Down-regulation of photosynthesis via sink limitation is linked to foliar soluble sugar content in high- and low-yielding varieties of sweet cherry

The objective of this study was to investigate the basis for yield differences between ‘Kordia’ and ‘Sylvia’, low- and high-crop load carrying sweet cherry cultivars, respectively. Branch girdling was effected on ‘Kordia’ and ‘Sylvia’ set up as Spanish Bush at Old Beach in the Derwent Valley, near Hobart, Tasmania, Australia. Photosynthesis was measured with a Li-Cor 6400 four times pre- and once post-harvest. Fruit set was 42% and 56% and fruitlet abscission was 69% and 34% in ‘Kordia’ and ‘Sylvia’, respectively. Maximum photosynthetic rate (A_sat) increased from a low or around 12 µmol m^-2 s^-1 in early Spring to a maximum of around 18 µmol m^-2 s^-1 early Summer and was similar between cultivars pre-harvest. However, post-harvest, A_sat of ‘Kordia’ was significantly lower at around 8 µmol m^-2 s^-1 relative to that of ‘Sylvia’ at around 16 µmol m^-2 s^-1. Girdling had a stronger effect on A_sat in the low-yielding ‘Kordia’ than in the high-yielding ‘Sylvia’ (-13% vs. -11%, respectively), and there was a greater increase in foliar soluble sugar (SS) content in ‘Kordia’ than ‘Sylvia’ (+18% vs. +13%), averaged across the season. There was a strong negative correlation between SS and A_sat in leaves, across the season, cultivars and treatments. There were no girdling or cultivar effects on foliar starch content. The results suggest that girdling-induced sink-limitation leads to down-regulation of photosynthesis that is linked to the accumulation of SS content in the leaves. Despite minor differences in A_sat between varieties, our results imply that the low-yielding ‘Kordia’ variety is not limited by source-supply potential during the growing season, but that low A_sat post-harvest could be linked to the subsequent season’s fruitlet abscission that limits potential yields. Further investigation is warranted regarding carbohydrate storage as a factor in the relatively high fruitlet abscission in ‘Kordia’, and other low-yielding varieties of sweet cherry.