Diurnal changes of photosynthesis in the leaves of grapevine (Vitis vinifera × V. labrusca) cultivars Campbell Early and Kyoho grown in the field were compared with respect to gas exchanges and actual quantum yield of photosystem 2 (Φ PS2 ) in late May. Net photosynthetic rate (P N ) of the two cultivars rapidly increased in the morning, saturated at photosynthetic photon flux density (PPFD) from 1200 to 1500 μmol m -2 s -1 between 10:00 and 12:00 and slowly decreased after midday. Maximum P N was 13.7 and 12.5 μmol m -2 s -1 in Campbell Early and Kyoho, respectively. The stomatal conductance (g s ) and transpiration rate changed in parallel with P N , indicating that P N was greatly affected by g s . However, the decrease in P N after midday under saturating PPFD was also associated with the observed depression of Φ PS2 at high PPFD. The substantial increase in the leaf to air vapour pressure deficit after midday might also contribute to decline of g s and P N .Additional key words: gas exchange, quantum yield, stomatal conductance, transpiration rate, vapour pressure deficit.
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