Bud fruitfulness in Vitis vinifera L. cv. Chardonnay in cool climate regions in South Africa

Abstract

Bud fruitfulness is a key determinant of the potential and the actual yield. The formation of the grapevine yield spans over a period of two consecutive growing seasons (Ferrara & Mazzeo, 2023). Flower induction and differentiation take place in the first season, followed by floral initiation and differentiation, development of the clusters until the physiological maturation of berries and seeds in the following season (Monteiro et al., 2021).  Bud fruitfulness varies depending on intrinsic (scion cultivar, rootstock, node position and shoot orientation, hormones – gibberellins and cytokinins) and extrinsic parameters (light intensity, air temperatures, nutritional status, water supply and carbohydrate reserves) (Cox et al., 2012; Li-Mallet et al., 2016).

Grapevine buds comprise a primary bud, secondary and tertiary bud (May, 2000). The primary bud is the most fruitful, but the secondary bud takes over the role of the primary bud if it is killed or damaged. Generally, secondary and tertiary buds are less fertile compared to primary buds (Dry, 2000 & May 2000). Primary buds can also become necrotic, which is detrimental for grapevine yield. Primary bud necrosis (PBN) is strongly correlated to the level of carbohydrates in the grapevine tissue (Collins et al., 2006; Holzapfel et al., 2018). Smith and Holzapfel (2009) reported reduced fruit set and fruitfulness in Semillon grapevines with lower carbohydrate reserves. Chardonnay is an early ripening variety that is subjected to extreme temperatures during spring. The cultivar is versatile and used to produce still and sparkling globally. In the South African context, most Chardonnay plantings are found in Robertson, Stellenbosch and Paarl wine regions. Recently there has been a shift from these traditional wine regions to cool-climate areas like the Walker Bay and Elgin wine regions. However, a lack of knowledge about the relationships between fruitfulness, bud necrosis and carbohydrate reserve levels on canes in Chardonnay in these cool-climate regions is stated. Through this knowledge, it is possible to better manage the most appropriate management techniques in these production areas.

The aim of this study was to determine the fruitfulness with cane elemental analysis in Chardonnay over two seasons 2021-2022 and 2022-2023 in cool climatic conditions. Additionally, the correlation between primary bud necrosis, elemental analysis (carbon %, hydrogen%, nitrogen % and sulphur %) and yield components was determined.