The Molecular Basis of ABA-Mediated Plant Response to Drought - Abiotic Stress: Plant Responses and Applications in Agriculture - page 115

can occur at different periods of the crop cycle and with different intensities. Consequently,
plants have developed various strategies in response to drought: tolerance, escape and
avoidance. Ludlow [122] defined three strategies plants use to cope with drought stress:
drought tolerance is the ability of a plant to cope with water deficit through low tissue water
potential, drought escape is defined as completion of the life cycle just before a severe
drought starts, and drought avoidance is plant maintenance of high tissue water potential
by minimizing water loss or maximizing water uptake. The final mechanism conveys the
ability to survive and recover rapidly after a severe stress through protective mechanisms,
such as cell wall folding, membrane protection, and the accumulation of antioxidants
[123-124].
In order to incorporate traits that confer drought tolerance into molecular breeding pro‐
grams, phenotyping protocols are extremely important [125]. With the wide availability
of genetic resources, such as mutant populations (TILLING) or mapping populations,
high-throughput phenotyping will become an essential asset in closing the gap between
plant physiology and genetics [126- 127]. It is worth noting that a complex set of both
abiotic and biotic stresses shapes the natural environment during plant development
drought stress is just one of many factors. It is hard to exclude one of the stress path‐
ways and to analyze it in isolation from others because the cascade of stress response is
a complicated web of overlapping pathways. When studying drought tolerance in plants,
it is very difficult to control and monitor the level and onset of water deficit, since it is a
dynamic process and a combination of the available water in the soil and the plant wa‐
ter status. Continuous measurements are needed in order to link the level of drought ex‐
perienced by the plant with the physiological changes occurring in response to it [125].
Under greenhouse conditions, water use can be monitored by weighing the pots or us‐
ing TDR (Time Domain Reflectometry) soil moisture meters [128]. The water supply can
be regulated at high-throughput automated screening facilities by using the classical wa‐
ter withdrawal approach [14] and maintaining a constant soil water status [129].
Another difficult issue is how to describe plant response to drought at the physiological lev‐
el using properly chosen physiological, but also morphological, traits. In breeding programs
for improved drought tolerance, crop traits associated with the conceptual framework for
yield drought adaptation have been proposed by Passioura [130]. This framework has three
important drivers: (1) water uptake (WU), (2) water-use efficiency (WUE) and (3) harvest in‐
dex (HI). Several traits are highly associated with these three aspects of Passioura model.
With regard to WU, the best method would be direct selection for variation in root architec‐
ture but since this is hard to perform, stomatal conductance, mainly the canopy tempera‐
ture, is measured. This provides indirect indicators of water uptake by roots [131]. To
estimate WUE, carbon isotope discrimination is used. A high affinity of Rubisco for the
more common 12 C isotope over the 13 C indicates a lower WUE, whereas a lower discrimina‐
tion value indicates a higher WUE [131]. In the case of HI, the extreme sensitivity of repro‐
ductive processes to drought may result in reproductive failure, which is associated with a
low HI value [132].