98 I. M. Ahmed et al.
5.6.2 Yield
Many yield-determining physiological processes in plants respond to water stress.
Yield is a quantitative trait and many physiological processes are involved. For
water stress, severity, duration and timing of stress, as well as responses of plants
after stress removal, and interaction between stress and other factors are extreme-
ly important (Plaut 2003 ). For instance, water stress applied at preanthesis re-
duced time to anthesis, while at postanthesis it shortened the grain-filling period
in triticale genotypes (Estrada-Campuzano et al. 2008 ). In barley ( H. vulgare),
drought stress reduces grain yield by decreasing the number of tillers, spikes,
and grains per plant and individual grain weight. Postanthesis drought stress was
detrimental to grain yield regardless of the stress severity (Samarah 2005 ). In
maize, water stress reduced yield by delaying silking, thus increasing the anthe-
sis-to-silking interval. This trait was highly correlated with grain yield, specifi-
cally ear and kernel number per plant (Cattivelli et al. 2008 ). Following heading,
drought had little effect on the rate of kernel filling in wheat, but its duration (time
from fertilization to maturity) was shortened and dry weight reduced at maturity
(Wardlaw and Willenbrink 2000 ).
Crop growth in saline medium is severely affected at different stages of the
plant life cycle. It was suggested by Shannon et al. ( 1994 ), that overall plant
response depends upon the concentration of salts in the tissue, composition of
salts, the exposure time, and climatic conditions as well. The commonly ob-
served adverse effects of salinity on Brassica species include the reduction in
plant height, yield, as well as deterioration of the quality of the product (Kumar
1995 ). In barley and wheat, salinity stress lowered grain yield by reducing grain
number and individual grain size (Harris et al. 2010 ). The plasticity of grain num-
ber and stability of grain size was found in another study in response to salinity
(Sadras 2007 ). Ahmed et al (2013b) observed that the reduction in spike length
was noticeably less in Tibetan wild barley than cultivated barley treated with sin-
gle or combined stress of salinity and drought. Moreover, the 1000-grain yield
and the filled grains per spike measurements were correlated, which may explain
the yield loss in cultivated barley compared to Tibetan wild barley under com-
bined drought and salinity during the anthesis stage. The decline in yield decline
was possibly associated with the reduction in spikelet fertility and grain filling
(Ahmed et al. 2013b).
In summary, prevailing drought and salinity reduce the plant growth and
development, increase flower abscission, reduce grain size due to poor grain filling
which arises due to the reduction in the partitioning of photosynthetic assimilate,
and decrease carbohydrate metabolism.