4. Adaptation strategies to climate change
Climate change can affect the growing conditions of grapevine,
and therefore impact the grape yield and quality. In order to overcome those
effects, some adaptation strategies can be implemented at the scale of the
vineyard. Those solutions can be either short-term or long-term oriented, based
on their effectiveness. This part will present different adaptation solutions
against climate change.
4.1 Long-term solutions
Some solutions are rather long-term oriented, since grapevine
is a perennial plant and cannot be replanted easily and quickly.
4.1.1 Row orientation
As seen earlier in this report (2.3.6.2), row
orientation is an important driver of grapevine sunburn. Reorienting the
vineyard to avoid grape sunburn can be efficient in terms of yield losses
reduction. Based on a conducted study at Château Margaux (Porte, 2020),
the best orientation to avoid grape sunburn is the North-East/South-West
orientation.
Consequently, changing the row orientation of every parcel
would be an ideal solution to diminish grape sunburn. However, this solution is
long-term oriented and hardly applicable to perennial plants such as grapevine.
Moreover, not every parcel in the vineyard can be reoriented because of
practical and geographical reasons, forcing Château Margaux to find both
short-term and long-term solutions.
4.1.2 Grape variety
Château Margaux historically implants specific grape
varieties in their vineyard, as they produce a certain wine typicity when
implemented on their terroir. However, some grape varieties have the capacity
to resist to higher temperatures than others, making them more adapted to
certain types of climates (2.3.2).
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As we can expect that climate change will cause a raise in
temperatures, implementing new temperature-resistant grape varieties into the
vineyard could represent a potential solution against grape sunburn. However,
by doing so, it would affect the wine profile and quality, and therefore change
the typicity of the renowned Château Margaux.
4.2 Short-term solutions against grape
sunburn
Some solutions are short-term oriented due to the fact that
they aren't considered as answers to grape sunburn, but as preventive ways to
reduce yield and quality losses.
4.2.1 Irrigation as a response to climate
change
Climate change could impact the water supply for grapevine
production. Irrigation based on weather forecasts and water potential measures
could be used to meet grapevine's water requirements. If the plant's water
needs are met before important heat waves, it could avoid or reduce grape
sunburn (Lal and Sahu, 2017).
This solution is already used in other wine regions in the
world, where temperatures during the growing season are higher, such as in New
Zealand or in the United States of America. However, irrigation isn't yet
authorized by the PDO registry, and therefore cannot be implemented at the
scale of the vineyard.
4.2.2 Shade netting to reduce sun exposure
Shade netting consists of applying a net at the scale of the
vineyard to provide protection from high solar radiation. This solution seems
ideal to reduce the radiation received directly by the berries and can also
consequently reduce their temperature. Direct sunlight being the primary cause
of sunburn (Lal and Sahu, 2017), shade netting could effectively and
significantly reduce the damages linked with sunburn.
However, just like irrigation, shade netting isn't authorized
by the PDO registry, and cannot be implemented in the vineyard yet.
4.2.3 Kaolin: a preventive solution against different
radiative and thermic stresses During summer days, Vitis vinifera
photosynthetic activity decreases due to stomatal and non-stomatal limitations
(Chaves et al., 1987). In order to mitigate those effects caused by extreme
temperatures and high irradiance, organic compounds can be applied to the
canopy to increase grapevine physiology, productivity and quality (Ou et al.,
2010).
When maturing, the Cabernet Sauvignon grape berries change
color. As a consequence, they reflect less light and absorb more, resulting in
the raise of temperature of the berry and leading to sunburn vulnerability. To
reduce the part of absorbed radiation by the berry, some studies suggest that
we can apply a kaolin-based particle film to the canopy.
Kaolin is a white clay made out of aluminum phyllosilicate
that, when mixed in water and adjuvant and sprayed on the canopy, is capable of
leaving a thin white layer on the leaves and fruits allowing a higher light
reflectance capacity (Yazici and Kaynak, 2009). Kaolin increases the
reflection of incident radiation on leaves and bunches, and consequently lowers
their temperatures, reducing the plant's thermic stress (Brillante et al.,
2016).
A study evaluated that kaolin treated plants reflect between
26 to 155% more UV and PAR throughout the growing season of the grapevine than
the non-kaolin-treated plants (Lobos et al., 2015). As UV and PAR are the main
two components of light that cause sunburn development (2.2.2), their
reflection should lower the plant's sensitivity to sunburn. Thanks to this
reflective action, kaolin treatment can improve the plant's water conditions,
and diminishes its hydric stress (Glenn and Puterka, 2010; Glenn et al.,
2010).
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Moreover, another study demonstrated that kaolin spraying on
leaves can increase the photosynthetic activity of the berries growing under
low light conditions inside the canopy due to higher reflection of PAR in the
inner zones (Garrido et al., 2019).
4.2.4 Early leaf defoliation to increase grape berries
sun resistance
The grape berries are partially protected from light and heat
stress thanks to their epicuticular waxes. The waxes protect the berries from
PAR and UV radiation by reflection, absorption and scattering mechanisms,
allowing to reduce the exposure levels in the tissues of the berry. However,
the epicuticular wax layer of the berries' capacity to scatter light depends on
parameters such as the berry's size and the wax crystals distribution and
orientation. There are two types of wax crystals: plate-like and amorphous. The
plate-like wax crystals have a tendency to scatter a higher proportion of light
than amorphous waxes (Jenks and Ashworth, 2010).
According to a study, plate-like wax crystals prevail in
light-exposed grape berries, while amorphous wax crystals prevail in grape
berries grown in the shade of the canopy (Muganu et al., 2011). Sunburn causes
the degradation of the plate-like wax crystalline structure into amorphous
masses, leading to higher levels of dehydration and water permeability of the
berry (Greer et al., 2006; Bondada and Keller, 2012).
Other studies evaluated that sun-exposed berries possess
thicker cell walls and an epicuticular wax layer than the shaded ones, thanks
to their higher capacity to reflect light (Muganu et al., 2011; Rosenquist and
Morrison, 1989; Jenks and Ashworth, 1999; Verdenal et al., 2019), proving that
early berry sun-exposure can be beneficial against sunburn.
Accordingly to the fact that sun-exposed berries retain more
plate-like wax crystals, we can conclude that exposing berries to sun under
normal conditions leads to a higher production of plate-like wax crystals, and
therefore protects the berries from sunburn by scattering a higher proportion
of light.
Besides, the exposure of grape berries to sunlight increases
their levels of sugars, anthocyanins and phenolics. Early leaf removal on the
canopy allows higher exposure of the berries, and therefore induces their skin
thickening, providing more epidermal layers destined to the storage of
anthocyanin compounds for protection against sunburn. The increase in sunlight
exposure of berries on early defoliated vines also induces the expression of
cell wall metabolism, resulting in an increase in berry skin thickness (Pastore
et al., 2013).
Consequently, as a response to light exposure, the cuticle of
the exposed berries synthetizes and accumulates higher levels of polyphenols.
Moreover, light-exposed berries possess a thicker epidermis than shaded
berries, due to their increased polyphenols accumulation (Pastore et al., 2013;
Solovchenko, 2010), providing protection for the berry against sunburn.
In order to expose the berries to sun rays progressively, we
can perform early defoliation on the canopy at a moderate intensity, and on the
morning sun-exposed side of the canopy, so that the sun rays remain
moderate.
All the presented solutions aim to reduce the effects of
climate change. However, two of them are focused on reducing the damages linked
with grape sunburn: kaolin and early defoliation. As we decided to focus on the
sunburn issue in this report, only those two solutions will be explored.
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