Drought tolerance indices and variability in leaf abscisic acid accumulation among accessions of cowpea (Vigna unguiculata L. Walp) under drought stress

Abstract

This study was done to assess the level of variability for drought tolerance in accessions of 
cowpea based on tolerance indices of plant height, dry root weight and accumulation of 
abscisic acid (ABA). The 23 accessions of cowpea from International Institute of Tropical 
Agriculture (IITA), Ibadan, Nigeria, were grown under well watered and drought stressed 
conditions at seedling stage for three weeks in a Completely Randomized Design (CRD) 
with three replicates. Drought was imposed at two weeks after planting for three weeks in 
the screen house. At 21 days after imposition of stress, terminal leaves of each plant from 
each pot were excised for ABA extraction and quantification. Data were also recorded for 
plant height and dry root weight and were used for drought tolerance indices estimation. 
Significant variability was observed among accessions under both conditions. Plant height 
(4.61 to 55.87%) and dry root weight (3.33 to 82.47%) were significantly reduced by 
drought stress in cowpea. Tolerance index based on plant height and dry root weight 
separated the 23 accessions into 5 groups of tolerance. There were contradictory responses 
among accessions for leaf ABA content under both conditions. Generally, accessions with 
leaf ABA of up to 15mol and above under control condition were all susceptible accessions 
except AC15, while the accessions with leaf ABA content of up to 15mol and above under 
drought stressed condition were mostly tolerant accessions except AC09. These 
contradictory responses suggest that the constitutive nature of ABA biosynthesis and the 
genes involved in the pathway of its synthesis in cowpea genotypes may depend on other 
factors present in the soil to be up regulated or down regulated and also on all traits which 
confer drought tolerance on plants which are also constitutive in nature rather than stress 
induced