Water Use and Drought Response in Cultivated and Wild Apples - Abiotic Stress: Plant Responses and Applications in Agriculture - page 250

under water deficit) and/or 'drought tolerance' (capacity to sustain plant function in a
dehydrated state) [6,7]. Drought resistance in a genetic/physiological context refers to the
ability of one genotype to yield 'better' than another during severe drought stress. On the other
hand WUE is defined as the ratio between diffusion of CO 2 into the leaf (photosynthesis) and
loss of H 2 O through transpiration, indicated as WUE = A/E, where A is carbon assimilation
and E is transpiration. It is positively correlated with carbon isotope discrimination (Δ 13 C)
based on the stable carbon isotope ratio, δ 13 C ( 12 C/ 13 C relative to a standard, i.e. PeeDee
Belemnite), in the plant tissue relative to the atmospheric ratio and is calculated as: Δ 13 C = δ 13 C
in air - δ 13 C of the plant/1- δ 13 C of the plant. Since most gas exchange occurs via the stomata,
it is expected that guard cell function would be closely associated with WUE. Indeed, the size
and density of stomates correlates well with water use efficiency [8-10]. For drought resistance,
yield is not necessarily adversely affected by resistance, whereas for WUE reduced transpira‐
tion through stomatal closure is often accompanied by reduced yield potential through
reduced carbon assimilation, particularly in herbaceous C 3 plants (however, see below). Other
parameters, such as root depth, leaf size, and trichome size and density have also been linked
to water use efficiency [2], but they have also been linked to drought resistance as well [6].
Different methods have been used to measure drought resistance and WUE [11]. These
methods measure the energy status of water in plant tissues and the trans-port processes into
and out of the soil-plant-atmosphere continuum. In general, these methods isolate specific
plant tissues at instantaneous moments in time, whereas Δ 13 C represents a time-integrated
value of the ratio of C i (intercellular CO 2 concentration) to ambient CO 2 which, as previously
indicated, reflects the plant's capacity for gas exchange via stomata [12] and discrimination of
rubisco against 13 C. The use of carbon isotope discrimination to select in-dividuals with higher
WUE has been applied successfully to cereal breeding programs (13). The extent of δ 13 C varies
substantially among wheat genotypes, and heritability is high because genotype X environ‐
ment interactions are relatively low [14, 15]. Rebetzke et al. [16] reported on the selection of
plants with greater biomass, harvest index and kernel weight using results from contrasting
high and low Δ 13 C groups in combination with a backcrossing program. There were significant
correlations between Δ 13 C and yield and between Δ 13 C and biomass. The resulting high
yielding strain, 'Drysdale', produces around 10% more grain under drought conditions than
other dry-area wheat varieties.
3. Adaptation and the relationship of δ 13 c to yield
Adaptive changes in populations growing in different environments have been amply
demonstrated in a variety of plants [17]. Divergence among populations associated with
different environments provides the raw material for speciation and differentiation among
closely related species. Higher fitness of genotypes in their native environment compared to
genotypes transplanted from contrasting environments provides evidence of local adaptation
[9]. For example, when two populations of Boechera holboellii growing in xeric and wet
environments were grown in reciprocal transplant experiments, significantly higher survival
was observed with plants growing in their native habitat [18]. Furthermore, genes identified