Effect of Photosynthate Source‐Sink Manipulation on Dinitrogen Fixation of Male‐Fertile and Male‐Sterile Soybean Isolines1

Abstract

The effect of carbohydrate availability upon nodule activity was studied using male‐sterile (ms2 ms2) and male‐fertile (Ms2 Ms2) soybean (Glycine max L. Merr.) isolines. At 63 days after emergence (DAE), treatments intended to increase or decrease the supply of carbohydrate to nodules were imposed. Treatments were: control, partial shading, partial defoliation, and partial depodding. At 70 DAE, partial sha,ding decreased photosynthetic photon flux density 75%, partial defoliation decreased leaf dry weight 60%, and depodding reduced pod dry weight 40 to 87%. Specific nodule activity (SNA) was determined during seed‐filling using acetylene reduction assay. Specific nodule activity of both isolines changed very little until 83 DAE, then declined rapidly during the next 14 days as male‐fertile plants approached physiological maturity. Control male‐fertile and male‐sterile plants had similar SNA until 83 DAE when SNA of sterile plants was greater than SNA of male‐fertile plants. Treatments intended to decrease the supply of carbohydrate to nodules decreased SNA throughout seed‐filling. Depodding increased SNA of male‐fertile plants, but decreased SNA of male‐sterile plants at 83 DAE. Depodding increased total root N concentration. Other treatments did not affect root N concentration. Control male‐sterile plants had higher total root N concentrations than control male‐fertile plants. On most dates, concentrations of starch and total soluble sugars were greater within roots of control male‐sterile plants than within roots of control male‐fertile plants. At 70 DAE, partial shading and partial defoliation of male‐fertile plants resulted in decreased root soluble sugars but had no affect on root starch concentration. Depodding increased the concentration of starch and total soluble sugars within roots of both isolines 77 and 83 DAE. Treatment and isoline effects on SNA were not consistently related to treatment effects on root carbohydrate concentration. Male‐sterility and depodding treatments which decreased the reproductive sink for carbohydrate also decreased the reproductive sink for newly reduced N. Data indicate that factors other than carbohydrate availability may be responsible for the decline in SNA during late seedfilling.