S01 - Session O4 - Bunchstem and bunch structure of grape varieties and its implication for abiotic and biotic stress

Authors: Bruno Holzapfel *, Gerhard Rossouw, Bianca Bittau, Albane Meunier, Ian Dry

The bunchstem (BST) structure is the main contributor to bunch compactness. Consisting of peduncle, rachis and pedicels, there are considerable differences between varieties in size. The BST provides the vascular connection from the shoot to the berries, with the function being crucial for the supply of water and metabolites to the berry and consequently its composition. Extreme temperatures can result in dysfunction of the BST and berry shrivel, resulting in yield and quality losses. Bunch diseases are common in cooler conditions and more pronounced in compact bunches due the specific BST structure of varieties. The BST structure was assessed in several common grape varieties and compared with new mildew resistant selections, with the aim to provide indications of the potential impact of biotic and abiotic stress on the bunches and BSTs. For this purpose, 10 important grape varieties (red/white) grown in Australia were assessed together with 20 mildew resistant selections (also half red/white), which all were grown under the same conditions in one vineyard. From each variety and selection 10 bunches were collected close to harvest and several bunch and BST measurements were conducted, including length and width, bunch volume and the weight of BSTs, bunches and berries. The varieties Shiraz and Semillon showed 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 varieties, but Pinot Noir and Chardonnay had wider BST in the bottom third. The BST weights were highest in Riesling and Malbec, with the other grape varieties being similar. The new selections had a much wider range in most parameters assessed, suggesting that there is most likely a larger potential to determine their suitability for wet and hot climatic conditions with respect to BST and bunch structure.