Soybean is a C3 plant, those that fix CO2 initially into three carbon sugars visa the carboxylation of ribulose-1,5-bisphosphate (RuBP). The primary products of photosynthesis are partitioned between two major pathways, one leading to chloroplastic starch synthesis, the other to cytoplasmic sucrose synthesis (Shibles et al. 1987). Starch and sucrose are the most heavily labelled products from either pulse-chase or steady-state 14CO2 uptake, with starch often accounting for most of the radioactivity (Fisher 1970a,b). Monosaccharides such as glucose, fructose, and hexose phosphates are present, but usually in concentrations of < 1% (Streeter and Jeffers 1979). There is a daily rhythm in leaf starch content, with accumulation occurring during daytime and hydrolysis and translocation resulting in a progressive reduction at night. Under a normal photoperiod, the diurnal in crease in leaf starch content occurs at a rate of approximately 3 to 3.5 mgdm-2h-1 regaedless of nodal position (Shibles et al. 1987).