Effects of Partial Defoliation on Petiole Phloem Area, Photosynthesis, and 14C Translocation in Developing Soybean Leaves1

The effect of altered photosynthate supply on crosssectional phloem area in soybean [Glycine max (L.) Merr.l leaf petioles was examined. Leaf. and leaflet removal were used to alter photosynthate production by a given leaf. Measurements of CO2 exchange rate (CER), 14C translocation, and phloem area were made over a period of 21 days as the first trifoliolate leaf developed. CER and translocation were maximum near day 10 then decreased while petiole phloem area increased throughout the 21‐day period. When unifoliolate leaves and cotyledons were removed from young plants, leaf area and phloem area of the first trifoliolate leaf were less than those of controls although a higher CER on a leaf area basis occurred in leaves of plants previously defoliated. Reduction in photosynthetic area by removal of one or two leaflets from a young unfolding leaf had no effect on petiole phloem area. The lack of changes in phloem area in the petiole in response to changes in photosynthesis rate or leaf area indicates that photosynthate supply does not affect the phloem area.