Patterns of assimilate distribution in soybeans at maturity. I. The influence of reproductive development stage and leaf position

Leaves at upper, middle and lower positions in the canopies of soybeans were allowed to assimilate 14CO2 at one of several stages of reproductive development from pod set to maturity (49, 35, 25, 7 and 0 days before maturity respectively), and the distribution of 14C activity in the plants was determined at maturity. Pods in the axils of the treated leaves were always the major location for final assimilate accumulation, and together with those at the two nodes above and two below, averaged over the three leaf positions, accounted for 60–70% of the activity from all treatment stages. More remote pods were much less important sinks. Higher leaves contributed relatively more of the total final activity to their adjacent pods than those lower in the canopy, except when treated towards pod maturity. Little activity was found in stems, except in the segment immediately adjacent to the treated leaf in mid canopy, and then only at the earliest stage before rapid seed growth was occurring, when the stem segment could act as a sink. The corresponding result for the lower leaf was a relatively large amount of activity in the root, an adjacent active sink; otherwise recovery in the roots was low for all occasions and leaf levels. Treated leaves accounted for some 10% of residual activity, although much more was retained by them when treated towards maturity.