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Sucrose Concentration Gradients in Wheat Leaves¹

The movement of sucrose from leaves of crop plants may be limited by diffusion from photosynthetic sites to the veins, and this diffusional transport may, hi turn, limit photosynthesis and growth. If sucrose moves to veins by diffusion, concentration gradients must be established that favor diffusion. Experiments were conducted with wheat leaves to assay sucrose concentrations in tissue cross-sections using cryostatic techniques. Fresh-frozen leaf sections, 30 fan thick, were cut serially between veins and assayed for sucrose. Plastic embedded leaf tissue was sectioned in an ultramicrotome for anatomical study of tissue analogous to that assayed for sucrose. Air space was estimated to comprise about 9% of the tissue crosssection in the vein region, 23% midway between veins, and 38% of the mesophyll adjacent to the vein. The volume of tissue, minus air space, and tissue dry weight were nearly constant between veins. The absolute amount of sucrose per 30 µm section was higher in the vein than in the interveinal regions. Since vein sections were nearly twice as heavy as interveinal sections, sucrose per unit of dry weight was higher in the interveinal regions, except for leaves with no light pretreatment. There was also a tendency for sucrose expressed on a molar basis to be higher in the interveinal regions. Sucrose concentration increased in the mesophyll from the region nearest the veins to the mid-interveinal region, especially after onehalf and three hours in the light. Sucrose concentrations increased for the first 3 hours in the light, but there was little further increase up to 24 hours. Using a sucrose concentration gradient in the mesophyll of 2.03 M cm^-1 after 3 hours of light, a potential sucrose flux rate of 3,533 p mole cm^-2 sec^-1 was calculated. This rate assumes the whole leaf cross-section, minus air space, serves as a pathway of sucrose movement. A flux rate calculated from an assumed apparent photosynthesis of 1.26 n mole CO₂ cm^-2sec^-1 (20 mg dm^-2hour^-1) and the accumulation of sucrose in 3 hours of light is 14.16 p mole cm^-2sec^-1 suggesting that the pathway for sucrose transport is only about 0.4% of the leaf cross-section.

Key Words: Photosynthetic product movement • Transport of sucrose in leaves • Triticum aestivum L.

² Assistant professor of agronomy, College of Agriculture, Seoul Univ., Suweon, Korea, and professors of agronomy, Univ. of Georgia.

Received for publication June 13, 1979.