Grain and nitrogen accumulation by mungbean under various water supply conditions

Abstract

Grain legume crops grown under rainfed conditions in semi-arid regions can be subjected to water stress at any time during crop growth. The study of water relations in mungbean (Vigna radiata), especially the effect of water stress on N accumulation and the occurrence of multiple flushes of flowers is limited. The objectives of this study were: (l) to examine the timing and severity of water stress on biomass production, grain yield and quality and nitrogen accumulation of mungbean, (2) to examine the occurrence and importance of second flush of flowering in contributing to total grain yield in mungbean under various water conditions, (3) to calibrate and test the APSIM-Mungbean model by using experimental data from this study and use the model to assess variation in mungbean productivity at several locations in Queensland and, (4) to compare mungbean with soybean (Glycine max) and blackgram (Vigna mungo) in their response to water stress.Four field experiments were conducted at Gatton, southeast Queensland. The water supply conditions included well-watered, rainfed, rainfed+irrigation and rainout shelter. In experiments 1 and 2, two mungbean cultivars (Berken and Emerald) and one soybean cultivar (A6785) were sown in November and January and grown under well-watered, rainfed, rainfed+irrigation and rainout shelter conditions. In experiment 3, three sowing dates, 2 weeks apart, were made in order to be able to subject mungbean crops to water stress at different growth stages. Water stress was imposed at mid vegetative to early flowering, late vegetative to mid pod filling and flowering to late pod filling by using rainout shelters. In experiment 4, mungbean (cv. Emerald), soybean (cv. A5939) and blackgram (cv. Regur) were grown under well-watered, rainfed and rainfed+irrigation conditions.The timing of water stress determined mungbean grain yield. Water stress during mid vegetative to early flowering had a smaller effect than that during flowering to pod filling. Terminal water stress from flowering to maturity resulted in no second flush of flowers, while under intermittent stress and well-watered conditions, mungbean produced a second flush of flowers and hence had higher yield than that under terminal stress.Indeterminate crops (mungbean and blackgram) showed a yield advantage compared with the determinate soybean crop under rainfed+irrigation conditions. Irrigation at late pod filling stage did not increase soybean yield, while in mungbean and blackgram, the second flush of flowers was produced and hence yield was increased. There was a negative correlation between size of the second flush and size of the first flush, where high yield of the first flush tended to suppress the yield of the second flush. There was a risk of weathering damage of seed from allowing a crop to produce a second flush. With cumulative rainfall of 35 mm after maturity of seeds from the first flush of flowers, seed quality was likely to deteriorate.Total N accumulation in soybean was greater than in mungbean because of the greater capacity of soybean in N fixation; however N fixation of soybean was more sensitive than mungbean to water stress. Nitrogen accumulation efficiency (g N per g biomass) was higher in soybean than mungbean and blackgram. Greater N accumulation efficiency of soybean was reflected in the higher N concentration in soybean tissues than in mungbean and blackgram.Harvest index at maturity of mungbean varied with different timing and severity of water stress. Water stress during the vegetative stage could increase harvest index by 15%, while terminal water stress could reduce it by 10% compared to well-watered conditions. However, compared with biomass production, harvest index was a reasonably conservative parameter. The pattern of harvest index increase until maturity of pods from the second flush of flowers could not be simply expressed as a linear function of time.