CHARACTERISTICS AND CARBON METABOLISM OF MESOPHYLL AND PARAVENOUS MESOPHYLL PROTOPLASTS FROM LEAVES OF NONNODULATED GLYCINE-MAX

The paraveinal mesophyll (PVM) is a spatially, morphologically and ultrastructurally specialized tissue in soybean leaves. It accounts for about 18% of the mesophyll volume as determined by stereological measurements and represents a significant cellular compartment, equal in size to that of the spongy mesophyll. Mesophyll cells (MC), their protoplasts (MP) and PVM protoplasts (PVMP) were isolated from leaves of non-nodulated plants and their physical characteristics and relative activity in some aspects of carbon metabolism were determined. PVMP were about twice the diameter and seven times the volume of MP: they had smaller and 2 3 fewer chloroplasts, and about one third the chlorophyll content of MP. Ribulose 1,5-biphosphate carboxylase (RuBPCase) activity and 14CO2 fixation rates of PVMP were 40% and 30%, respectively, of the values measured in MP on a protoplast basis. PVMP showed net oxygen consumption (240 nmol O2 (106 protoplasts)-1 h-1) in the light with 10 mM bicarbonate, while MP showed the expected photosynthetic oxygen evolution (89 nmol O2 (106 protoplasts)-1 h-1) under similar conditions. The dark respiration rate of PVMP was about six times greater on a chlorophyll basis, three times greater on a protoplast basis and one point seven times greater on a cytoplasmic volume basis than the rate measured in MP. The data show that the PVM plays a minor role in carbon fixation in soybean leaves and supports the hypothesis that one of its primary roles is as a cellular pathway for metabolite transport within the leaf. © 1990.