5 Tolerance to Combined Stress of Drought and Salinity in Barley 101
5.7.4 Chlorophyll Fluorescence
Chlorophyll fluorescence analysis has proven to be a sensitive method for the de-
tection and quantification of changes induced in the photosynthetic apparatus. The
chlorophyll fluorescence is based on the measurement of fluorescence signal of
dark-adapted plants exposed to continuous light (Govindjee 1995 ). The dark-adapt-
ed samples show characteristic changes in the intensity of chlorophyll fluorescence
during the illumination by continuous lights and this effect is called fluorescence
induction of Kautsky’s effect. When barley plants are exposed to drought, the val-
ues of maximal quantum yield of PSII ( Fv/Fm) decrease, which is a reliable sign of
photoinhibition (Guo et al. 2009 ).
Salt stress leads to a decrease in the efficiency of photosynthesis and is known
to influence the chlorophyll content and chlorophyll a fluorescence of barley leaves
(Fedina et al. 2003 ). Chlorophyll a fluorescence parameters have been used to
study high salt-induced damage to PSII. By measuring 77 K fluorescence emis-
sion spectra in dark grown wheat leaves under high salt conditions, it was shown
that salt stress inhibits the chlorophyll accumulation by restraining several steps in
porphyrin formation (Abdelkader et al. 2008 ). Delayed fluorescence measurements
in Arabidopsis thaliana seedlings have also proved to be useful as a marker for
detecting damage caused by salt stress (Zhang et al. 2008 ). A significant decrease in
Fv/Fm by combined drought and salinity (D + S) suggested a possible inhibition of
PSII photochemistry, which could be due to insufficient energy transfer from light
harvesting chlorophyll complex to the reaction center. Compared with Tibetan wild
barley (XZ5), greater decrease in Fv/Fm in cultivated barley (CM72) indicated that
PSII of the latter was more sensitive to D + S, suggesting that a higher protective
capacity for PSII could be an important tolerance mechanism for barley genotypes
(Ahmed et al., 2013a).
5.7.5 Plant Nutrition
Decreasing water availability under drought generally results in limited total nutri-
ent uptake and their diminished tissue concentrations in crop plants. An important
effect of water deficit is on the acquisition of nutrients by the root and their transport
to shoots (Farooq et al. 2009 ). In general, moisture stress induces an increase in N,
a definitive decline in P and no definitive effects on K (Garg 2003 ). Influence of
drought on plant nutrition may also be related to limited availability of energy for
the assimilation of NO / NH , PO , and SO :3 44−+−−^324 they must be converted in energy-
dependent processes before these ions can be used for growth and development
of plants (Grossman and Takahashi 2001 ). As nutrient and water requirements are
closely related, fertilizer application is likely to increase the efficiency of crops
in utilizing available water. This indicates a significant interaction between soil
moisture deficits and nutrient acquisition. It was shown that N and K uptake was
hampered under drought stress in cotton (McWilliams 2003 ). Likewise, P and PO 43 −