Joanna M. Gambetta - School of Agricultural and Wine Sciences, National Wine and Grape Industry Centre, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2678, Australia. | South Australian Research and Development Institute, Waite Research Precinct, Urrbrae, SA, 5064, Australia
Leigh M. Schmidtke - School of Agricultural and Wine Sciences, National Wine and Grape Industry Centre, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2678, Australia
Bruno Holzapfel - School of Agricultural and Wine Sciences, National Wine and Grape Industry Centre, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2678, Australia. | New South Wales Department of Primary Industries, Wagga Wagga, NSW 2650, Australia
Email contact: joanna.gambetta[@]adelaide.edu.au
Sunburn is a physiological disorder that leads to yield and quality losses in a range of fruits such as grapes and apples. It affects the visual appearance and the composition of the fruit, leading to irreversible changes and ultimately, cell death in extreme situations. Sunburn is caused by a combination of excessive radiation and temperature that lead to photo- and thermal stress, the formation of reactive oxygen species and oxidative stress. A series of factors, such as environmental conditions, grape variety and development stage modulate the final amount of damage. In turn, berries have evolved a series of mechanisms to protect themselves that are dependent on developmental stage1. Secondary metabolites such as the carotenoids, polyphenols and even the aroma compounds can act as antioxidants and light screens2–4, however, the ability to upregulate their production depends on ripening stage5. This project aimed to evaluate the changes in secondary metabolism generated by varying degrees of sunburn damage in Chardonnay grapes, and how exposure of berries through defoliation at two different stages in development could modulate sunburn intensity.
Field trials were conducted in two vineyards in the Orange region (NSW, Australia) during 2019. Treatments consisted of vines defoliated after the end of flowering, at véraison, and a non-defoliated control. Basic chemistry, carotenoids, polyphenols and free volatile analysis were conducted. Canopy mesoclimate, ultraviolet and photosynthetically active radiation, as well as berry temperature and radiation were monitored throughout the season using a range of sensors and light sensitive tapes.
Sunburn damage was modulated by the specific meteorological conditions at each vineyard, and was higher at the warmest vineyard. Changes in grape composition were statistically significant between the different levels of sunburn damage, all of which could be clearly separated using MBSOPLSLDA analysis. Among four different levels of damage studied, undamaged berries were the most distinct category and contained the highest levels of terpenes and lowest levels of polyphenols. As sunburn damage increased, an upregulation of compounds from the xanthophyll cycle was observed as well as of the flavonoids and flavan-3-ols, while a distinct destruction of chlorophyll a and b was also evident. Changes to concentrations of terpenes seemed to be mainly affected by temperature than radiation, and changes to specific aroma compounds such as the GLVs are reported for the first time. Comparison of defoliation treatments revealed that late defoliation led to a higher level and intensity of sunburn damage. Distinct biosynthetic mechanisms were apparent with regards to defoliation timing.
1. Gambetta, J. M., Holzapfel, B. P., Stoll, M. & Friedel, M. Sunburn in grapes: A review. Frontiers in Plant Science (2020).
2. Joubert, C., Young, P. R., Eyéghé-Bickong, H. A. & Vivier, M. A. Field-grown grapevine berries use carotenoids and the associated xanthophyll cycles to acclimate to uv exposure differentially in high and low light (shade) conditions. Front. Plant Sci. 7, (2016).
3. Young, P. R. et al. Grapevine plasticity in response to an altered microclimate: Sauvignon blanc modulates specific metabolites in response to increased berry exposure. Plant Physiol 170, 1235–1254 (2016).
4. Rustioni, L. et al. Oxidations in white grape (Vitis vinifera L.) skins: Comparison between ripening process and photooxidative sunburn symptoms. Plant Physiology and Biochemistry 150, 270–278 (2020).
5. Pastore, C. et al. Selective defoliation affects plant growth, fruit transcriptional ripening program and flavonoid metabolism in grapevine. BMC Plant Biol 13, 30 (2013).