The use of short-term solutions against grape sunburn within a context of climate change in the Médoc vineyard

2.2 Medium to long term factors of grape sunburn

2.2.1 Temperature: the main cause of grape sunburn

Temperature can affect the plant and the fruit, as it is a major source of abiotic stress. Fruits possess an intracellular signaling mechanism that gets activated in response to heat (Gambetta et al., 2021). Thermal stress mainly targets the photosynthetic apparatus of the plant, causing its modification to adapt to heat. Those changes are usually reversible, unless the heat is excessive, in which case the photosystems can be severely and irreversibly damaged (Araújo et al., 2018).

When exposed to high temperatures, plants can be subject to an imbalance between their light energy absorption and usage. Consequently, the fruit's respiratory mechanisms are modified. Higher temperatures (> 30°C) cause higher levels of respiration that can result in the accumulation of Reactive Oxygen Species¹ (ROS) (Jiang et al., 2015).

Apart from altering the regulation of metabolic pathways, the accumulation of ROS can cause membrane destabilization, protein denaturation, and berry pericarp cell death. High temperatures can cause early cell death, and therefore sunburn (Bonada et al., 2013).

2.2.1.1 Temperature at different scales

A reminder on the different levels of climate can be found in Annex 1.

Temperature at the regional scale (macroclimate) is a major component of the vineyard (mesoclimate) and fruiting zone (microclimate) temperatures. In order to qualify the climates of viticultural regions and their ability to implant certain grape varieties, climate indices were created.

¹ An unstable molecule type involved in normal metabolism reactions containing two unpaired electrons from dioxygen that can easily react with other molecules in a cell. An excess generation of ROS in plants' cells can react with its DNA and gene expression, ultimately resulting in cell death (Bayr, 2005).

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The Winkler Index (WI) classifies regions based on the accumulation of heat summation units, by adding up temperatures above 10°C during the growing season. This index attributes growing degree-days (GDD) to regions during the growing season (Amerine and Winkler, 1944).

The Huglin Index (HI) uses the heliothermic potential, calculating the sum of the temperatures above 10°C from April to September (growing season). It varies from the WI as it takes a the latitude of the location in consideration as it affects the duration of the day, and is therefore more precise (Morata, 2018). The day length coefficient of Bordeaux is 1.04.

Viticultural climates were classified in six categories, based on the HI calculation, from very cool regions to very warm regions (Tonietto and Carbonneau, 2004).

Table 1: Viticultural climates classification based on the Huglin Index (Tonietto and Carbonneau, 2004; Liviu Mihai et al.,

2013)

Formula	Categories	Climate type	Grape varieties
	HI < 1500	Very cool	Muller-Thurgau, Pinot Blanc, Gamay
	1500 < HI < 1800	Cool	Riesling, Pinot noir, Chardonnay, Merlot, Cabernet Franc
T: maximum temperature Tx: average temperature d: day length coefficient
	1800 < HI < 2100	Temperate	Cabernet-Sauvignon, Ugni Blanc, Syrah
	2100 < HI < 2400	Temperate warm	Granche, Mourvèdre, Carignan
	2400 < HI < 3000	Warm	/
	3000 < HI	Very warm	/

According to the HI, Bordeaux's climate went from temperate between 1956 and 1986 (HI = 1814), to warm temperate between 1987 and 2017 (HI = 2125). Cabernet Sauvignon and Merlot, the principal grape varieties of Margaux, have an HI between 1500 and 2100, and were therefore more adapted to a cool to temperate climate, than to the current warm temperate climate (CNRS, 2020).

The fruiting zone temperature (microclimate) depends on the bunch exposure. Indeed, the unexposed berries microclimate tends to mimic the parcel's under shelter mesoclimate, and is close to the air temperature (Spayd et al., 2002). However, the fruiting zone temperature can be very variable in one vine stock as it is very precise. It can therefore be said that the microclimate of the fruiting zone temperature is primarily defined by the air temperature, then modulated by other factors such as solar radiation, wind or air humidity (Gambetta et al., 2021).