RELATIONSHIPS OF SOLAR-RADIATION AND VAPOR-PRESSURE DEFICIT WITH PHOTOSYNTHESIS AND WATER RELATIONS IN DRY-LAND PIGEON PEA

A study was undertaken to compare the relationships of photosynthetica lly active radiation (PAR) and vapour pressure deficit (VPD) with carb on assimilation and water relations of dry-land pigeon pea at the vege tative and reproductive phases. Photosynthetic rate (P(n)), transpirat ion rate (T), leaf water potential (psi(L)), and stomatal conductance (g(s)) were measured at 7- to 10-day intervals from 1 month after seed ing until a fortnight before harvest during two seasons. Generally, P( n), T, and g(s) were higher and psi(L) was lower during the reproducti ve than during the vegetative phase. At high PAR and VPD, P(n), T, psi (L), and g(s) decreased. The decrease in the T at high PAR was smaller during the reproductive phase. The results showed that growth of dry- land pigeon pea was affected not only during periods of water stress w hich was associated with high PAR and high VPD but also under conditio ns of favourable plant water status which were associated with less th an optimal levels of PAR. It also showed transpiration efficiency (TE) was lower during the pod-filling than during the vegetative phase, wh en PAR was optimum.