Linking Vegetative Growth, Stomatal Traits, and Yield Performance of Cowpea (Vigna Unguiculata L.) Varieties Under Dry-Zone Conditions of Sri Lanka

Cowpea (Vigna unguiculata L.) is a vital legume crop in the Low Country Dry Zone of Sri Lanka, contributing to protein security and sustainable farming under moisture-limited conditions. This study evaluated vegetative growth, leaf anatomical traits, and yield performance of three cowpea varieties (MI 35, Dawala, and Waruni) in Batticaloa district, Sri Lanka. The experiment was laid out in a randomized complete block design with three replicates of 25 plants each, cultivated on 30 m² plots with 50 cm row spacing and 30 cm plant spacing. Growth parameters: plant height, number of branches, number of leaves, and leaf area index (LAI) were measured at 2, 4, and 6 weeks after planting (WAP). Grain yield was recorded at two harvests, and stomatal traits; density, guard cell dimensions, pore size, and stomatal ratio were assessed to evaluate varietal differences in adaptation. Data were subjected to analysis of variance (ANOVA), with mean comparisons performed at the 5% significance level using Minitab 17 version. Significant differences were observed among the cowpea varieties for vegetative growth, stomatal traits, and yield (P<0.05). MI 35 exhibited the greatest early vegetative vigor, reaching plant heights of 16.88 ± 0.43 cm at 2 WAP, 31.01 ± 0.85 cm at 4 WAP, and 17.72 ± 1.92 cm at 6 WAP, with corresponding leaf numbers of 8.90 ± 0.93, 27.12 ± 1.85, and 36.92 ± 1.99, and LAI values of 12.62 ± 1.59, 12.75 ± 1.62, and 33.00 ± 1.33. Despite this early vigor, Dawala produced the highest grain yield, ranging from 1,255–1,525 g/plot in the first harvest and 1,175–1,300 g/plot in the second harvest, likely due to its balanced canopy development, moderate stomatal density (2.440 ± 0.021 stomata/mm²), and conservative water-use traits. The variety Waruni reported the highest stomatal density (2.738 ± 0.123 stomata/mm²) and largest guard cell length (25.96 ± 1.25 μm), indicating potential for rapid gas exchange, but achieved intermediate grain yields of 830–870 g/plot and 915–950 g/plot in the first and second harvests, respectively. These findings highlight the importance of integrating vegetative vigor, stomatal functionality, and reproductive allocation when selecting cowpea varieties for dry-zone conditions