Increasing sink size does not increase photosynthesis during seed filling in soybean

Sink size at phloem-isolated nodes in soybean (Glycine max L. Merrill) was increased to determine if a larger sink would stimulate photosynthesis. A larger sink was created by increasing the number of isolated nodes fed by a single leaf from one to three. Greenhouse grown plants (cultivar Elgin 87) were girdled below node 7 (one-node treatment) or below node 5 (three-node treatment) when the first flowers appeared at node 7. The stem above node 7 was removed at the time of girdling as were the leaves and petioles from nodes 5 and 6 on the three-node treatment. The three-node treatment produced approximately 40% more pods and seeds than the one-node treatment, but this increase in sink size had no effect on carbon exchange rate (CER) or leaf chlorophyll levels (estimated with a SPAD chlorophyll meter) during seed filling. There was no treatment effect on leaf soluble sugars, but starch levels tended to be slightly higher in the three-node treatment. These data provide no evidence that increasing the size of the sink exerts any affect on source activity. Mature weight per seed of the three-node treatment was reduced, but yield was the same as the one-node treatment. Our data are consistent with the concept that the seed sink is soybean is relatively passive and increasing sink size above its normal level does not exert a direct influence on leaf photosynthesis.