5 Tolerance to Combined Stress of Drought and Salinity in Barley 105
secondary products. Generally, precursors of secondary metabolic pathways are the
products of the primary metabolism. To a large extent, secondary metabolites derive
from three biosynthetic routes, namely the phenyl propanoid, isoprenoid, and alka-
loid pathways. The major source of aromatic secondary metabolites in plants is the
phenylpropanoid pathway (Irti and Faoro 2009 ).
Elevated phenol and flavonoid content were observed under single and combined
stresses in the two Tibetan wild genotypes (Ahmed et al. 2014 ). In salt stressed
H. vulgare, significantly higher concentration of flavonoids was observed (Ali and
Abbas 2003 ). The content of protochatechuic acid, caffeic, and chlorogenic acids
was increased following drought stress in Matricaria chamomilla (Kováčik et al.
2009 ). Ahmed et al. (2013c) also observed that the increase of phenolic compounds
in the tissue prevented the formation of ROS in Tibetan wild and cultivated bar-
ley under combined drought and salinity stresses. In addition, the induced expres-
sion of genes related to secondary metabolism ( GST, PPO, SKDH, PAL, CAD, and
chi2) was demonstrated under all stress conditions in wild barley and accompanied
an increase in the activities of the respective enzymes, with the greatest increase
observed in XZ5. During rehydration and recovery, the activities of all enzymes
increased except for phenylalanine ammonialyase (PAL) and cinnamyl alcohol
dehydrogenase (CAD), which increased only in XZ5 (Ahmed et al. 2014 ).
5.7.9 Ultra-Morphology of Plants
Drought and salt stress leads to disintegration of fine structure of chloroplast, insta-
bility of the pigment protein complexes, destruction of chlorophylls, and changes in
the quantity and composition of carotenoids (Dubey 1997 ). A wide array of varia-
tion has been observed in many studies regarding the effects of salinity stress on
chloroplast ultrastructure like swelling of thylakoid membranes of chloroplast in
the mesophyll cells of sweet potato leaves (Mitsuya et al. 2000 ) and also reduced
numbers and depth of the grana stacks, and enlargement of starch grains in the
chloroplasts of potato (Bruns and Hecht-Buchholz 1990 ). Hernández et al (1995)
observed disorganized thylakoid structure of the chloroplasts, increased number
and size of plastoglobuli, and decreased starch content in chloroplasts of plants ex-
posed to drought and salinity stress. Whereas, chloroplasts aggregation, distortion
of cell membranes with no signs of grana or thylakoid in chloroplasts were observed
in tomato plants exposed to salt stress (Khavari-Nejad and Mostofi 1998 ). Eleva-
tion in the level of NaCl increased swelling of thylakoids and reduced chlorophyll
fluorescence in barley seedlings (Zahra et al. 2014 ). Chloroplasts and mitochondria
were affected in a variety-specific manner under all adverse treatments. The organ-
elles of the drought-tolerant wheat cultivar Katya were better preserved than those
in the sensitive variety Sadovo. Leaf ultrastructure can be considered as one of the
important characteristics in the evaluation of the drought susceptibility of different
wheat varieties (Grigorova et al. 2012 ). The effect of drought and salinity alone and
in combination on endosperm starch and protein composition varied with genotypes
and treatments. Under drought stress, the endosperm of CM72 grains had smaller