Biotechnology approaches to overcome biotic and abiotic stress constraints in legumes - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 251

15.3.2.2 Drought and waterlogging
Fluctuation in water levels in agricultural land is a major
source of reduced legume yields. Drought alone does
not present a major challenge to the plants. However, a
combination of drought with other physical constraints
like heat and salinity, prove to be a lethal combination
for plant growth and development. For instance,
drought in combination with high temperature results
in a decrease in stomatal opening, making the overall
stress-resistance mechanisms of the plant less efficient
(Reynolds-Henne et al., 2010). This process results in the
physiological stress and, ultimately, death of the plant.
Soya, alfalfa, Medicago spp. and a number of other
legume species tend to show the greatest negative
growth patterns in response to drought conditions.
At the other extreme, waterlogging results in
significant physiological impact and yield loss (Palta et al.,
2010). Moreover, waterlogging promotes the growth of
weeds and grasses in cultivable land, resulting in an
increase in biotic stress for the already strained legumes
(Rodenburg et al., 2011). One important detriment of
waterlogging is the decrease in the soil population of rhi-
zobia, resulting in diminished nitrogen fixation capability
of the plants (Lindström & Mousavi, 2010). Soybean,
mungbean and grain legumes are the most important
legumes affected by waterlogging.

the unavailability of appropriate nutrients or nutrient-
depleted edaphic conditions result in lack of essential
nutrients, resulting in poor growth.

15.3.2.4 Natural disasters and other
abiotic constraints
The most significant of the natural disasters that affect the
normal growth of legumes are tornadoes, wild fires, floods
and high-speed winds (Pagano, 2013). These elements
cause widespread destruction of agricultural lands. In
addition, other eco-physiological agents adversely affecting
legume output include variations in atmospheric carbon
dioxide and the amount, duration and intensity of light
(Ross et al., 2013; Wang et al., 2012). All these factors
affect the nitrogen fixation capability, photosynthetic
apparatus and anatomical features of the plants.
Biotic and abiotic factors are, hence, responsible for
causing significant losses to the growth and yield of
leguminous plants. The plants adapt a number of cellular
and molecular pathways to neutralize the effects of
these stress factors. Utilizing these mechanisms to obtain
economic benefits can help in decreasing the overall
disease burden on the agricultural system.

15.4 host defence mechanisms

15.3.2.3 Salt stress and nutrient depletion
Salinity, acidity and heavy metal stress belong to a group
of abiotic stresses that affect plant growth by altering the
soil characteristics, ultimately resulting in inappropriate
growth conditions. As with other abiotic factors, the
detrimental effects of these constraints are enhanced
when occurring in combination with other factors. The
most significant of these synergistic abiotic constraints
are extremes in temperature. Heat, for instance, results
in increased transpiration resulting in enhanced uptake
of water loaded with salt, acid or heavy metal (Hamidou
et al., 2013). Salt stress, in particular, has a significant
effect on a number of physiological properties of
legumes, including nodulation, symbiotic nitrogen fixa-
tion and plant growth capability (López-Gómez et al.,
2013). Among the legumes, various varieties of peas,
beans, lentils and forage legumes have been extensively
studied for their responses in salt-stressed conditions.
Similarly, acidic and metal-stressed soil results in inade-
quate growth conditions for many legumes. Conversely,

Legumes, like other plants, resist changes to their
normal living conditions. Plants have evolved a number
of mechanisms to defend themselves against harsh
environmental conditions and various biological threats.
Production of certain chemicals, regulation of normal
physiological processes and modification of anatomical
features are some of the chief strategies in this respect.
Underlying all types of responses is modification of the
genetic and molecular assemblage (Figure 15.2).
Legumes may present a number of physiological
changes on exposure to an abiotic stress. In response to
drought, for instance, a leguminous plant shows a
variety of different protective mechanisms. These
include changes in membrane composition and loss of
turgidity, which help in the osmoprotection of the plant.
Allied modifications include changes in anatomical
characteristics like root thickness and soil penetrating
capability of roots, alterations in photosynthesis and
carbohydrate metabolism, and increased accumulation
of organic acids and osmolytes. Similarly, in response to