Relationships of solar radiation and vapour pressure deficit with photosynthesis and water relations in dryland pigeon pea

A study was undertaken to compare the relationships of photosynthetically active radiation (PAR) and vapour pressure deficit (VPD) with carbon assimilation and water relations of dry-land pigeon pea at the vegetative and reproductive phases. Photosynthetic rate (Pn), transpiration rate (T), leaf water potential (?L), and stomatal conductance (gs) were measured at 7- to 10-day intervals from 1 month after seeding until a fortnight before harvest during two seasons. Generally, Pn, T, and gs, were higher and ?L was lower during the reproductive than during the vegetative phase. At high PAR and VPD, Pn, T, ?L, and gs 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 which was associated with high PAR and high VPD but also under conditions of favourable plant water status which were associated with less than optimal levels of PAR. It also showed transpiration efficiency (TE) was lower during the pod-filling than during the vegetative phase, when PAR was optimum.