Agriculture Reference
In-Depth Information
Generally, the root component accounts for 20 to 90% of the total resistance (reciprocal of
conductance) of the plant [38]. This variability largely reflects differences in the proportion of
roots, their anatomy and the depths at which they grow [39; 41]. The resistance to water
transport in roots is initially relatively high as water has to pass a complex anatomical structure
before reaching the conduits of the xylem [40; 42].
The importance of roots for plant water relations increases with the onset of drought for several
reasons. First, root growth is typically favored over leaf growth early on during drought, thus
growth of the organ exploiting the most limiting resource is favored [43]. Second, under more
severe conditions of drought, root layers may shrink or lateral roots may die from dehydration
causing deteriorated contact with soil particles holding water, thus increasing the resistance
of hydraulic water transport from soil to roots [44; 46]. Third, roots seem to be particularly
prone to suffer cavitation of conduits. In many species, including poplar [45; 47], willows and
walnut [212] roots are more vulnerable to xylem cavitation than shoots.
2.12. Root and shoot water content
Tissue water content may be expressed in several ways, including the amount of water per
unit dry or fresh weight and per unit weight of water at full hydration. Fresh weight seems to
be the less accurate of them to measure tissue water content because is highly influenced by
changes in tissue dry weight [213]. Sometimes decreases in tissue water content may be more
important than decreases in water potential or pressure potential in terms of influencing
growth.
The vast majority of land plants, including all major horticultural plants, would be classified
as drought avoiders. Although vascular plants do produce specialized structures capable of
withstanding severe stress (e.g. pollen, seeds and spores), few species can survive substantial
loss of water from their vegetative tissues [34 -36]. Tolerance is the ability to withstand a
particular environmental condition. Under water-limiting conditions, plants will experience
a net loss of water to the environment and cells will dehydrate (i.e. Ψw and relative water
contents, RWC, will decline). Land plants can be classified based upon how they respond to
this water deficit. Drought-avoiding plants strive to maintain elevated Ψw. Drought-tolerant
plants are able to tolerate extended periods of water deficit. However, both drought-avoiding
and drought-tolerant plants will reach a 'permanent wilting point' where Ψw has declined to
such a degree that the plant cannot recover upon rewatering.
Under stress condition, derangement in the leaf water potential and its components takes place
[31]. It is reported that the water relation and transpirational parameters are closely correlated,
and in the laboratory, where equipment to quantify plant water potential are not available,
determination of the RWC is still a valid parameter to quantify the plant water status [32-33].
RWC is a measure of the relative cellular volume that shows the changes in cellular volume
that could be affecting interactions between macromolecules and organelles. As a general rule,
a RWC about 90-100% is related to closing of the stomata pore in the leaf and a reduction in
the cellular expansion and growth. Contents of 80-90% are correlated with changes in the
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