Response of field-grown soybean to saturated soil culture 1. Patterns of biomass and nitrogen accumulation

Abstract

Soybean ( Glycine max (L.) Merr.) has been shown to acclimate to, and produce high seed yield in, continuously saturated soil. In trials conducted to clarify the physiological mechanisms underlying this potentially valuable trait, soybean cv. Fitzroy was grown on beds in two years at Lawes, S.E. Queensland, and irrigated either continuously (saturated soil culture, SSC) or after 60 mm cumulative open-pan evaporation (conventional irrigation, CI). Shoot growth was initially lower in SSC but subsequently greater than that in CI, particularly during seed growth. Root and nodule growth, nitrogenase activity, and leaf conductance to water vapour were all greater in SSC. Seed yields were 24% and 52% higher in SSC in the two years, and these responses were associated with greater numbers of pods through reduced pod abortion, and higher individual seed weights, in SSC. Nitrogen assimilation by shoots in SSC during seed growth was approximately double that in CI, and during senescence the loss of leaf area and nodule function, and the withdrawal of nitrogen from the leaves, was delayed. It is suggested that enhanced leaf water-status in SSC resulted in greater photosynthesis and sustained nodule during pod growth, which in turn enabled continued nitrogen supply to the seeds and thus delayed the impairment of leaf function which follows nitrogen depletion.