Effects of post-bloom low light and girdling on fruit set of Vitis vinifera (L.) cv. 'Riesling' and Vitis labruscana (L.) cv. 'Concord'

Abstract

Aim: In grapevines, similarly to many woody crops, berry set is a crucial determinant for vine productivity. We reduced vine carbohydrate supply through shading and preventing phloem flow by girdling to investigate the effects on fruit set and berry weight, in field-grown 'Concord' (Vitis labruscana L.) and 'Riesling' (Vitis vinifera L.) grapevines. Methods and results: Carbon supply to the fruit was manipulated by shading (30 % light transmissivity) individual shoots or shading and girdling. Girdling isolated the shoot and its fruit from possible carbohydrate supply from the rest of the vine. Shading was accomplished by covering shoots with a woven strip of shade cloth for four consecutive days in the period between flowering and 28 ('Concord') or 16 ('Riesling') days after flowering. The percentage of fruit set was calculated from estimated flower numbers using a photographic method and actual berry counts. Conclusions: In both 'Concord' and 'Riesling', short-time shading caused reductions in fruit set, although this effect was more severe in 'Concord'. Both species were most sensitive to reductions in carbon supply during the period between 5 and 12 days after flowering. In 'Concord' vines, the effects caused by shading were greater with girdling probably because 'Concord' vines had higher crop level. Significance and impact of the study: The results can be used as basis for modelling fruit set in grapevines.