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Grapevine bud fertility under conditions of elevated carbon dioxide

Aims: Microscopic bud dissection is commonly used to assess grapevine bud fertility and thereby predict yield for the following season. Grapevine yield has been shown to increase in response to elevated carbon dioxide concentration (eCO2), and this yield gain has been demonstrated under Free Air Carbon dioxide Enrichment (FACE). However, the effects of eCO2 on bud fertility and their relation to yield gain have not been investigated. Little is known about which stages of development and which yield components are affected. The aim of this study was to determine the number of inflorescence primordia (IP) per node, the cross-sectional area of the IP, and the incidence of primary bud necrosis (PBN) in grapevine compound buds grown under conditions of ambient CO2 concentration (aCO2) and eCO2, and to relate the data to yield parameters of field-grown vines at harvest.Methods and results: Plant material was collected in February 2016 and February 2017 from two Vitis vinifera L. cultivars, Riesling and Cabernet-Sauvignon, growing at the VineyardFACE experimental site of Hochschule Geisenheim University (49°59¢N, 7°57¢E) in the Rheingau wine region, Germany. Bud dissections were performed at the University of Adelaide’s Waite Research Institute, Australia. Canes were stored at 4°C until dissection at room temperature. The first eight nodes of each cane were dissected and the compound buds assessed; IP number, IP cross-sectional area (evaluated by image analysis) and PBN incidence were recorded. In Riesling, no differences were found between plants grown under the two CO2 treatments in terms of IP number per node or subsequent number of bunches per shoot. Compound buds of Riesling plants grown under eCO2 had larger IP cross-sectional area; however, this did not result in higher bunch weight or yield in response to eCO2 over the 2 years. In Cabernet-Sauvignon, IP number per node was higher in plants grown under eCO2 but no changes in bunch number per shoot were found in either season. In contrast, the larger IP cross-sectional area in Cabernet-Sauvignon in response to eCO2 translated into higher bunch weight and yield in both seasons. PBN incidence of both cultivars was highest at basal node positions along the fruiting cane, except for Cabernet-Sauvignon in 2017, where both treatments showed higher incidence at the distal ends. In both cultivars, average PBN incidence along the cane was unaffected by eCO2.Conclusions: Microscopic bud dissection can be used at an early stage of vine development to predict increased bunch weight. There was evidence of a cultivar-dependent response to eCO2 in terms of bud fruitfulness. In future, it would be interesting to investigate whether higher carbohydrate levels are responsible for the increase in IP area detectable at a very early stage of development in plants grown under eCO2.Significance and impact of the study: The findings of this study contribute to our understanding of grapevine bud fertility and yield potential, particularly under changing climatic conditions.