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).