Nitrogen accumulated and partitioned at maturity by grain legumes grown under different water regimes in a semi-arid tropical environment

Abstract

The nitrogen accumulated and partitioned above ground at maturity by soybea, green gram, black gram, cowpea, lablab bean and pigeon pea grown under different soil water regimes during the dry season in semi-arid tropical Australia was compared. There were marked differences in total nitrogen accumulated ( N), average nitrogen accumulation rate ( R N), and efficiency of nitrogen accumulation per unit of dry matter production ( E N) between species and water regimes, as well as significant water regime × species interactions. N and R N decreased in response to prolonged water deficits ∗, while E N either remained unchanged or increased in all species except soybean. Under well-watered conditions, R N for all species save pigeon pea was similar. The proportion of nitrogen partitioned to the seeds tended to decrease as a result of prolonged water deficits in all species save green gram which showed an increase in nitrogen harvest index. Differences in nitrogen distribution to the seed, hull, stem and leaf fractions as a result of water deficits were examined using a partitioning index (PI), defined as the ratio of the proportions of above-ground nitrogen and dry matter partitioned to a given plant part at maturity. Under prolonged water deficits an early-maturing soybean such as cv. Buchanan produced as much nitrogen directly utilizable by man, as cowpea and green gram despite the higher seed yields and higher R N of these latter species. For the semi-arid tropics, soybean offers the additional guarantee of highest seed nitrogen yield should water be non-limiting in a particular season.