Salt stress and leguminous crops: Present status and prospects - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 22

Salinity often leads to the development of other prob-
lems in soils such as soil sodicity and alkalinity. Soil
sodicity is the result of the binding of Na + to the nega-
tively charged clay particles, which leads to clay swelling
and dispersal. Hydrolysis of the Na-clay complex results
in soil alkalinity. Salt-affected lands occur in practically
all climatic regions, from the humid tropics to the polar
regions. Saline soils can be found at different altitudes,
from below sea level (e.g. around the Dead Sea) to
mountains rising above 5000 m, such as the Tibetan
Plateau or the Rocky Mountains. However, the saliniza-
tion process is more widespread in arid and semi-arid
regions. Over 800 million hectares of land throughout
the world is salt-affected, either by salinity (397 million
ha) or the associated condition of sodicity (434 million
ha) (FAO, 2005). This is over 6% of the world's total
land area. Out of 230 million ha of irrigated land, 45
million ha is affected by salinity or sodicity (20%) (FAO,
2005). Out of 1500 million ha of land farmed by dry-
land agriculture, 32 million ha (2%) is affected by
secondary salinity to varying degrees. Irrigated land has
at least twice the productivity of rain-fed land; it pro-
duces one-third of the world's food (Munns, 2005). This
problem is likely to become more severe in the future as
we face climate change at a global level.
Climate change is distressing ecosystems, freshwater
supplies and human health (Raman et al., 2012).
Emissions of heat-trapping GHGs such as CO 2 , methane
(CH 4 ) and nitrous oxides (NO x ) are responsible for
large increases in global temperatures, observed in the
second half of the 20th century, as reported by the
Intergovernmental Panel on Climate Change (IPCC,
2007). The higher the atmospheric concentrations of
GHGs, the more the earth will warm in the decades and
centuries ahead. Scientists have also observed that
during the 20th century, the mean global surface tem-
perature increased by 0.8 °C per year (IPCC, 2007). The
effects of global warming will not only be restricted to
increasing mean annual temperatures around the globe,
but also will have profound consequences on rainfall
patterns, natural ecosystems, agriculture, wildlife and
life in general (WWF, 2008). Because of climatic changes
and global warming, oceanic water is intruding into
coastal regions of arid and semi-arid regions and caus-
ing salinization (Kundzewicz et al., 2005). Thus,
understanding salt stress responses is essential in
attempts to breed crops with salt resistance to feed the
growing population.

Anticipated dehydration and increases in temperature
will cause high evapotranspiration. The drier regions of
the world may become even drier. Accordingly, it will
become very difficult for water-limited countries to face
these challenges. Surface water shortage will increase
pressure on use of groundwater, the major part of which
is not of a safe and usable quality. Increased soil and
water salinity/sodicity negatively affects the chemical
and physical characteristics of soil and subsequently
reduces the growth and yield of crops. Legumes are
the most salt sensitive group in this regard and are
anticipated to be significantly affected. The changing cir-
cumstances will also necessitate prudent decisions
around the selection of crops that can endure salinity
stress and addition of legumes in crop rotations.
Understanding of genetic variability with respect to salt
tolerance will be required. This will involve imple-
menting robust breeding programmes to achieve this
objective supported by modern approaches such as bio-
technology, mutagenesis and genetic engineering.
This chapter provides an overview of 'salt stress and
leguminous crops: present status and prospects' and
also briefly discusses the effects of salinity and responses
of plants to salinity. Efforts are also made to review the
lessons learned from studies of leguminous crops and
to highlight recent progress in understanding the
physiological, biochemical and molecular mechanisms
of salt stress responses and tolerance. Finally we sum-
marize research on 'omics' approaches to a better
understanding of salt stress responses and tolerance in
leguminous crops.

2.2 effects of salinity

Natural soil salinity predates human civilization. When
early humans, looking for better sources of livelihood,
moved into arid lands along the riverbanks, they
resorted to irrigated agriculture. With the practice of
irrigation came salinity, the first environmental problem
caused by humans. The earliest written account of
salt lands dates back to 2400 bc and was recorded in
the Tigris-Euphrates alluvial plains of Iraq. Salinity
destroyed the ancient societies that had thrived for
several centuries (Hillel, 2005; Rengasamy, 2006).
Sustained salinization of arable land is becoming more
widespread and thus decreasing crop yields from for-
merly productive soils throughout the world.