Relationship of Enhanced Sink Demand with Photosynthesis and Amount and Activity of Ribulose 1,5-bisphosphate carboxylase in Soybean Leaves

Abstract

Summary The presence of reproductive sinks is known to enhance the photosynthetic rate of soybean [ Glycine max (L.) Merr.] leaves. Thinning the plant stand at early podding to 25 % of normal plant density increased pod set and pod (plus seed) growth rate per plant, thereby increasing sink size of the remaining plants. CO 2 -exchange rate (CER), the contents of soluble protein and ribulose 1,5-bisphosphate carboxylase (Rubisco), as well as Rubisco activity, were determined for leaves of differing ontogeny. Leaf CER of plants with enhanced sink demand peaked at a higher level during the season, or the rates declined less rapidly during senescence than for control plants (normal density). CER was positively correlated with that of soluble protein and Rubisco content per unit leaf area. The percentage of protein that was Rubisco did not change in comparison with controls and, as the soluble protein increased after thinning, this led to an increased Rubisco activity per unit leaf area that was not due to increased Rubisco activity on a soluble protein basis. Thus, the greater sink demand increased CER or delayed its decline in soybean in parallel with the amount of Rubisco protein, not with the activity of the enzyme.