INFLUENCE OF CROP LOAD ON PHOTOSYNTHESIS AND DRY-MATTER PARTITIONING OF SEYVAL GRAPEVINES .1. SINGLE LEAF AND WHOLE VINE RESPONSE PREHARVEST AND POSTHARVEST

Crop load on two-year-old, own-rooted Seyval grapevines was adjusted to 1 (low crop load = LCL), 3 (moderate crop load = MCL), or 7.7 clusters/vine (high crop load = HCL), with all laterals removed to eliminate intra-vine shading; or a fourth treatment, 3 clusters/vine with laterals retained (LAT). Vines with strong fruiting (HCL) or vegetative (LAT) sinks had reduced main shoot growth, node number, internode length, rate and extent of shoot maturation, and leaf size. Total leaf area was inversely related to crop load, except when laterals were retained, then LAT vines had the greatest leaf area/vine. Leaf area/fruit was correlated with the vine's ability to accumulate storage carbohydrates. Total dry weight/vine was not significantly different among treatments when compared by ANOVA. Photosynthesis (Pn) was measured at harvest. Single leaf Pn (SLPn) was measured on leaves at four node positions: opposite the basal cluster (BAS); one node above the basal cluster (BAS + 1); mid-shoot leaf (MID); and the most recent fully expanded leaf (MRFE). SLPn increased from the BAS to the MRFE position for all crop loads. Crop load was positively correlated with SLPn at the MID and MRFE positions. Whole vine Pn (WVPn) per unit leaf area (WVPn/L) was also correlated with crop load, however, only SLPn at the MRFE position was significantly correlated with WVPn/L. WVPn per vine (WVPn/V) was not significantly different among treatments and not correlated with SLPn. LCL significantly enhanced CO2 fixed/g fruit but not CO2 fixed/g total vine dry weight. Crop load did not directly affect WVPn/V. Rather, the effect was indirect, mediated through allocation of assimilates to meet carbon demands, and expressed morphologically and physiologically by the various vine components.