Dinitrogen fixation in soybean in response to leaf water stress and seed growth rate

Late season declines in N2 fixation by soybeans [Glycine max (L.) Merr.] frequently are observed under field conditions but do not always occur under glasshouse and growth chamber culture where water stress is avoided and photoperiod is manipulated to alter rate of seed growth. To evaluate the effects of water stress and photoperiod during reproductive growth declines in N2-fixation, nodulated 'Ransom' soybeans dependent entirely on N2-fixation and growing under controlled environment conditions were divided into two groups at the beginning seed (R5) stage. At R5, photoperiod was utilized to experimentally alter the rate of seed growth as a sink for photosynthate and N by imposing a short-day photoperiod (SD) on half of the plants and a long-day photoperiod (LD) on the other half. Within each photoperiod treatment, half of the plants were subjected to a single episode of leaf water stress between -1.2 and -1.8 MPa at the full seed (R6) stage, and half served as nonstressed controls. Plants were sampled at 1- to 5-day intervals between R5 and full maturity (R8) stages and analyzed for N and total nonstructural carbohydrates (TNC). The rate of N accumulation from N2-fixation declined during reproductive growth for the stressed plants but not for the nonstressed plants. Thus, a water stress can actuate a late season decline in N2-fixation. When rate of seed growth was slowed under LD, N composition in leaves remained higher following water stress than when a faster rate of seed growth was promoted under SD. Dinitrogen-fixation activity and dry matter production recovered after rewatering for stressed plants under LD but not under SD. Concentration of TNC in leaflets was greater under LD for both stressed and nonstressed plants. Thus, concentration of TNC in leaflets does not appear to be as important in recovery from water stress as concentration of N.