The Molecular Basis of ABA-Mediated Plant Response to Drought - Abiotic Stress: Plant Responses and Applications in Agriculture - page 117

leaves, it allows a plant to recover faster following rehydration [145]. Photosynthetic re‐
covery following rehydration plays a pivotal role in drought-tolerance mechanisms and
prevents a dramatic decline in crop yields [146]. It was shown that recovery from a se‐
vere stress is a two-step process. The first phase occurs during the first hours or days af‐
ter rewatering and corresponds to an improvement of leaf water status and the
reopening of stomata [147]. The second stage lasts a few days and requires the de novo
synthesis of photosynthetic proteins [148-149].
It is also worth noting that other phenotype analyses should be performed in order to obtain
a complete picture of the stress response of a given plant. Relative Water Content (RWC),
which was proposed by Sinclair and Ludlow [12], is the most often used assay to assess
plant response to a water deficit. This simple test allows the establishment of relative water
content in a leaf of control and drought-treated plants. Detached leaves are weighed and sa‐
turated with water for 24 h, then again weighed and dried for 48 h and weighed again. RWC
is calculated from the following formula: RWC (%) = [(FM - DM)/(TM - DM)] * 100, where,
FM, DM, and TM are the fresh, dry and turgid masses of the tissue weighted, respectively.
The degree of cell membrane stability (CMS) is considered to be one of the best physiologi‐
cal indicators of drought-stress tolerance. It can be evaluated using measurements of solute
leakage from plant tissue [150-151].
In response to drought stress, plants are able to adjust osmotic pressure by synthesizing os‐
moprotectants such as proline, the water soluble carbohydrates that behave like a molecular
weapon against dehydration within the cell. There are several methods used in order to esti‐
mate the accumulation of endogenous proline or sugars in drought-treated plants [152].
Several morphological traits that have an impact on drought tolerance have been ob‐
served. Growth inhibition resulting from drought-induced ABA biosynthesis was ob‐
served in plants exposed to stress [153]. A number of studies have shown that wax
deposition on the leaf surface increased in response to drought and an associated im‐
provement in drought tolerance was observed in oat, rice, sorghum, wheat and barley
plants that had an increased wax layer [154 -157]. Enhanced drought tolerance was also
gained by plants having a reduced number of stomata, which was probably dependent
on the accumulation of waxes [158]. Yang et al [158] performed analysis on an ox -win1 /
shn1 (overexpressor wax inducer 1 / shine 1 ) mutant. WIN1 / SHN1 encodes a transcription
factor that regulates the expression of genes that control the accumulation of cuticular
wax. Analyses performed by Yang et al [158] showed that induction of WIN1 / SHN1 ex‐
pression by drought is correlated with an increased expression of the genes involved in
wax accumulation, and on the other hand, a decreased expression of the genes involved
in stomatal development. These results suggest that the drought-tolerant phenotype of
analyzed by Yang et al [158] forms caused by induction of WIN1 / SHN1 may be due to a
reduced number of stomata as well as wax accumulation.
There are now several high-throughput phenotyping techniques available for the measure‐
ment of some of the traits described above. One of these is thermal infrared imaging, or in‐
frared thermography (IRT), which is used to measure the leaf or canopy temperature.