14CO2 Fixation, Translocation, and Carbon Metabolism in Rapidly Expanding Leaves ofPopulus deltoides

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Abstract
To examine 14CO2 fixation, potential translocation, and carbon flow among leaf chemical fractions of young developing leaves, the shoot tip of 24-leaf cottonwood (Populus deltoides Bartr. ex. Marsh) plants were cut off under water, placed in artificial xylem sap, and treated with 14CO2 in continuous and pulse-chase experiments. Additional leaves on whole plants were spot treated on the lamina tip to follow export from the tip only. The analysed leaves ranged in age from leaf plastochron index(LPI) −5 to 3, the spot treated leaves from LPI 2 to 5. After 30 min fixation, the specific activity in the lamina tip increased linearly with leaf age from LPI −5 to 1 (0.5 to 4.5 kBq mg−1). Specific activity in the lower lamina increased slowly with leaf age and did not reach 500 kBq mg−1 until LPI −1. Total 14CO2 fixed in the lower lamina exceeded that fixed in the tip by LPI −2 because of the large amount of tissue present in the lower lamina. Although the lamina tip fixed high levels of 14CO2, pulse-chase studies coupled with autoradiography indicated no vein loading or translocation from the tip until about LPI 4 or 5. The 14C fixed in both tip and lower lamina was incorporated at the site of fixation and was not distributed to younger tissue or translocated from the lamina. Although the percentage distribution (14C in each chemical fraction compared with the total in all fractions) of 14C among certain chemical fractions, e.g. sugars, amino acids and proteins, indicated that the mesophyll of the tip was more mature than the lower lamina, physiologically both leaf sectors were immature based on the expected 14C distribution in mature tissue. Information from this and other studies indicates that the extreme tip of a developing cottonwood leaf first begins to export photosynthate about LPI 4 or 5 on a 24-leaf plant. The first photosynthate translocated may be incorporated into the vascular tissues and mesophyll directly below the tip. However, as the tip continues to mature photosynthate is translocated past the immature lower lamina into the petiole and out of the leaf.