MODELING CARBON EXPORT OUT OF MATURE PEACH LEAVES

The characteristics of sorbitol and sucrose export out of mature leave s in seedlings of peach (Prunus persica L. Batsch cv GF 305) were inve stigated by simulating carbon fluxes through the leaf. Three treatment s were employed: a control treatment and two treatments modifying leaf export, the latter using either shading or girdling. Photosynthesis a nd C-14 partitioning into sorbitol and sucrose were measured during ca rbohydrate pool buildup at the beginning of the photoperiod, and the e xport rate of sorbitol and sucrose was modeled using a PSPICE (Simulat ion Program with Integrated Circuit Emphasis) simulator. The simulatio n allowed prediction of the resulting sorbitol and sucrose contents, w hich were compared to experimental carbohydrate contents. The apparent K-m for sorbitol and sucrose phloem loading, estimated by carbon flux modeling, was 6.6 and 4 mol m(-3), respectively. The predicted export capacity of the leaf, characterized by the estimated V-max values for phloem loading of sorbitol and sucrose, was similar to the photosynth etic carbon flux measured under the leaf growth conditions. This expor t capacity was enhanced in plants in which all leaves except those stu died were shaded. The mature leaf had a higher storage capacity for so rbitol than for sucrose in control plants, especially in the girdled t reatment. Sucrose content appears to be tightly regulated.