Physiology of yield determination of soybean ( Glycine max (L.) Merr.) under different irrigation regimes in the sub-humid zone of Sri Lanka

Abstract

Water stress is a major constraint to increasing soybean yields in the sub-humid zone of Sri Lanka. Therefore, the present study was conducted with the objective of analyzing the yield variation of soybean in terms of leaf area growth, radiation interception, radiation use efficiency (RUE) and harvest index (HI) under a range irrigation regimes at Maha-Illuppallama in the sub-humid zone of Sri Lanka (7–8°N and 80–81°E) in 1995 and 1996. Eight irrigation regimes were defined as all possible combinations of irrigation/non-irrigation during three growth stages, i.e. vegetative (V 1–R 1), flowering (R 1–R 4) and pod-filling (R 4–R 8). Consequently, the maximum potential soil water deficit (PSWD) ranged from 69 mm in the ‘fully irrigated’ treatment to 462 mm in the rainfed treatment. The maximum leaf area index (LAI, L m), the fraction of incoming radiation intercepted ( F i) and the maximum total biomass ( W m) increased significantly with the number of stages irrigated, with irrigation during the vegetative stage having the highest positive effect. Reductions in L m, F i and W m due to the absence of irrigation during the vegetative stage, could not be recovered completely by irrigation during the two subsequent stages. RUE also increased significantly with the number of stages irrigated, with irrigation during the flowering and pod-filling stages showing a greater positive effect. Although irrigation during the vegetative stage promoted leaf area growth and radiation interception, absence of irrigation during the subsequent stages decreased RUE significantly because of the substantial water deficits due to higher water loss from the larger canopies. The maximum number of pods (NP) initiated, the mean pod growth rate (PGR) and HI increased significantly with the number of stages irrigated with irrigation during the flowering stage having the highest positive effect. Mean PGR during the main pod-filling period exceeded the corresponding overall crop growth rate (CGR) in all treatments, indicating re-translocation of assimilates from vegetative organs. The highest re-translocation was observed in the treatment which was irrigated during vegetative and flowering stages, but was kept rainfed during the pod-filling stage.