Water Use and Drought Response in Cultivated and Wild Apples - Abiotic Stress: Plant Responses and Applications in Agriculture - page 252

water and 'normal' water and to differences in diffusivity with air. However, the relationship
between E and isotopic enrichment is complex. For example, variation in E can be caused by
changes in evaporative demand and/or changes in stomatal conductance, gs [31]. If the source
of variation is evaporative demand, then as E increases, 18 O enrichment increases. On the other
hand, if stomates are the source of variation, then as E decreases (stomates close), leaf water
enrichment increases. How does this relate to 18 O enrichment of plant organic matter? Plants
accumulate 18 O in their tissues as a result of the exchange of oxygen between water and
carbonyl oxygens in triose phosphates. An enrichment of about 27 parts per thousand (ppt)
has been observed in the organic material of several different plants relative to leaf water [32].
This suggests that differences in 18 O enrichment can be used to distinguish genotypes with
favorable yields and stomatal function. In fact Barbour et al. [33] recently demonstrated a
reliably negative correlation between yield and δ 18 O in wheat which was used to identify water
use efficient varieties for breeding.
5. Specific genes associated with WUE and/or drought responses in apple
and other plants
The recent advent of global gene expression methodology has spawned a number of studies
of abiotic stress responses, including drought, in several plant species [34-38]. In Arabidopsis,
a compilation study of microarray analyses on plants subjected to a variety of stress treatments
highlighted overlap among genes up-regulated in the early stages of all the stress responses
[39]. Studies on WD stress in cereals and dicots have cataloged a large number of genes up-
regulated during treatment [35-37, 40, 43]. Comparisons among these studies reveal that a
number of genes are reproducibly up-regulated in response to WD regardless of how the stress
was imposed or what plant system was involved, including apple (Table 1) [44].
6. Genes associated with wue
Recent reports of genes associated with regulation of transpiration demonstrate the complexity
of water use and transport, as well as the overlap in gene response to other stresses. ESKI‐
MO1 , which was originally associated with cold response, has recently been shown to affect
both drought and salt responses in Arabidopsis. Insertional mutation lines inactivating ESK1
had reduced transpiration rates and were only slightly less drought tolerant than wild type
[45]. Furthermore, there was a reduction in biomass when mutant plants were grown without
stress, suggesting that alterations in WUE were reducing both transpiration and CO 2 exchange;
biomass differences between WT and esk1 lines were negligible under stress. Another example
of pleotrophic gene effects on plant physiology was reported in a study by Masle et al. [46]. A
leucine-rich repeat receptor kinase ( ERECTA ) previously associated with inflorescence
development was also shown to regulate transpiration in different races of Arabidopsis. The
gene is implicated in epidermal cell expansion, cell-cell contact and mesophyll cell prolifera‐
tion, but its relationship to reduced transpiration may be linked to differences in stomatal