Accumulation of End Products in Source Leaves Affects Photosynthetic Rate in Peach via Alteration of Stomatal Conductance and Photosynthetic Efficiency

In 'Beijing 24' peach [Prumis persica (L.) Batch] trees, a series of source leaves with differing levels of end products were created by retaining fruit ("+fruit"), removing fruit ("-fruit"), or reducing the light period. To alter the light period, leaves were covered with a bag made of brown inner paper and outer silver paper, which was then removed at different times the next day. The highest level of end products were obtained by fruit removal, while reducing the light period resulted in a lower level than "+fruit." Net photosynthetic rate (P-n) and stomatal conductance (g(s)) decreased, but leaf temperatures (T-leaf) increased, following an increase in end product levels in leaves. After the "-fruit" treatment, reduced P-n was correlated with lower g(s), and T-leaf increase was concomitant with decreases in maximal quantum yield of photosystem II (F-v/F-m), actual photochemical efficiency of photosystem II (Phi PSII), and photochemical quenching, and with an increase in nonphotochemical quenching. However, there were no significant differences in chlorophyll fluorescence between "+fruit" and the two treatments reducing the light period. The Phi PSII decreased following an increase in foliar sorbitol level, and it linearly decreased as sucrose and starch increased. Although fruit removal resulted in a significant accumulation of sucrose, sorbitol, and starch in leaves throughout the day, the extractable activities of several important enzymes involved in carbohydrate leaf storage and translocation did not decrease. Therefore, instead of feedback regulation by the accumulation of end products in source leaves, a high T-leaf induced by decreased stomatal aperture may play a key role in regulation of photosynthesis by limiting the photochemical efficiency of the PSII reaction centers under high levels of the end products in peach leaves.