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CO₂assimilation and photosystem Ⅱ (PSⅡ) activities under various photosynthetic photon flux density (PPFD) conditions were compared in one-year-old cuttings of Vitis vinifera×V. labrusca cv. ´Campbell Early´ and ´Kyogo´. Miximum CO₂assimilation rates in ´Campbell Early´ and ´Kyoho´ were 13.1 and 11.0 μmol·m^(-2)·s^(-1) respectively. Net CO₂assimilation rate was higher in ´Campbell Earlu´ than in ´Kyoho´ by 1-3 μmol·m^(-2)·s^(-1) depending on PPFD ranged from 500 to 2,000 μmol·m^(-2)·s^(-1). However, apparent quantum yield, light compensation point, and dark respiration rate were not significantly different in the two cultivars. Quantum yields of PSⅡ in both cultivars decreased linearly with increasing PPFD, except abrupt increases at 70 μmol·m^(-2)·s^(-1) PPFD. Electron transport rate (ETR) was higher in ´Csmpbell Early´ than in ´Kyoho´ beyond 500 μmol·m^(-2)·s^(-1) PPFD. Under PPFD conditions beyond light compensation point, net CO₂assimilation rate was curvilinearly related to stomatal conductance, indicating that CO₂assimilation was affected by both stomatal and non-stomatal factors. Curvilinear relationship was also found between the responses of net CO₂assimilation rate and ETR to incident PPEF. Our results demonstrated that both cultivars had similar photosynthetic characteristics at low PPFE. At high PPFE, however, ´Campbell Early´ exhibited higher CO₂assimilation rate.
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