2.2.1.2 Air temperature and its impact on berry
temperature
Berry temperature is a function of radiative heat transfer and
air temperature. Both factors cannot be separated as the relationship between
temperature and absorbed light is linear. Direct sun exposure increases fruit
surface temperature by 12 to 15°C above air temperature on the berry's
sun-exposed side. Fruit surface temperature can therefore vary depending on the
bunch location in the canopy, as well as the level of solar exposure (Spayd et
al., 2002).
The berry temperature can also be affected by the bunch
compactness, berry size, wind velocity, and its color. Dark-colored berries
exposed to the sun and under low wind conditions can be up to 15°C above
air temperature, explaining why sunburn mainly occurs during the
veraison2 stage (Dry, 2009).
Sunburn is mainly observed when berry temperature is above
45°C due to a combination of low wind-velocity, high temperatures, and
high light causing radiative heat transfer (Schrader et al., 2009).
2 Ripening stage, when the berries start to change
colors.
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2.2.2 Solar radiation
Solar radiation plays an essential role in mechanisms such as
plant morphogenesis regulation and photosynthesis. It is one of the most
important environmental factors as it represents both a source of energy and
information that interact with the plant the most. Solar radiation is capable
to raise the temperature of a surface using the energy issued by the sun (Smart
and Sinclair, 1976). Consequently, solar radiation and temperature are closely
linked variables.
In the vineyard, the solar radiation received by the grapevine
can be declined in three components: the direct solar radiation, the diffuse
radiation, and the reflected radiation (Riou et al., 1989). The direct
radiation comes from the sun and is directly oriented towards the grapevine.
Part of this radiation is diffused by gazes and vapor in the atmosphere, while
another part can be reflected by the ground to create an albedo
(Gutiérrez-Jurado and Vivoni, 2013).
Figure 2: Types of solar radiations (Mallon et al.,
2017)
Solar radiation can be divided into three categories:
- Ultraviolets (UV): UV-A (400-315 nm) and UV-B (315-280 nm) -
Visible: 400-780 nm, including PAR3 (400-700 nm)
- Infrared Radiation (IR): > 780 nm (Gambetta et al., 2021)
According to an article on UV irradiance, the intensity of the
components of solar radiation depends on altitude, longitude, season, time of
the day and cloud coverage (McKenzie et al., 2003).
Light can act as both a source of heat as well as a driver of
photochemical and oxidative reactions for the berry. Photooxidation plays a
major role in the appearance of sunburn browning symptoms. In well-shaded
bunches in the field, neither sunburn necrosis nor sunburn browning can be
observed, meaning that solar radiation is the major actor in the apparition of
sunburn symptoms (Rustioni et al., 2014).
Scientifically speaking, light has an effect on berry sunburn
as it promotes the production of chlorophyll and reactive oxygen species, both
promotors of oxidative stress in the photosystems of the plant and the fruit.
Sunburn development is mainly caused by two components of light: PAR and UV.
When highly exposed to PAR, NPQ4 increases in order
to protect the photosystem of the plant. This process works temporarily, as
when the PAR overexposure continues, the NPQ can be photo-inhibited, causing
sunburn damages (Glenn and Yuri, 2013).
3 Photosynthetically Active Radiation, amount of light
available for photosynthesis, and therefore needed for plant growth (Fondriest,
2010).
4 Non-photochemical quenching (NPQ) is a process
that takes place in the photosynthetic membranes of plants and algae, in which
excess absorbed light energy is dissipated into heat. This mechanism is
employed by the plants to protect themselves from high light intensity (Ruban,
2016).
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UV being a high-energy form of radiation, it can collapse
membrane integrity, depending on the duration, dose, and wavelength of
exposure. As previously said, sunburn is mainly caused by the combination of
both light and temperature. However, in areas with relatively low average
temperatures such as New Zealand and Chile, grape and apple sunburn is mainly
due to their high UV index (Schrader et al., 2009).
Additionally, the interaction between PAR and UV can result in
greater changes in fruit composition than when separated. Even if PAR plays a
bigger role in the degradation of the berry's photosystems, this interaction
plays an important part in the apparition of sunburn damage (Glenn and Yuri,
2013).
Finally, no studies have yet reported a potential influence of
IR-radiation on the apparition and development of sunburn.
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