Response of field-grown soybean to saturated soil culture 2. Effect of treatments to alter photosynthesis and leaf nitrogen supply

Abstract

This paper is the second in a series describing the growth and accumulation of nitrogen by soybean ( Glycine max (L.) Merr.) cv. Fitzroy, grown on beds at Lawes, S.E. Queensland, and irrigated either continuously (saturated soil culture, SSC) or after 60 mm of cumulative open-pan evaporation (conventional irrigation, CI). Rates of growth and N accumulation were initially reduced in SSC, but after a brief acclimation period were enhanced such that seed yield was higher that in CI. This paper describes the consequences for patterns of growth and N accumulation of various treatments which were imposed to increase the interception of photosynthetically-active radiation ( Q pa) by, or the aviability of nitrogen to, the plants. Increased interception of Q pa almost invariably resulted in increased dry-matter ( DM) production and seed yield, accompanied by enhanced nitrogen fixation. The effects of treatments which aimed to directly increase nitrogen availability were less consistent, particularly in relation to seed yield. However, while the extent of translocation of DM to seed was variable, all treatments depleted shoot N to an apparently minimum level, suggesting that seed yield was limited by nitrogen uptake. This conclusion agrees with that of the first paper, and with the concepts of the ‘selfdesctruct’ hypothesis. The responses of plants in the two irrigation regimes to manipulation treatments were surprisingly similar, considering the extent of the differences in growth between control plants. The results were c onsistent the extent the hypothesis that the response to SSC is primarily due to enhanced leaf water status, which enables increased photosynthesis and, in turn, nitrogen fixation, rather than a direct positive effect of the wet-soil environment on the symbiosis.