Agriculture Reference
In-Depth Information
Fig. 10.5
Sunscald symptom
on Bell Pepper, cultivated in
a glasshouse in the south of
Germany. A cellular death,
a collapse of the tissue and
papery thin skin are clearly
seen in fruits that were
directly exposed to solar
radiation and were not shaded
from leaves. (Source: Gruda
2003, private collection.)
Further disorders caused by high light intensity are uneven ripening, the oc-
currence of green shoulder, and blossom-end rot in tomato as well as cracking in
tomato and pepper fruits.
Measures to Mitigate the Adverse Influence of High Radiation Intensity
The most common methods to reduce incoming solar radiation include the white-
washing and the use of shade screens. Natural and forced ventilation systems, as
well as evaporative cooling devices, are often installed to remove excess heat due to
supra-optimal radiation in protected cropping systems. Effective crop transpiration
and active evaporative cooling in the form of fog and sprinkling systems, convert
plant sensible heat into latent heat. The preferred system depends very strongly on
outside climate conditions, greenhouse types and available facilities. In Mediterra-
nean countries whitewashing or shade screens, as well as evaporative cooling, can
be successful, whereas in hot humid areas an evaporative cooling system may not
be as efficient.
Shading is necessary to limit the temperature rise in the greenhouse. The actual
shading percentage of products such as traditional whitewash can be influenced by
different climate conditions, the type of the greenhouse construction, plant cultiva-
tions and the applied settings and can decrease during the year.
Villegas et al. (
2006
) reported a positive effect of shading when cyclamen (
Cy-
clamen
spp.) plants, were cultivated in the Mediterranean area under double shade
cloths with an accumulative 50 % of shading. These plants had better quality and
were more compact whereas plants under grey shade cloths at the same shading rate
had a higher number of flowers. The authors recommend treating the results with
caution when growing plants under cool and cloudy environments. For instance,
Marcelis (
1993
) reported that shading can affect cucumber weight by reducing the
distribution of photosynthate to the fruits, resulting in a strong decrease in fresh
and dry fruit weight. Young fruits are usually relatively more sensitive to a reduc-
tion in assimilate supply (irradiance) than older fruits on the same plant. Cockshull
et al. (
1992
), stated that 23 % shade was sufficient to reduce the yield of tomatoes
by 20 % in England. Consequently no general recommendations can be made here.
