Stomatal and non-stomatal limitations to photosynthesis in field-grown grapevine cultivars

Diurnal changes of photosynthesis in the leaves of grapevine (Vitis vinifera x 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 (I broken vertical bar(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 A mu mol m(-2) s(-1) between 10:00 and 12:00 and slowly decreased after midday. Maximum P-N was 13.7 and 12.5 A mu mol 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 I broken vertical bar(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.