Agriculture Reference
In-Depth Information
drought stress might therefore be significant. This will increase for crops with high
requirements on air humidity, such as cucumbers.
Interestingly, humidity is often neglected in protected cultivation, as long as dis-
eases and pests do not appear. There are two main reasons for that: firstly, high
humidity seldom causes any direct negative effect on plant growth and development.
Grange and Hand ( 1987 ) found that vapor pressure deficit (VPD, 0.2-1.0 kPa) had
almost no effect on the growth and development of horticultural crops. Secondly,
until one or two decades ago, optimization of the greenhouse environment has
been achieved traditionally by focusing on productivity, while product quality and
quality parameters only given prominence in recent research studies (Mortensen
2000 ; Gruda 2005 ). On the other hand, according to Holder and Cockshull ( 1990 )
VPDs smaller than 0.2 kPa can only be induced for extended periods in modern
glasshouses.
The management of humidity has two main purposes: maintaining crop tran-
spiration within boundaries and preventing condensation on the crop. Excessively
high or low rates of transpiration may result in local calcium deficiencies, loss of
turgor, partial stomatal closure and loss of assimilation. Condensation is known to
increase the incidence of disease causing organisms such as mildew and botrytis
grey mould (Köhl et al. 2007 ; Stanghellini and Kempkes 2008 ). Whereas a high RH
or respectively low VPD is successfully used for plant propagation and grafting.
Three potentially harmful effects of extreme humidity on plants, can occur with
heat damage is likely to occur because of the reduction of transpirational cooling,
increased injury by air pollutants due to changes in stomatal resistance, and reduc-
ing the translocation of some ions from roots to shoots due to reduced transpiration
rate under high humidity.
In the literature the information on the average weight of marketable fruits
and fruit size of tomato and sweet pepper plants is inconsistent and contradictory
(Gruda 2005 ). Mulholland et al. ( 2001 ), for instance, reported that fewer tomato
fruits growing under low VPD may be due an increased rate of flower abortion and
a reduction of pollen viability. In addition, the authors interestingly state that VPD
of 0.1 kPa can severely reduce the K concentration in young leaves compared with
standard air humidity.
Gruda ( 2005 ) has shown that controlling VPD could influence the excess or de-
ficiency of calcium content in the fruits or in some fruit parts and consequently the
occurrence of at least two related physiological disorders: “gold specks” and “blos-
som-end-rot” (BER). The physiological disorder known as “gold specks,” is a con-
sequence of an increased movement of calcium into the fruit and an accumulation
of an excess of calcium, deposited as calcium oxalate, in cells below the epidermis
(De Kreij et al. 1992 ; Adams 2002 ; Gruda 2005 ).
Different authors have shown that under conditions of low VPD, a reduction
in the incidence of BER in tomato and sweet pepper is achieved (De Kreij 1996 ;
Paiva et al. 1998 ; Li et al. 2002 ). High air humidity, especially during the night
when the stomata are normally closed, appears to prevent calcium deficiency in
lettuce (Collier and Tibbitts 1982 ). Cariglia and Stanghellini ( 2001 ), with Li et al.
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