Leaf starch accumulation and seed set at phloem-isolated nodes in soybean

There is usually a linear relationship between pod and seed number per unit area and assimilate supply in soybean (Glycine max L. Merrill) communities . In contrast, the relationship is curvilinear at single phloem-isolated no des. To further investigate this curvilinear relationship, we evaluated pho tosynthesis, leaf starch levels and pod and seed number at phloem-isolated nodes of cultivar Elgin 87 in three greenhouse experiments. The main stem w as girdled between the fifth and sixth nodes (unifioliolate node was node o ne) when the first flowers opened at the sixth node. The main stem above th e sixth node was removed and defoliation (five or six levels from 0 to 100% ) created a range in assimilate supply Girdling increased leaf starch level s 3- to 7-fold over non-girdled plants within 7 days. in two experiments st arch decreased to control levels within 14-27 days after girdling, but in a third experiment the increase was maintained for 28 days. Defoliation redu ced leaf starch levels, with 66-91% defoliation lowering it to the level in the non-girdled controls. Pod and seed number were directly related to ass imilate supplies at low levels of assimilate availability when there was no accumulation of starch. There were only relatively small increases in pod and seed number at high levels of assimilate availability, but there were l arge accumulations of starch in the leaves. Flower and pod abortion was alw ays above 50%, so pod set was not limited by Bower availability. The failur e of pod and seed number to respond to high levels of assimilate availabili ty suggests that there may be other processes involved in determining pod a nd seed number at isolated nodes in soybean. (C) 2000 Elsevier Science B.V. All rights reserved.