Influence of leaf ageing, leaf area and crop load on photosynthesis, stomatal conductance and senescence of grapevine (Vitis vinifera L. cv. Pinot noir) leaves

Treatments varying the leaf area (source) to crop load (sink) balance of pot-grown Pinot noir vines caused differences in the photosynthesis (Pn) rates of the fourth leaf, 48 h after they were applied. Stomatal conductance was only affected by leaf removal, not by the presence or absence of crop. The vines with and without crop were subject to a range of leaf removal treatments. All treatments retained leaves at nodes 1-4 from the shoot base and then had 100 % (control), 66 %, 33 %, or 0 % leaves retained from node 5 to the apex of the shoot. Leaf removal elevated the Pn rate of the fourth leaf, but there was no difference in Pn rate between vines with or without crop pre-veraison. From veraison on the photosynthetic rate of vines with crop and 100 % leaves retained increased. Similar, high Pn rate was also observed for vines without crop and 0 % leaves retained. The lower Pn rate of vines with crop pre-veraison suggests that there is potential to increase vine productivity in this period. The Pn rate of vines without crop, 100 % and 66 % leaves retained declined from 15 d after treatment. Average Pn of all treatments over the 4 measurements prior to harvest was positively correlated with the vine leaf area (source) to total vine dry weight (sink) ratio. Leaves of vines with a high source:sink ratio (without crop, 100, 66 or 33 % leaves retained) senesced i.e. decreased in chlorophyll content more rapidly than leaves of the low source to sink ratio treatments. Results indicate that the decline in grapevine leaf Pn, previously associated with advanced leaf age is actually caused by a progressive increase in leaf area to fruit weight (source:sink) ratio, as leaves emerge on the developing vine.