Photosynthesis in response to sink-source manipulations during different phenological stages of fruit development in peach trees: regulation by stomatal aperture and leaf temperature

Abstract

SummaryFruit was removed (‘– fruit’) or retained (‘+ fruit’) on 1 year-old shoots of Prunus persica L. Batch cv. ‘Okubo’ with the same number of leaves, and on the limbs of cv. ‘Yanfengyihao’ peach trees during different phenological stages of fruit development in 2002 and 2003, respectively. Fruit removal significantly decreased the net photosynthesis rate, stomatal conductance and transpiration rate, and increased leaf temperature around midday, compared to the ‘+ fruit’ treatment. No significant difference in the sub-stomatal CO2 concentration in source leaves was observed between the ‘– fruit’ and ‘+ fruit’ treatments. Decreased carbon-sink activity due to fruit removal significantly reduced the net photosynthesis rate only when photosynthetically active radiation was high and exceeded 900-1,000 µmol m–2 s–1.The net photosynthesis rate was significantly and positively correlated with stomatal conductance, especially at small stomatal apertures, and was significantly lower in the ‘– fruit’ treatment compared to the ‘+ fruit’ treatment. This decrease in net photosynthesis rate was often accompanied by significantly decreased stomatal conductance and transpiration rate. Moreover, the net photosynthesis rate peaked when the leaf temperature was between 36°–38.5ºC, then decreased sharply when the leaf temperature continued to increase in the presence or absence of fruit. However, the net photosynthesis rate in the ‘– fruit’ treatment was lower than in the ‘+ fruit’ treatment at the same leaf temperature, especially when the leaf temperature exceeded 38.5ºC. It is suggested that increased leaf temperature, followed by a reduced stomatal aperture, may be critical for reduced photosynthesis with decreased sink strength after fruit removal. Regulation of stomatal aperture, the first important physiological reaction of leaves, may be considered the trigger, and increased leaf temperature as being instrumental in regulating photosynthesis during low sink-demand in peach trees. Similar results were found for both peach cultivars at two experimental sites.