Response Of Photosynthetic Apparatus Of Apple Leaves To High Temperature At Different Growth Stages

Prompt fluorescence, delayed fluorescence and modulated820nm reflection weremeasured simultaneously in ‘Malus micromalus’ and ‘Honeycrisp’ leaves, to explorephotosynthetic performance during leaf expansion and effects of high temperature on PSII,PSI and the whole photosynthetic electron transport chain during leaf expansion and thefollowing deveopment. The main results are as follows:1. As leaves expanded, the antenna size per reaction center for the two systems becamesmaller, and the energetic connectivity of PSII units decreased gradually. Meanwhile, the lighttrapping efficiency of PSII, electron transfer capacity at the donor side of PSII, exchangecapacity of PQs at the QBsite and the reoxidation capacity of PQH2were all increased asleaves expanded. However, the capacity of PQH2reoxidation increased at a slower rate thanthe exchange capacity of PQs at the QBsite. In general, during leaf development, thephotochemical activity of both PSII and PSI increased, although the increase in PSII activitywas faster relative to PSI.2. Under high temperature, the photochemical activity of both PSII and PSI, the contentof P680and P700, and the activity of OEC decreased during leaf expansion. Meanwhile, thedegradation rate of PSII antenna pigments was lower than the reaction center. Damage to PSIIelectron donor side was more severe than that to PSII electron acceptor side under slightlyhigh temperature stress. Heavy high temperature significantly inhibited the photochemicalactivity of PSII, PSI and the photosynthetic electron transport chain, with the inhibition ofPSII activity being higher than PSI. The amount of PSII antenna pigments per reaction centerincreased slowly with leaf expansion. In general, thermal tolerance of photosyntheticapparatus inceased with leaf expansion.3. Thermal tolerance of photosynthetic apparatus increased in the fully expanded leavesat different developmental periods. The results suggest that after high temperature acclimation,the thermostability of both PSII donor and acceptor side increased, the relative degradation rate of PSII reaction center pigments was slow down, the light trapping capacity of PSII forelectron transfer also increased. The photochemical capacity of PSI dreased in mature leavesduring leaf development, but there was no remarkable difference of the thermotolerance ofPSI.