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

To examine ¹⁴CO₂ fixation, potential translocation, and carbonflow 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 artificialxylem sap, and treated with ¹⁴CO₂ in continuous and pulse-chaseexperiments. Additional leaves on whole plants were spot treatedon the lamina tip to follow export from the tip only. The analysedleaves ranged in age from leaf plastochron index(LPI) –5to 3, the spot treated leaves from LPI 2 to 5. After 30 minfixation, the specific activity in the lamina tip increasedlinearly with leaf age from LPI –5 to 1 (0.5 to 4.5 kBqmg^–1). Specific activity in the lower lamina increasedslowly with leaf age and did not reach 500 kBq mg^–1 untilLPI –1. Total ¹⁴CO₂ fixed in the lower lamina exceededthat fixed in the tip by LPI –2 because of the large amountof tissue present in the lower lamina. Although the lamina tipfixed high levels of ¹⁴CO₂, pulse-chase studies coupled withautoradiography indicated no vein loading or translocation fromthe tip until about LPI 4 or 5. The ¹⁴C fixed in both tip andlower lamina was incorporated at the site of fixation and wasnot distributed to younger tissue or translocated from the lamina.Although the percentage distribution (¹⁴C in each chemical fractioncompared with the total in all fractions) of ¹⁴C among certainchemical fractions, e.g. sugars, amino acids and proteins, indicatedthat the mesophyll of the tip was more mature than the lowerlamina, physiologically both leaf sectors were immature basedon the expected ¹⁴C distribution in mature tissue. Informationfrom this and other studies indicates that the extreme tip ofa developing cottonwood leaf first begins to export photosynthateabout LPI 4 or 5 on a 24-leaf plant. The first photosynthatetranslocated may be incorporated into the vascular tissues andmesophyll directly below the tip. However, as the tip continuesto mature photosynthate is translocated past the immature lowerlamina into the petiole and out of the leaf. Populus deltoides Bartr. ex. Marsh, eastern cottonwood, translocation, leaf development, ¹⁴C fixation, carbon metabolism