Agriculture Reference
In-Depth Information
Fig. 10.12
Cold shock of
actively growing young
cucumber plants due to cold
water (< 10 °C). (Source:
Technical University Munich,
Germany, 1999)
Furthermore, irrigation with cold water makes the plants more predisposed
to diseases, such as those caused by
Pythium
ssp., and
Rhizoctonia solani
(Jansen et al.
1989
).
Humidity Modifications Under Cover and their Influence
on Plant Growth and Product Quality of Horticultural
Plants
Humidity is an important environmental factor which influences the water status
of greenhouse vegetable plants and consequently affects all processes that are as-
sociated with transpiration such as the water balance, transpirational cooling and
ion translocation (Bakker
1984
). In the scientific literature, apart from
r
elative
H
u-
midity (rH) in percent, very often the term
V
apor
P
ressure
D
eficit (VPD) in kPa
(kilo-Pascal) is used to characterize greenhouse humidity. VPD is the difference
between the amount of water in the air of current air humidity and the amount by
saturation at the same temperature. There are some factors that influence the hu-
midity in greenhouses e.g. ET (i.e. soil evaporation plus plant transpiration) as well
as air exchange with the atmosphere, water condensation at the roof level, as well
as on plants. The air and crop temperatures also play a role in the control of VPD
level. For example, warm air can hold more water vapor and it is more difficult to be
saturated than cold air. High or low VPD will adversely influence the plant growth,
yield and product quality, depending on the availability of water in the root zone,
because in the end it's a question of plant water balance. For instance, in green-
houses with a high VPD, e.g. high outside temperatures in arid regions, the risk of
