Modification of Solution pH as a Mechanism of Tolerance to Acidity and Aluminium Stress in Selected Cowpea (Vigna unguiculata L. Walp) Cultivars

Abstract

Aluminium toxicity is a major constraint to crop production in soils with a pH below 5.5. However, plant species exhibit varied tolerance mechanisms. Nine (9) cowpea cultivars that were coded; UOE-COWPEA-1, UOE-COWPEA-2, UOE-COWPEA-3, UOECOWPEA-4, UOE-COWPEA-5, KEN-KUNDE-1, K-80, M-66 and KVU-27-1 were assessed for modification of solution pH and tolerance to acidity and aluminium stress in solution culture. Cowpea seeds were sterilized and pre-germinated in paperlined trays and the seedlings were transferred to constantly aerated growth trays containing 1 5 X Hoagland Nutrient solution with a starting pH of 4.3, supplemented with 0 µM and 185 µM Al. The seedlings’ initial root and shoot lengths per cultivar were measured and recorded. pH measurements of the nutrient solutions were recorded daily for seven (7) days without adjusting. The final root and shoot lengths and number of lateral root branches per cultivar and treatment were assessed and recorded. Fresh root and shoot biomass were also measured and recorded. The data collected were subjected to analysis of variance (ANOVA), and the means were compared at significant level of P≤ 0.05 and separation of means was done using Tukey’s test. All the nine (9) cowpea cultivars progressively increased the pH of the solution culture. The growth of cowpea cultivars at 0 µM Al induced a higher change in pH compared to when grown in 185 µM Al concentration. UOE-COWPEA-4 caused the highest increase in pH from 4.3 to 5.13 while K-80 cultivar induced the least change in pH from 4.21 to 4.58 at 0 µM Al. UOE-COWPEA-5 induced the highest increase in pH when compared to others from 4.03 to 5.06 while K-80 cultivar induced the least increase in pH change from 4.32 to 4.53 when grown in solution culture supplied with 185 µM Al. UOE-COWPEA-4, KVU 27-1, KEN-KUNDE-1 and UOECOWPEA-2 had higher relative net root length. UOE-COWPEA-3 produced significantly higher number of lateral root branches in low pH without Al compared to the other cultivars. UOE-COWPEA-3 produced a significantly higher number of lateral branches at 185 µM Al. The findings of this study show that cowpea exhibits genotypic variation in tolerance to acidity and aluminium stress. Furthermore differences in modification of pH varied among the tested cowpea cultivars. It was concluded that acidity and aluminum tolerance were associated with alteration of pH of the solution, suggesting that cowpea adapts to acidity and Al stress by raising the solution pH.