Mitigating berry cracking through modulation of berry skin elasticity

PROJECT TITLE:    Mitigating Grape Berry Cracking through modulation of Berry Skin Elasticity

Project leader:        Justin Lashbrooke (Stellenbosch University)

Contact:                     jglash@sun.ac.za

Duration:                 1 January 2021 – 31 December 2023

Problem identification and project objectives:

Berry cracking is a major problem for grape growers worldwide, and for an export dominated industry, like that of South Africa, the problem is further amplified. Cracking typically occurs due to the physical failure of the waxy cuticle layer or skin of the grape berries. Cracking is an extremely complex phenomenon, and while it is linked to environmental conditions, such as fungal infection or heavy rainfall (especially in the harvest period), it is also under genetic control, evidence by the fact some cultivars are more susceptible to cracking than others. Currently growers must either plant appropriate cultivars for their region or apply various biological growth agents (such as cytokinin and gibberellin) to their crops to reduce cracking. However, both research and anecdotal evidence in grape (and other crops) has found that the application of these agents does not consistently improve resistance to fruit cracking. This is, at least in part, because the mode of action of these regulators is poorly understood. Indeed, while it was originally presumed that these agents acted through stimulation of fruit skin cell division, and increase in cuticle deposition, it has recently been discovered that this is not the case. The cuticle layer is composed of a matrix of fatty acid molecules embedded with wax crystals, and the relative abundance of the various molecules (in excess of one hundred types) has been found to impact the elasticity of the cuticle. The complexity of the cuticle, and its underlying genetics, contributes to the complexity of the berry cracking phenotype. Unfortunately, in grapevine our knowledge of the molecular composition of this layer remains limited. It is now assumed that cultivars which show differences in cracking potential, differ in their cuticle composition, while the application of growth agents stimulates a molecular change in the cuticle structure. This project aims to investigate these assumptions, generating data to support the role that diverse cultivar physiology (specifically cuticle composition) has on cracking susceptibility, and how growth agents may modify this phenomenon. This work therefore will provide a much more comprehensive understanding of the underlying causes of berry cracking, potentially leading to several future studies. While the data generated here would be of immediate relevance to the table grape industry in two major spheres. Firstly, knowledge of which of the chemical constituents (and indeed their relative abundance) of the cuticle layer leads to a more elastic berry skin can be used to identify and screen for crack resistant cultivars. While this knowledge can also be used directly in breeding programs as a measurable trait. Secondly, by identifying how plant growth regulators modulate cuticle molecular structure we can devise a more precise and targeted application protocol to reduce berry cracking in existing vineyards.