Chapter 1
Legumes and breeding under abiotic stress: An overview

Arafat Abdel Hamed Abdel Latef1 and Parvaiz Ahmad2

1 Department of Botany, Faculty of Science, South Valley University, Qena, Egypt

2 Department of Botany, S.P. College, Srinagar, Jammu and Kashmir, India

1.1 Introduction

The present world population of 7.2 billion is expected to reach 9.6 billion by the middle of the 21st century due to the high growth rate, particularly in developing countries. There is a need to produce about 70% more food to feed this excessive population (Varshney & Roorkiwal, 2013).

Legumes belong to the family Fabaceae/Leguminosae (with about 700 genera and 18,000 species). Legume crops can be divided into two groups according to their ability to grow in different seasons, namely cool season food legumes and warm or tropical season food legumes (Miller et al., 2002; Toker & Yadav, 2010). The cool season food legumes include broad bean (Vicia faba), lentil (Lens culinaris), lupins (Lupinus spp.), dry pea (Pisum sativum), chickpea (Cicer arietinum), grass pea (Lathyrus sativus) and common vetch (Vicia sativa) crops (FAOSTAT 2009; Andrews & Hodge, 2010). These are among the world's oldest cultivated plants (Materne et al., 2011). Dry pea, chickpea, broad bean and lentil are the four major cool season grain legume crops produced for human consumption. They are grown on all continents except Antarctica. Lupin species - e.g. Lupinus albus (white lupin) and Lupinus luteus (yellow lupin) - and vetches - in particular, common vetch - are important for animal feed (Andrews & Hodge, 2010). On the other hand, the warm season food legumes include pigeon pea (Cajanus cajan), cowpea (Vigna unguiculata), soybean (Glycine max L.), mung bean (Vigna radiata var. radiata) and urd bean (Vigna mungo) crops, which are mainly grown in hot and humid climatic conditions. Warm season food legumes are popular in different parts of world; for example, pigeon pea is mainly grown in India and African countries, cowpea and soybean are important crops in the USA, while mung bean and urd bean are important crops in Southeast Asian countries, especially in the Indian subcontinent (Singh et al., 2011).

Legumes rank third after cereals and oilseeds in world production and have major effects on the environment, agriculture, and animal and human nutrition and health (Graham & Vance, 2003; Dita et al., 2006; Mantri et al., 2013). Legumes are a primary source of amino acids and provide around one-third (20-40%) of all dietary protein (Zhu et al., 2005; Kudapa et al., 2013). Legumes produce secondary metabolic compounds that can protect the plant against pathogens and pests (Kudapa et al., 2013).

Legumes are second to cereals in providing food for humans worldwide (Kamal et al., 2003; Ashraf et al., 2010; Kudapa et al., 2013). In comparison with cereal grains, legume seeds are rich in protein, and thus are a source of nutritionally rich food (Ahlawat et al., 2007; Ashraf et al., 2010; Kudapa et al., 2013). Grain legumes such as chickpea, pigeon pea, cowpea, dry pea, lentil, mung bean, urd bean, bean (Phaseolus vulgaris L.), broad bean and grass pea are the main source of dietary protein for vegetarians, and are an integral part of the daily diet in several forms worldwide. In addition, grain legumes, predominantly peanut (Arachis hypogaea L.) and soybean are also major sources for vegetable oil, providing more than 35% of the world's processed vegetable oil (Sharma et al., 2010).

Legumes play an important role in diet and they are often referred to as 'poor man's meat'. Legumes are an important source of protein, oil, fibre and micronutrients, and play a vital role in cropping cycles due to their ability to fix atmospheric nitrogen (El-Enany et al., 2013; Mantri et al., 2013).

Under conducive environmental conditions, legumes develop symbiotic associations with arbuscular mycorrhizal (AM) fungi, leading to the formation of sites of phosphorus nutrient exchange called arbuscules (Parniske, 2008; Mantri et al., 2013).

Biological fixation of nitrogen (N) is considered more ecofriendly than industrial N fixation because the NH3 produced in the former process is readily assimilated into organic forms by the plant (Valentine et al., 2011). Biological nitrogen fixation (BNF) in legume nodules occurs with differentiated forms of rhizobia, termed bacteroids, within specialized structures called symbiosomes, inside the host plant cells (Arrese-Igor et al., 2011). Thus, these symbiotic associations have strongly driven the investigation and application of biotechnology tools for legumes (Dita et al., 2006).

It is estimated that crops grown on 90% of arable lands experience one or more environmental stresses. Abiotic stress causes more than 50% of crop loss worldwide (Rasool et al., 2013; Rodziewicz et al., 2014). 'Abiotic stress' is a broad term that includes multiple stresses (drought, waterlogging, salinity, heat, chilling and mineral toxicities) and negatively affects the adaptability and yield of legumes. Application of biotechnology tools to legume crops can help in solving or reducing the problems resulting from abiotic stress.

This chapter aims to review the main abiotic stresses that have a negative impact on the production of some important food legumes. It also summarizes the selection criteria and available genetic resources for stress resistance under abiotic stress conditions.

1.2 Legumes under abiotic stress

1.2.1 Legumes under drought

Drought is a type of water stress that is imposed due to lack of rainfall and/or inadequate irrigation. About 60% of all crop production suffers from drought conditions (Grant, 2012; Naeem et al., 2013). For legumes, drought stress has adverse effects on total biomass, pod number, seed number, seed weight and quality, and seed yield per plant (Toker et al., 2007b; Charlson et al., 2009; Khan et al., 2010; Toker & Mutlu, 2011; Impa et al., 2012; Hasanuzzaman et al., 2013; Pagano, 2014). Drought alone resulted in about a 40% reduction in soybean yield (Valentine et al., 2011). Faba bean and pea are known to be drought-sensitive, whereas lentil and chickpea are known as drought-resistant genera (Toker & Yadav, 2010). Singh et al. (1999) arranged warm season food legumes in increasing order of drought tolerance: soybean?<?black gram?<?green gram?<?groundnut?<?Bambara nut?<?lablab?<?cowpea. Sinclair and Serraj (1995) reported that legumes such as faba (broad) bean, pea and chickpea export amides (principally asparagine and glutamine) in the nodule xylem are generally more tolerant to drought stress than cowpea, soybean and pigeon pea, which export ureides (allantoin and allantoic acid).

The symbiotic nitrogen fixation (SNF) rate in legume plants rapidly decreased under drought stress due to (i) the accumulation of ureides in both nodules and shoots (Vadez et al., 2000; Charlson et al., 2009), (ii) decline in shoot N demand, (iii) lower xylem translocation rate due to a decreased transpiration rate, and (iv) decline of metabolic enzyme activity (Valentine et al., 2011). Several reports have indicated that drought stress led to inhibition in nodule initiation, nodule growth and development as well as nodule functions (Vadez et al., 2000; Streeter, 2003; Valentine et al., 2011). The decrease in SNF under drought conditions was associated with the reduction of photosynthesis rate in legumes (Ladrera et al., 2007; Valentine et al. 2011).

In many nodules of legumes, water stress resulted in stimulation of sucrose and total sugars (González et al., 1995, 1998; Ramos et al., 1999; Streeter, 2003; Gálvez et al., 2005; Valentine et al,. 2011). This was consistent with a study on pea mutants, which showed that sucrose synthase (SS) is essential for normal nodule development and function (Craig et al., 1999; Gordon et al., 1999).

Drought stress induces oxidative damage in legumes and this has a harmful effect on nodule performance and BNF (Arrese-Igor et al., 2011). Some reports suggest that nodules having an increment in enzymatic antioxidant defence can display a higher tolerance to drought/salt stress in common bean (Sassi et al., 2008) and chickpea (Kaur et al., 2009). In addition to this, Verdoy et al. (2006) reported improved resistance to drought stress in Medicago truncatula by overexpression of ?-pyrroline-5-carbolyate synthetase resulting in accumulation of high proline levels.

Generally, the mechanisms of drought tolerance include (i) escape, (ii) avoidance, or (iii) resistance (Ishitani et al., 2011; Toker & Mutlu, 2011; Impa et al. 2012; Rapparini & Peñuelas, 2014). There are several screening and selection techniques for drought tolerance in food legumes; however, few techniques have been successful under field conditions (Toker & Mutlu, 2011):

1. Line source sprinkler irrigation systems (Saxena et al., 1993).
2. Root trait...