The through either stomatal and/or non-stomatal components (Chaves et

The number of pods per plant, grains per
pod, and the weight of individual grains are the principal parameters in
quantifying grain yield in pulses. Salinity leads to reduction in flower
numbers and pollen production (Dhingra and Varghese, 1993), which subsequently
reduces pod numbers, grains per pod, and grain weight (Mamo et al., 1996). In
chickpea, 50-100 mM salt concentration resulted in reduced pollentube length,
grain numbers, and substantial decline in grain yield (Turner et al. 2013).
Similarly all yield-related traits were equally responsible for the
salinity-induced yield reduction in soya bean (Ghassemi-Golezani et al., 2009).
However in mung bean only fewer grain/pod trait contributed to reduction in
grain yield. (Ahmed, 2009). Salt stress also caused 80–100% yield loss in
mungbean, particularly during the rainy season due to salinity-induced
desiccation, flower shedding and pod shattering (Sehrawat et al.,
2015). The effect of salinity stress on growth of mung bean was investigated by
Saha et al. (2010). They concluded that salinity stress suppressed the early
growth of mung bean seedlings by 50%.

Salt stress affects the availability,
competitive uptake, and translocation of nutrients to aboveground plant parts.
Under salt stress, the presence of excessive concentrations of Na and Clions in
the root zone will cause imbalanced nutrition in legumes as these ions
interfere with other elements, including boron (B), zinc (Zn), calcium (Ca),
copper (Cu), magnesium (Mg), iron (Fe), nitrogen (N), phosphorus (P) and
potassium (K) (Dahiya and Singh, 1976; Doering et al., 1984; Yadav et al.,
1989; El Sayed, 2011). For example, the K+/Na+ ratio
decreased significantly in chickpea (Garg and Bhandari, 2016), faba bean (Ullah
et al., 1993) and mungbean (Nandwal et al., 2000), due to competition for
intercellular Na+ and K+ ion flux, resulting significant
yield reductions (Sekeroglu et al.,1999).

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Another important physiological response
induced by salinity is reduction in net photosynthesis in grain legumes (Flexas
et al., 2004; Chaves et al., 2009; Khan et al., 2015) which occurs through
either stomatal and/or non-stomatal components (Chaves et al., 2009; Khan et
al., 2015). Under salt stress, carbon fixation in legumes (C3 photosynthesis)
decreases due to a reduction in the availability of CO2 caused due to stomatal
limitations (Flexas et al., 2004). Non-stomatal factors include mesophyll
conductance to CO2 and oxidative damage to the photosynthetic apparatus.
Studies conducted in chickpea, revealed that decrease in photosynthetic
efficiency under salt stress is due to non stomatal factor: damage to
photosystem II (PS II).  Similarly in
mungbean, reduction in photosynthesis was due to a decline in levels of
photosynthetic pigments and damage to electron transport in PS II (Khan et al.,
2010).

Salt stress has diverse effects on the
quality and composition of grain legumes (Manchanda and Garg, 2008). For
instance, salt stress substantially reduced grain protein content in chickpea, mungbean
and faba bean due to imbalance in N metabolism (Ghassemi-Golezani et al., 2010;
Qados, 2011). In contrast, salt stress reportedly increased protein content in
mashbean (Kapoor and Srivastava, 2010) and common beans (Qados,2011). In
soybean, salt stress severely decreased the protein yields per plant with
increasing salt concentrations compared with non-saline conditions, while the oil
percentage/ plant increased (Ghassemi-Golezani et al., 2009, 2010).

Salt stress interferes with biological fixation
and uptake of nitrogen (Frechilla et al., 2001; Rabie and Almadini, 2005),
thereby limiting nitrogen supply in grain legumes. Salinity significantly
reduced the density and activity of nodule formation in faba bean (Cordovilla
et al., 1994; Rabie and Almadini, 2005) and pigeon pea (Garg and Manchanda,
2008) as a result of premature senescence (Matamoros et al., 1999) thus
inhibiting biological N fixation in these grain legumesDrought
or water deficit hampers cell divison , expansion and differentiation . Loss of
turgor pressure and xylem water content as a result of water deficit are
primarily responsible for reduction in plant growth (Taiz and Zeiger,2006). Drought
stress has adverse effects on total biomass, pod number, seed number, seed
weight and quality, and seed yield/plant in chickpea(Toker & Yadav, 2010);
soyabean(Valentine et al., 2011), mung bean( Sherawat et al., 2015); pigeon pea
and pea (Kang et al., 2017).

Desmukh
and Mate (2013) evaluated 11 genotypes of pigeon pea grown in rain shelter
under drought conditions. They reported that lower drought susceptibility index
(DSI), increased dry matter biomass and harvest index(HI) contributed towards
drought tolerance as observed in the drought tolerant cultivar (JSA-59)  In
a study on pea, drought stress impaired the rate of germination and early
seedling growth of five cultivars tested (Okcu et al., 2005). Water deficit
stress in chickpea, field pea (Pisum sativum L.), faba bean (Vicia
faba L.) and lentil, and 35–400C resulted 20–70% yield
reductions(Abid et al., 2017; Kumar et al., 2016)  ,whereas terminal drought stress in chick pea
reduced the seed yield by 60%. Similar studies have been carried out among the
major agro economic pulses.  A
comparative analysis in grain legumes differentiated faba bean and pea as
drought-sensitive from lentil and chickpea (drought-resistant) (Toker &
Yadav, 2010). In another yield related analysis in legumes (Daryanto et al.,
2015) it was observed that lentil(21.7% ) and groundnut (28.6% ) exhibited the
lowest rate yield reduction while faba bean reported  highest yield reduction (40%) when subjected
to  water reduction (i.e., >65%). When
the same legumes were subjected to moderate water reduction (i.e., 60–65%), the
yield reduction was as follows: pigeon pea (21.8%),  soybean (28.0%), chickpeas (40.4%), cowpeas (44.3%)
and common beans (60.8%).

 

The physiological response to drought
depends on its severity and duration of exposure. Apart from reducing crop
growth and grain yield, it reduces biological nitrogen fixation capacity in
terms of uptake and assimilation by the grain legumes due to reduction in
leghaemoglobin in nodules and number of nodule under severe water deficit
conditions. Other important physiological traits includes rate of CO2
assimilation, transpiration efficiency and stomatal conductance. All these
traits contribute towards drought avoidance as well as drought tolerance

The symbiotic nitrogen fixation (SNF)
rate drastically declines in legumes as ureides accumulated heavily in nodules
and shoots (Vadez et al., 2000; Charlson et al., 2009) and also decrease in
shoot nitrogen demand. The decrease in SNF under drought conditions contributed
to reduction of photosynthesis rate in legumes (Ladrera et al., 2007; Valentine
et al. 2011). Pulses (common bean and cowpea) under water deficit maintain
their leaf water content and avoid tissue dehydration by regulating their
stomatal conductance, leaf abscission and stomatal closure (Pinheiro et al.,
2001; Choudhary et al., 2014). This subsequently leads to decrease in internal
CO2 concentrations, thereby limiting photosynthesis and shoot
growth.

Relative water content, leaf water
potential, stomatal resistance, rate of transpiration, leaf temperature and
canopy temperature are set of important physiological parameters that helps in
quantifying plant water relations under drought (Choudhary et al.,2017).  Exposure to drought stress alters the water
status of the crop plants and is characterized by steady decline in stomatal
conductance as well as transpiration efficiency (Ribas-Carbo et al., 2005). Leaf water potential and water use
efficiency (WUE) are therefore important physiological responses that
contribute to drought avoidance strategy. WUE is measured either with respect
to the whole plant or the leaf area. In case of the whole plant across the
complete growth season, WUE is measured as a ratio of harvested yield to plant
transpiration rate (Chaves and Oliveira, 2004. In relation to plant leaf, the
ratio between net rate of CO2 assimilation and transpiration is
reported as the WUE value. The WUE value reflects on the plant photosynthetic
activity. Kashiwagi et al(2006) evaluated the WUE status of  drought stressed chickpea cultivars using the
carbon isotope discrimination technique(CID) and reported that lowered WUE
significantly reduced grain formation during reproductive growth phase.  

Drought
leads to increased ethylene production in roots due to oxidation of
1-amino-cyclopropane 1-caboxylate (ACC) oxidase. Increased ACC oxidase
accumulation is responsible for decreased root nodulation and root biomass
(Glick et al., 2007).  Biofertilizers
composed of plant growth-promoting rhizobacteria (PGPR ) such as Bacillus subtilis, Pseudomonas stutzeri  etc
effectively solve this problem. These microbes hydrolyses the accumulated ACC
into ?-ketobutyrate  and ammonia., which
can then be used as carbon and nitrogen sources as observed in  drought stressed chick pea( Swarnalakshmi et
al., 2016).  In pea plants, the fungi arbuscular
mycorrhizal improved the WUE by 11%–24% under drought (Kumar et al., 2016).

 The plant water status is also dependent
on the stomatal conductance. It is calculated using the stomatal size, and
average size of stomatal opening. A lower stomatal conductance leads to
decrease in water loss thereby improving plant biomass (Lawlor and Tezara,
2009a)  as observed in common bean (Miyashita
et al., 2005) . Several pulses (chickpea, cowpea, common
bean, pigeon pea) maintain cellular water content and turgor by lowering stomatal
conductance while in other beans (pea, faba bean, mung bean) it is achieved by
lowering the osmotic potential in response to drought(Amede et al., 2003).Abiotic stresses are responsible for
extensive loss in crop production worldwide. Enhancing salt and drought
tolerance is of prime importance for plant breeders in order to strike a
balance between crop production and population growth. Salinity and drought are
the major obstacles in the path of sustainable crop productivity. Legumes serve
as rich sources of protein and these crops have been earmarked to solve the
protein requirements of the future generation. Legumes are more susceptible to
salinity and drought than cereals and oilseeds. Both these stresses
significantly reduce growth, grain yield and quality, turgor, water potential,
photosynthesis, carbon fixation and stomatal conductance in the major pulses.
In the last few decades extensive studies have been conducted on legumes in
order to understand the physiological responses of these legumes under salinity
and drought. These physiological traits when coupled with superior crop, soil
fertility and pest management practices have the potential to enhance stress
tolerance and pulse productivity thereby mitigating the global food crisis.
Moreover with the advent of molecular biology and genomics, these physiological
traits or parameters can serve as markers for stress tolerant legumes, whose genotype
can be introduced in other plants or legumes thereby developing transgenic
varieties with enhanced stress tolerance.

Therefore, future studies should focus
on identifying potential salinity and drought responsive genes and their
expression pattern in legumes in all their developmental stages. Such studies
are essential for unraveling their stress tolerance mechanisms and
incorporating these insights to improve legume production and grain quality
under existing abiotic stresses.