COWPEA WATER RELATIONS AND GROWTH-RESPONSE ON A TOPOSEQUENCE WATER-TABLE GRADIENT

Cowpea sown after wetland rice in tropical monsoon climates is frequen tly subjected to soil saturation and a high water table during early g rowth. We compared the effects of variation in water table regime on t he shoot and root growth, and plant water relations of two medium-matu ring (cv. TVX3236-01G and TVX3410-02J) and two early maturing (cv. BS6 and IT82D-889) cowpea [Vigna unquiculata (L.) Walp] cultivars in thre e water table depth regimes on a Typic Tropudalf toposequence. Our obj ective was to elucidate the possible mechanisms of differential cultiv ar adaptation to shallow water table conditions. The shallow water tab le regime (SWT) significantly reduced the total dry matter accumulatio n and harvest index for BS6, an early maturing cultivar (reductions in stems, leaves, pods, and total dry matter by 82, 77, 88, and 84% resp ectively), while the least reductions (reductions by 44, 3, 57 and 55% , respectively) was observed for TVX 3410-02J, a medium-maturing culti var. Roots of the medium cultivars were more concentrated in the top 0 .2m in the SWT Site than those of the early-maturing cultivars. Roots of the medium cultivars later penetrated to 0.8-m maximum depth compar ed to 0.6-m maximum depth for the early cultivars, and extracted soll water from 20 cm deeper in the soil profile than the early maturing cu ltivars. The medium-maturity cultivars maintained higher leaf water po tentials and lower canopy temperatures throughout the season. This was associated with greater end-of-season total soil profile water extrac tion. The medium-maturing cultivars showed better adaptation to shallo w water table conditions due to superior plant water status, rooting p attern, and biomass production and partitioning.