Agriculture Reference
In-Depth Information
Either too much or too little water can induce plant stress. For instance, a typical
disorder of tomato fruits is blossom end rot—usually occurring due to water deficit,
whereas cracking is due to an excess of water supply. According to Peet and Willits
(
1995
), the application of excess irrigation water to greenhouse tomatoes induced a
two-fold higher incidence of radical cracking in fruit compared to the recommend-
ed water regime. Photosynthesis and transpiration are negatively affected by water
and/or drought stress. Most of the water consumed by plants is used in the transpi-
ration process, as well as regulation of internal temperature. Plants react to water
fluctuations firstly by stomata regulations with stomata closing during the night
and opening at dawn (light-induced stomatal opening). Transpiration increases with
increasing temperature until midday and decreases significantly by cooling, due to
a gradual closing of stomata.
With inadequate water supply, or extreme heat and drought, the stomata close
much earlier with negative consequences for gas exchanges and CO
2
assimilation.
Because of the reduction in CO
2
assimilation in leaves, the metabolic processes
are impacted resulting in many of the integrated physiological and biochemical
processes that cause yield and quality reduced. The loss of turgor pressure in the
cells leads to wilting that initially manifests in a withering of leaves followed by
leaf necrosis and plant desiccation.
According to Bolla et al. (
2009
) greenhouse water shortages in roses can have
a negative effect on photosynthesis with a simultaneous reduction in the photosyn-
thetic rate and a significantly lower quantum yield of photosystem II, without any
limitation made on the intercellular CO
2
concentration levels. This, as well as the
increase in carbohydrate content (glucose, fructose and sucrose) and inorganic sol-
utes (potassium) of the stressed plants during the dry-down period indicate that the
plants are able to maintain their metabolic and physiological function. Apart from
the stomatal closure the ability to continue functioning also plays a role, by means
of turgor maintenance and osmotic adaptation.
Niu et al. (
2008
) also indicated that during dry-down, fluorescence measure-
ments indicated some damage in the photosystem II of four clones of oleander
plants (
Nerium oleander
). In addition, shoot dry weight was reduced, while root-to-
shoot dry weight ratio was increased; as substrate volumetric moisture content de-
creased from 30 %, leaf net photosynthetic rate, ET rate, and stomatal conductance
decreased in all clones.
Plants express a response to water stress by changes in their morphology such as
decreasing leaf area, in order to regulate water loss and prevent further dehydration
(Gruda and Schnitzler
2000a
), or by an adaption in their root system. According
to Kulkarni and Phalke (
2009
), under drought stressed conditions, plants would
increase their water uptake from deeper soil layers by restricting the horizontal pro-
liferation of lateral roots in the topsoil and allocating more resources to the growth
of primary roots. The plant can be considered a hydraulic system, connecting water
in the soil, or in the case of SCS the substrate or nutrient solution, with the water
vapor in the atmosphere (Taiz and Zeiger
2010
). In the literature, sometimes the
term “soil-plant-atmosphere continuum” is used to characterize the water pathway.
The water movement processes are explained using the water potential concept
