Agriculture Reference
In-Depth Information
(negative pressure). The factors influencing water movement along this pathway
are water tension in the root zone, soil or substrate type, physiological plant status
and atmospheric water demand. Hence, greenhouse environmental conditions, such
as air temperature, radiation, air movement and air VPD will directly or indirectly
influence water movement. This sub-section only considers soil water and its effect
on protected crops.
In greenhouses, plants obtain water mainly from the soil. Thus, the regulation
of soil moisture is very important. Excessive amounts of water, due to an incorrect
irrigation can result in nutrient losses by leaching, particularly of nitrogen. More-
over, overwatering should be avoided, because of a likely oxygen deficiency and a
probable production of other adverse gases in the soil such as methane and/or car-
bon dioxide. In this case the roots may not successfully uptake water and nutrients.
The optimum water content in greenhouses is in the range of field capacity, and
sometimes higher, with substrate cultures with a high pore volume. The regulation
of soil moisture is also important for microbial transformation in the soil, for mo-
bilization of nutrients from the soil organic matter, and for solubility of nutrients.
For instance, adequate soil moisture is necessary to facilitate diffusion of potassium
(K
+
) to plant roots for uptake which according to Lester et al. (
2010
) accounts for
> 75 % of K
+
movement. Therefore, soil moisture deficits can limit soil K transport
as well as uptake into the plant, thereby causing K deficiency.
Simulated drought conditions are sometimes used as a tool to control plant habit
and generative development. As it was stated earlier, one of the quality require-
ments for ornamentals is to be compact plants and to possess a good number of buds
and blooms at sale time and where the stature of the plant benefits the consumer
and grower; namely, a high-quality product, and savings in production and trans-
portation costs, less water and space requirements. Different methods have been
developed to induce compact morphology. The inhibition of growth with chemical
agents is certainly one of these methods. Very limited licenses as well as an in-
creased consumer consciousness however have led to increasing pressure in devel-
oping alternative methods for growth inhibition. The use of techniques for lowering
temperatures and/or “cool morning”, and thigmomorphogenesis and restrictive wa-
ter management are some of these alternative methods. Röber et al. (
1986
) demon-
strated that decreasing moisture leads to a reduction of height, diameter, and leaf
area of ornamental plant species. Precisely performed induced sub-drought stress
can produce compact high quality plants, comparable to those treated with growth
inhibitors. Liptay et al. (
1997
) reported that carefully regulated, moderate stress can
slow down growth of vegetable transplants under certain circumstances without
influencing yield or product quality loss. Gruda and Schnitzler (
2000b
) have proved
that size differences in head lettuce transplants, induced by variable irrigation levels
and different organic substrates, were no longer detectable 3 weeks after transplan-
tation. In addition, moderate stress can reduce susceptibility to pathogens, such as
Pythium ultimum
(Schnitzler and Gruda
2002
). A modification of water availability
can also be achieved by adjusting the concentration of nutrient solution in soilless
culture and this is used by growers in order to improve the quality of some fruit
vegetables, such as tomato.
