morphological characteristics and yield components.
In order to evaluate the effect of terminal drought stress on yield and yield components of 31 cowpea genotypes an experiment was conducted in randomized complete block design (RCBD) with three replications in normal irrigation (non- stress) and drought stress (from flowering stage until the end of the growing season) at research field of Tehran University in Karaj during 2011-2012 growing season. Each plot consisted of two rows with two m length and 50 cm row spacing and 5 cm for plant spacing on lines with planting depth of approximately 5 cm. Each genotype in the two lines was planted manually. For the water stress from flowering stage until the end of the growing season, irrigation was terminated. In this experiment plant height, number of branches per plant, number of pods per plant, number of seeds per pod, seed 100 weight, biological yield per plant and grain yield per plant were measured., Randomized complete block design was performed for simple analysis of variance and the combined analysis was performed for analysis of variance in two experiments (under drought stress and non-stress conditions).
The results showed that values of many of the traits decreased under drought stress. The most damage due to drought stress related to the number pods per plant (34.27%), which was caused a sharp decline in the grain yield (32.5%). The analysis of simple variance examined the both situations (stress and non-stress), showed that in the study varieties, there was a significant difference for all the characteristics. The results of the compound analysis in the treatment indicated that there was a definite distinction in the genotypes and also environment of all traits. The counter effect of environment × genotype for the reaction of the number of pods per plant, number of seeds per pod and grain 100 weight traits was significant, which showed the different reactions of the genotypes in different environments. The comparison between the average traits proved that in both conditions, genotype number 6 showed the highest performance.
The results showed that under both irrigation regimes the biological yield and pod number per plant were highly correlated with grain yield which can be used to improve the performance of their direct selection. Understanding the importance of direct selection of genotypes for biological yield and number of pods per plant is recommended.