Bunchstem and bunch structure of grape cultivars and its implication for abiotic and biotic stress

The bunchstem (BST) consists of the peduncle, rachis and pedicels, and is the main structure determining the bunch compactness among grapevine cultivars. BST provides the vascular connection between the shoot and the berries, being crucial for the supply of water and metabolites to the developing fruit. However, extreme temperatures can result in dysfunction of the BST leading to berry shrivel, and subsequent yield and quality losses. Bunch diseases are particularly common in cooler conditions and more pronounced in cultivars with compact bunches. The BST structure of several common grape cultivars was assessed and compared with that of new mildew resistant selections, with the aim of providing an indication of the potential susceptibility of bunches to biotic and abiotic stress-related impairment. For this purpose, ten important grape cultivars (red/white) and 20 mildew resistant selections (also half red/white), all grown in the same vineyard, were assessed. From each cultivar and selection, ten bunches were collected at fruit maturity. BST measurements, including length, width, bunch volume and weight, and berry weight were conducted. From the assessed cultivars, ‘Shiraz’ and ‘Semillon’ exhibited the longest BST, while ‘Pinot Noir’ and ‘Pinot Gris’ had the shortest. However, the width differences were relatively small in the top third of the bunch between all the cultivars, while ‘Pinot Noir’ and ‘Chardonnay’ had wider BST of the bottom third. The BST weights were highest for ‘Riesling’ and ‘Malbec’, with the other grape cultivars being similar. Overall, the new selections had a wider range in bunch compactness, suggesting that there is most likely a larger potential to determine their suitability for wet and hot climatic conditions in respect to BST and bunch structure.