Photosynthetic functioning of individual grapevine leaves (Vitis vinifera L. cv. Pinot noir) during ontogeny in the field

Field studies were conducted to investigate ontogenic changes in photosynthesis of a single grapevine leaf (Vitis vinifera L. cv. Pinot noir) subtending the fruit. A 40-day-old leaf was physiologically most active with regard to net photosynthetic (Pn) and electron transport rates. Variable to maximum fluorescence ratios of dark-adapted leaves (F-v/F-m = 0.77) were higher in mature leaves than in expanding (0.66) or-senescent ones (0.65). Lower F-v/F-m values in these stages seemed to be caused not by photoinhibition but by a low photochemical capacity as suggested from the chlorophyll a/b ratios. In isolated thylakoids, lower rates of whole chain and PSH activity were observed in expanding and senescent leaves, while higher rates were observed in mature leaves. A similar trend was noticed for Rubisco and total soluble proteins. The artificial exogenous electron donors Mn2+ failed to restore the loss of PSII activity in senescent leaves, while DPC and NH2OH significantly restored the loss of PSII activity. The marked loss of PSII activity in senescent leaves was primarily due to the loss of 33, 28-25, 23 and 17 kDa polypepides. A marked loss of Rubisco activity in senescent leaves is mainly due to the loss of 15 (SSU) and 55 (LSU) kDa polypeptides.