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Photosystem II (PSII) efficiency and CO 2  assimilation in response to photon flux density (PFD) and intercellular CO 2  concentration (Ci) were monitored simultaneously in leaves of apple, pear, apricot, and cherry with a combined system for measuring chlorophyll fluorescence and gas exchange. When photorespiration was minimized by low O 2  (2%) and saturated CO 2  (1300 ppm), a linear relationship was found between PSII efficiency and the quantum yield for CO 2  assimilation with altering PFD, indicating CO 2  assimilation in this case is closely linked to PSII activity. As PFD increased from 80 to 1900 μmol·m –2 ·s –1  under ambient CO 2  (350 ppm) and O 2  (21%) conditions, PSII efficiency decreased by increased nonphotochemical quenching and decreased concentration of open PSII reaction centers. The rate of linear electron transport showed a similar response to PFD as CO 2  assimilation. As Ci increased from 50 to 1000 ppm under saturating PFD (1000 μmol·m –2 ·s –1 ) and ambient O 2 , PSII efficiency was increased initially by decreased nonphotochemical quenching and increased concentration of open PSII reaction centers and then leveled off with further a rise in Ci. CO 2  assimilation reached a plateau at a higher Ci than PSII efficiency because increasing Ci diverted electron flow from O 2  reduction to CO 2  assimilation by depressing photorespiration. It is concluded that PSII efficiency is regulated by both nonphotochemical quenching and concentration of open PSII reaction centers in response to light and CO 2  to meet the requirement for photosynthetic electron transport.

588 Photosystem II Efficiency and CO2 Assimilation in Response to Light and CO2 in Leaves of Deciduous Tree Fruit