Influence of crop load on photosynthesis and dry matter partitioning of Seyval grapevines .2. Seasonal changes in single leaf and whole vine photosynthesis

Photosynthesis (Pn), leaf area/vine and dry matter partitioning were measured at four phenological stages of development on two-year-old, own-rooted Seyval grapevines adjusted to five different crop loads. Crop loads were 0, 1, 2, 4, or 6 clusters/vine with all laterals removed to eliminate intra-vine shading. Yield/vine was positively correlated and leaf area/vine negatively correlated with clusters/vine. Total dry weight/vine was similar for all crop loads, but increased at each phenophase. Whole vine Pn (WVPn) was measured in a whole plant chamber and expressed as WVPn per unit leaf area (WVPn/L) and WVPn per vine (WVPn/V). Single leaf Pn (SLPn) was also measured on leaves at four node positions. SLPn was highest at the basal node position (node four or five) at fruit set. SLPn at all other node positions was highest mid-season or veraison. WVPn increased through veraison, then declined. SLPn was positively correlated with crop load in at least one leaf position at each phenophase. WVPn/L was positively correlated with crop load only at harvest. WVPn/V was inversely correlated with crop load at mid-season, indicating that vegetative, as well as fruiting sinks, can strongly influence whole vine photosynthetic rates. Over the season, SLPn of the most recently fully expanded leaf was best correlated with WVPn/L. However, there was no general relationship between SLPn and WVPn/V.