Vitis 39 (1), 3136 (2000) Influence of leaf ageing, leaf area and crop load on photosynthesis, stomatal conductance and senescence of grapevine (Vitis vinifera L. cv. Pinot noir) leaves P. R. PETRIE 1) , M. C. T. TROUGHT 2) and G. S. HOWELL 3) 1, 2) Microbial and Plant Sciences Group, Soil, Plant and Ecological Sciences Division, Lincoln University, Canterbury, New Zealand 3) Michigan State University, East Lansing, USA Correspondence to: Dr. PAUL R. PETRIE, Microbial and Plant Sciences Group, Soil, Plant and Ecological Sciences Division, Lincoln Univer- sity, P.O. Box 84, Canterbury, New Zealand. Fax: +64-3-325 3843. E-mail: petrip@lincoln.ac.nz Summary 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 be- tween 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 in- crease 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 asso- ciated 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 develop- ing vine. Key w o r d s : source:sink ratio, leaf area, leaf age, crop load, photosynthesis, stomatal conductance, fruit maturity. Introduction Pn of a grapevine leaf may be influenced by ambient factors and leaf age (KRIEDEMANN 1968; KRIEDEMANN et al. 1970; LIU et al. 1978; ALLEWELDT et al. 1982; WILLIAMS and SMITH 1985; PONI et al. 1994). When the source or sink size is manipulated by defoliation (HOF˜CKER 1978; HUNTER and VISSER 1988), topping (KAPS and CAHOON 1989), or girdling (KRIEDEMANN and LENZ 1972; HOF˜CKER 1978), the leaves usually respond by increasing their pho- tosynthetic rate as the relative source:sink ratio decreases. The presence of fruit stimulates the Pn rate of individual leaves (HOF˜CKER 1978; CHAVES 1984; DOWNTON et al. 1987; KAPS and CAHOON 1989). KRIEDEMANN et al. (1970) and PONI et al. (1994) concluded that Pn reaches a maxi- mum 30 d after unfolding, and then declines as leaves age. However SCHULTZ (1996) has reported that the leaves op- posite the cluster maintain the highest Pn rates until the month prior to harvest. Cropping has also been associated with higher late season Pn in peach (CHARMERS et al. 1975; DE JONG 1986) and apple (PALMER et al. 1997). This sug- gests that the higher demand for photosynthates associ- ated with fruit ripening has the potential to maintain or in- crease grapevine Pn. It also suggests that the Pn decline attributed to leaf ageing may in part be caused by the in- creasing leaf area during the season, i.e. an increase in the source:sink ratio. This provides a cultural challenge to viti- culturists, suggesting that once an adequate leaf area has been developed early in the season, the vine has greater capacity to fix CO 2 than it can utilise. Under these circum- stances the vine could be considered to be sink-limited. However, post-veraison the demand for photosynthates may be greater than the total vine Pn, and under these circum- stances the vine could be considered to be source-limited. To achieve optimal yields of ripe fruit, without exces- sive vegetative growth, vines have to be manipulated to optimise supply and demand at the different stages of de- velopment. To investigate these factors, an experiment was undertaken to determine whether varying the source:sink ratio of fruiting, cutting grown grapevines would influence the leaf ageing response and rate of leaf Pn over a growing season. Material and Methods Rooted, fruiting grapevines (Vitis vinifera L, cv. Pinot noir), were grown from 6-node cuttings using the method described by MULLINS and RAJASEKARAN (1981). After 5 weeks, vines were selected for uniformity and planted into 1-l pots containing a 80:20 bark:sand mix, 5 kg m -3 of 9 month 16:3.5:10 slow release Osmocotefi fertiliser and