Gas Exchange and Chlorophyll Fluorescence Characteristics of Korla Fragrant Pear Leaves in Sun and Shade

An experiment was conducted to investigate the photosynthetic characteristics and response of sun and shade leaves of Korla Fragrant Pear trees, to elucidate difference between sun and shade leaves. Treatments were established according to leaf type in sun or shade situation. Sun leaves were chosen from those growing on. exterior portions of the tree canopy and exposed to full sunlight for most of the day (PPF?..1500p.mol"m-2. s-1). Shade leaves were those growing in interior parts of the tree canopy (PPM 5 gmol-m-2. s-1). Leaf SPAD value, light response curve, gas exchange and chlorophyll fluorescence parameters were also measured. Results indicated that the light condition significantly affected leaf SPAD value. There were no remarkable difference on light compensation between sun and shade leaves, but the light saturation, dark respiration rate and observed quantum yield were significantly lower in shade leaves than in sun leaves. The high intercellular CO2 concentration and low stomatal conductance (gs) of shade leaves benefited to increase net carbon assimilation under low water transpiration rate. High actual efficiency of PS II photochemistry (BPS II), maximum quantum yield of PS II (Fv/Fm) and electron transport rate (ETR) were observed in shade leaves than in sun leaves. These results indicate that, low sunlight density is main factor in inducing of minus net carbon assimilation in shade leaves.