2.2.3 Combination of high temperature and solar
radiation
Studies showed that grapevine sunburn that can be observed in
vineyard mostly results from the combination of high light and high
temperatures. When trying to induce sunburn in a greenhouse by exposing plants
to high light intensity but at low-moderate temperatures, little to no damage
was observed. However, when those temperatures were raised to 38°C,
sunburn damage on Semillon berries was observed at low light intensities and
was intensified at high light intensities (Hulands et al., 2014).
2.2.4 Wind and relative humidity
Fruits and leaves temperature is regulated by
evapotranspiration, thanks to the heat relieved by water vaporization. Under
windy conditions, berry transpiration is increased, and the sun-exposed berries
are cooled down by forced convection. Consequently, high wind velocities
diminish the appearance of grape sunburn.
When in contact with the plant, wind clears the air of
humidity produced by the plant's transpiration, forcing the plant to continue
to evaporate and cool down (Pereira et al., 1999). On a ripe berry, fruit
surface temperature is on average 5°C lower when wind velocity increases
from 0.5 to 2.0 m.s-1 (Smart and Sinclair, 1976).
Relative humidity also has an impact on grape sunburn as it
reduces the plant's evapotranspiration, which increases the risks of grape
sunburn (Gambetta et al., 2021).
2.3 Short term factors of grape sunburn
2.3.1 Vineyard soil management
Vineyard floor management can contribute to sunburn
development. Soil can increase the reflected radiation on grapevine
(2.2.2) depending on its characteristics. Indeed, bare and light-colored
soils reflect more light and heat than cultivated ones (Dry, 2009), resulting
in a potential increase in grape temperature.
Implementing cover crops in the vineyards can reduce the
reflected light but might also be a source of competition for water with vines.
Replacing light-colored soils or competitive cover crops by dark-colored
mulches can reduce both light reflection and water competition (Dry, 2009).
The soil composition also is an important factor as it plays a
central role for vine physiology. It conditions the water status for the plant
and contains mineral elements that are essential for plant growth. For example,
the soil's nitrogen status highly impacts plant vigor proven to impact sunburn
sensitivity (Chone et al., 2001) (2.3.3).
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2.3.2 Cultivar choice
The grape variety and the rootstock choice are essential for
the adaptation of the cultivar to the terroir. Grapevine cultivars express
different abilities to tolerate light and heat stresses. Rustioni et al. (2015)
conducted a study on 20 cultivars underlining the central role of radiation in
berry sunburn injuries. This study put into light that the response of
different cultivars to thermos-radiative stresses varies. The cultivars were
then classified based on their radiation susceptibility.
The rootstock choice as well as the cultivar choice can
modulate the plant's vigor, affecting the canopy's porosity. When the canopy
has a higher porosity, the shading is lighter, increasing the bunches'
exposition causing sunburn (Southey and Jooste, 1991).
The grapevine cultivar can also influence the plant's
sensitivity to sunburn. To start with, anthocyanin-containing fruits reach
higher temperatures than lacking anthocyanins due to a lower capacity to
reflect radiation (Smart and Sinclair, 1976), explaining why red grape
varieties are often more susceptible to sunburn than white grape varieties.
The cultivar's sunburn susceptibility also depends on the
bunch morphology. A study put into light that the berries' size is positively
correlated to its temperature, as large berries reach higher temperatures, and
are therefore more exposed to sunburn than bunches with smaller berries (Smart
and Sinclair, 1976).
By implementing varieties with higher Huglin Indexes into the
vineyard, the sensitivity to grape sunburn should be reduced.
2.3.3 Plant vigor
Plant vigor can be defined as an observed increase in plant
height and density through time (Short and Woolfolk, 1956). Plant vigor impacts
grape berries' volume and foliage porosity. Higher vigor also leads to higher
vegetation height and canopy density, and therefore a higher degree of shading,
allowing to reduce the temperature of the bunch microclimate by limiting the
direct received radiation.
As a consequence, plant vigor also modifies grape berry
composition by reducing its radiation-induced polyphenol production and
increases its sunburn sensitivity (Smart, 1985).
Modifying plant vigor is difficult, nearly impossible.
However, in order to mimic canopy density and a higher degree of shading, a
solution could be shade netting.
2.3.4 Developmental stage
Grape berry sunburn sensitivity varies according to the
developmental stages. A study reported that the grape berry susceptibility
increases as the bunches develop. The grape berry is less susceptible to
sunburn at early developmental stages (Hulands et al., 2014). Sunburn damages
can be observed on pre-véraison grapes but at a very low intensity. The
highest damages can be observed later, during véraison (Hulands et al.,
2014).
In contrast, other studies showed that the berries are
supposed to be more susceptible to thermal stresses earlier in the season due
to a higher ratio of photoprotective pigments to chlorophylls in the grape
berries, that decreases during berry development (Düring and Davtyan,
2002).
However, during the early developmental stages, the plant's
photoprotective mechanisms are at their highest due to a high chloroplast
activity. This capacity decreases during the plant's development, causing
higher sunburn sensitivity (Joubert et al., 2016).
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2.3.5 Water status and its impact on sunburn
Soil water status plays an essential role in determining the
yield potential and quality of crops. Water status of grapevine in the field
can be evaluated by measuring the predawn leaf water potential (Williams and
Araujo, 2002).
By maintaining a balanced soil moisture in the root zone,
plants can optimize their transpiration throughout the day, increasing the
relative humidity of the bunch zone (Suat, 2019). Higher canopy transpiration
also reduces the fruit surface temperature (Cook et al., 1964), reducing
sunburn risk and contributing to sunburn protection.
Important hydric stress promotes the production of ROS by
plants. As mentioned before (2.2.1), ROS accumulation weakens the
berries and can lead to cell death. Drought stress can also lead to smaller
canopies due to reduced vigor, reducing the shading, increasing bunch exposure,
and consequently increasing potential sunburn damage (Gambetta et al.,
2021).
Implementing irrigation systems into the vineyard could be a
solution to reduce water stress.
2.3.6 Vineyard management practices and operations to
modulate the sunburn risk 2.3.6.1 Vineyard operations
Vineyard management operations can directly influence the
sunburn sensitivity of cultivars (Rustioni et al., 2015). The pruning system
determines for example the density of the canopy, which affects the degree of
interception of solar radiation by the bunches. In hot winegrowing regions
where grapes suffer from sunburn, minimal pruning systems can be employed to
offer shelter and protect them (Gambetta et al., 2021). Avoiding excessive
pruning and leaf stripping could reduce sunburn by avoiding sudden exposure of
the fruit to direct sunlight.
Trellis systems usually used in central Europe were designed
to intensify fruit exposure but can consequently increase sunburn damage
(Gambetta et al., 2021). However, other suitable alternative trellising systems
reducing direct radiation exist, such as: high-wire cordon, head-training,
pergola, Geneva double curtain, and closing Y-shaped trellis (Palliotti et al.,
2014).
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