Abiotic stresses
Warm season food legume crops encounter unpredictable environmental conditions such as waterlogging, terminal drought, high tem-perature, heavy rains, etc. These factors taken together affect yield. Therefore, the develop-ment of plant types that can survive under different environmental conditions will be required to boost the crop production and productivity (Pennisi, 2008). Some impor-tant target traits in breeding programmes for improving the genotypes of these crops against abiotic stress are discussed below.
Short duration and photo-thermal insensitivity
These are important traits in soybean, mung bean and urd bean, because the development of short-duration and photo-thermally insen-sitivite genotypes creates plants suitable for different cropping systems, and also avoids terminal drought (Singh, 2010, unpublished report). In cowpea, photosensitive culti-vars not only flower early but also become extremely dwarf in habit when day length is under 12.5 h (Ishiyaku and Singh, 2001), and a complete association of photosensitiv-ity has been observed with dwarfing, which is controlled by a monogenic recessive gene (Ishiyaku and Singh, 2001). In urd bean, earli-ness and photo-thermosensitivity are reces-sive traits and are controlled by major genes (Sinha, 1988). Thus selection of genotypes with early vigour holds tremendous impor-tance in breeding programmes. As a result, some of the very popular early varieties, such as Narendra Urd 1, KU 300, Sarla, Vamban, and Urd 3, have been developed in India for commercial cultivation. Since urd bean is also cultivated in the spring/summer sea-son, Pant U 19, T 9, KM 1 and TMV 1 have
been developed as photo-thermoinsensitive varieties (Gupta and Kumar, 2006).
Leaf pubescence density
Suitability for soybean cultivation is improved by this trait in drought-prone areas, as it reduces leaf temperature and water loss by transpiration and enhances photosynthesis and vegetative vigour (Du et al., 2009). Two addi-tive genes control this trait in soybean (Pfeiffer and Pilcher, 2006). This is also an important trait of mung bean and urd bean; some lines of mung bean developed at AVRDC, e.g. V 2013, V 1281, V 3372, VC 1163D, VC 2750A, VC 2754A and VC 2768A, can withstand moisture stress (Tickoo et al., 2006), including long spells of rainfall causing flooding.
Seed dormancy
Reduced seed dormancy is found in mung bean, resulting in preharvest sprouting dur-ing the maturity phase in the monsoon (kharif) season, and therefore the identification of lines with tolerance to preharvest sprouting is highly desirable in this crop (Tickoo et al., 2006) and in urd bean.
Deep root system
Pigeon pea is cultivated mostly in rainfed zones, the deep and dense root system pro-viding inherent potential to counteract drought or water stress during the critical growth phases.
Biotic stress
Warm season crops are also affected by a number of important diseases, insect pests and nematodes, now discussed below.
Therefore, the development of cultivars resist-ant to these biotic stresses remains a target of breeding programmes in these crops.
Diseases
RUST. A devastating disease of soybean caused by Phakpspora pachyrhizi, yield losses of up to 95% have been reported in Brazil
(Hartman et al., 1997), 75% in Argentina (Yorinori et al., 2005) and 50% in the USA (Hartman, 2005). Inheritance studies sug-gest that four single dominant genes control this trait (Hartman, 2005). Although geno-type PI 459025, having a single dominant gene for resistance to all three rust isolates, has been identified, its use has been shown to be problematic due to the rapid break-down of resistance. Therefore, the develop-ment of genotypes having multiple genes of resistance is an important target of soybean breeding programmes.
BACTERIAL PUSTULES. In soybean this condi-tion is caused by Xanthomonas axonopodis pv.
glycines, which is very much favoured by hot and humid conditions. Studies have shown that a single recessive gene controls resistance to this disease (Hartwig and Lehman, 1951).
Molecular breeding has also been conducted, and SSR markers tightly linked to BLP resis-tance have been identified for using in breed-ing programmes (Kim et al., 2010).
FUSARIUM WILT (FW). In pigeon pea, FW is an important biotic stress causing significant yield losses of up to 20–25% in India (Dhar and Reddy, 1999) and Africa (ICRISAT, 1983).
Resistance to FW is a complex phenom-enon, studies suggesting variously that it is governed by multiple genes (Pal, 1934), two complementary genes (Shaw, 1936; Pathak, 1970) and a single dominant gene (Pawar and Mayee, 1986; Singh, I.P., et al., 1998). Many wilt-resistant varieties have been developed in India through pedigree and bulk-pedigree methods, e.g. Pusa 33, C 11, BDN 1, BDN 2, ICPL 8863, Jawahar Arhar 4, Birsa Arhar 1, ICPL 87119, KM 7 and MAL 13.
PYTOPHTHORA BLIGHT (PB). Caused by the fungus Phytophthora drechsleri f. sp. cajani, no resistant variety is available for pigeon pea (Singh et al., 2005). Studies have variously claimed that resistance is governed by a sin-gle dominant gene (Sharma et al., 1982) and two homozygous recessive genes (Singh et al., 2003a). Some tolerant lines, e.g. KPBR 80-2-1, KPBR 80-2-2, GAUT 82-55 and ICP 8103 have been developed. Some level of resistance
Breeding of Warm Season Food Legumes 71
has been found among accessions of Cajanus platycarpus against PB.
POWDERY MILDEW (PM). Of importance in mung bean and urd bean, in the former PM is caused by Erysiphe polygoni DC, and can cause yield losses of up to 20–40% in India (Grewal, 1978). The status of PM resistance in this crop has been reviewed (Reddy et al., 2008), an inheritance for resistance to PM has variously been reported as monogenic and polygenic (Yong et al., 1993; Sorajjapinun et al., 2005;
Reddy, 2009). The TARM 1 and TARM 18 lines are well-known varieties showing a high level of resistance to PM. It is also a serious disease in urd bean, causing 20–25% yield losses.
Resistance is controlled by a single recessive gene (Kaushal and Singh, 1989). Limited resis-tance sources are available for PM in mung bean and urd bean, e.g. Pant U 30, P 115, Line 6203 and LBG 642. Cultivar LBG 17, resistant to PM, is very popular in rice-fallow areas of India (Gupta and Kumar, 2006).
CERCOSPORA LEAF SPOT (CLS). In mung bean, CLS caused by Cercospora canescens Ell. and Mart. and Cercospora cruenta Sacc. is an impor-tant disease. Warm and humid weather condi-tions are very favourable for its appearance. It has been variously reported that resistance to CLS is governed by one or two genes (Singh and Patel, 1977; Mishra et al., 1998) and a single recessive gene (Yadav et al., 1981). ML 613 is a cultivated variety bearing resistance to CLS.
VIRAL MOSAICS. Viruses cause a number of diseases in warm season legume crops, includ-ing sterility mosaic virus (SMV) in pigeon pea, soybean mosaic virus (SMV) in soybean and mung bean yellow mosaic virus (MYMV) in mung bean. Inheritance studies have been conducted on these diseases; in soybean, SMV resistance is controlled by three independent genes (Moon et al., 2009). Bud blight disease of soybean is caused by a strain of groundnut bud necrosis virus (GBNV), and is an important viral disease in major soybean-growing areas of India. Some lines such as MACS 754, NRC 55, VLS 55 and JS-SH-96-04 have been identified as resistant to bud blight (Lal et al., 2002).
Sterility mosaic virus is an important viral disease of pigeon pea carried by an arthropod vector (Kumar et al., 2000). Inheritance to this disease in pigeon pea has been reported to be monogenic to oligogenic (Singh, B.V. et al., 1983; Srinivas et al., 1997; Singh, I.P. et al., 2003b). Some of the popular varieties in India such as Hy 3C, Bahar, Pusa 9, Narender Arhar 1, MA 3, MAL 13 and Asha have resistance to SMV.
Predominantly found across India, espe-cially in the rainy season, MYMV is spread by the vector white fly (Bemisia tabaci Genn.).
Resistance to MYMV is reported variously to be governed by a single recessive gene (Singh and Patel, 1977) and two recessive genes (Verma, 1985; Reddy, 1986). In India, a large number of varieties, e.g. Pant Moong 2, Narendra Moong 1, Meha, Samrat, IPM2-3, HUM 1 and PM 6 have considerable resist-ance to MYMV. MYMV is also the most com-mon threat to the urd bean. Under severe conditions, yield loss has been observed up to 100%. Resistance to this disease has variously been reported to be monogenic dominant (Dahiya et al., 1977) and digenic recessive (Singh, A., et al., 1998). Pant U 84, UPU 2, Pant U-19, UH 81-7, UG-700 and IPU 94-1 are among the most important genotypes resist-ant to MYMV.
Insect pests
POD BORER (HELICOVERPA ARMIGERA, MARUCA TESTULALIS,MARUCA VITRATA)AND PODFLY (MELANA
-GROMYZA OBTUSE). For pigeon pea, these are the most important insect pests. Pod borers cause damage in all mature groups, while podfly is prevalent in late-duration genotypes. High-density trichomes on the pod wall surface and their associated exu-dates play a major role in resistance to pod borers. The inheritance of trichomes is gov-erned by single dominant gene (Verulkar et al., 1997, Rupakula et al., 2005; Banu et al., 2007) in Cajanus scarabaeoides. C. scara-baeoides shows resistance to podfly due to trichomes, their expression governed by a single dominant gene (Verulkar et al., 1997), whereas for podfly resistance in cultivated species, two genes behave in both domi-nant and recessive fashion based on allelic
interactions (Singh and Lal, 2002). Annual losses due to M. vitrata have been estimated at US$30 million (ICRISAT, 1992). Very limited efforts have been made to identify a source for its resistance. Recently, ICPL 98003 and ICPL 98008 have been identified as donors for use in breeding programmes (Sunitha et al., 2008).
The pod borers M. testulalis and H. armig-era) also cause heavy losses in mung bean.
At AVRDC (Taiwan), resistant lines V 2019, V 4270, V 2106 and V 2135 have been used in breeding programmes. Only low levels of resistance have been observed for Maruca pod borers in cowpea; in this crop the P120 and C11 lines have been reported to be the least damaged (Jagginavan et al., 1995), and TV × 7 line has been shown to be the geno-type most resistant to these insects (Veeranna and Hussain, 1997).
BRUCHIDS (CALLOSOBRUCHUS CHINENSIS AND CALLOSOBRUCHUS MACULATES). Bruchids are the most important pests of stored grain in Vigna spp. Multiple seed factors are respon-sible for resistance against bruchids, i.e. the presence of a-amylase inhibitors, trypsin inhibitors, polyphenol and and tannin content (Ishimoto and Kitamura, 1989). Inheritance of resistance is variously reported as due to monogenic dominant (Tickoo et al., 2006) and digenic dominant duplicate (Souframanien and Gopalakrishna, 2007) gene actions. No effective resistant source has been reported for mung bean, whereas in urd bean lines Mash 59, VM 2011 and VM 2166, some resistance has been documented (Gupta and Kumar, 2006). Resistance to multiple insects has been found in cowpea, and several improved cowpea varieties with combined resistance to aphids, thrips and bruchids have been developed (Singh et al., 1996). The varieties IT97K-207-15, IT95K-398-14 and 98K-506-1 have a high level of bruchid resistance (Singh 1999), and the 7s-storage protein ‘vicillin’ has been reported to be responsible for bruchid resistance in cowpea lines related to TVu 2027 (Yunes et al., 1998).
THRIPS AND STEMFLY. These are major pests in urd bean, with yield losses under severe
attack amounting to up to 40%. A large degree of genotypic variation has been observed for resistance. Important donors against thrips include PDU 5, KB 63, UG 567 and UH 804.
The genotypes UG 218, PDU 1, PDU 5, LBG 707 and CO 305 are suitable donors for stem fly resistance (Gupta and Kumar, 2006).
APHID (APHIS GLYCINES MATSUMURA). For soybean, this is a major pest. Genotypes PI 200538 and PI 243540 have strong resistance to aphids, and a single dominant gene gov-erns resistance to this insect (Kang et al., 2008;
Hill et al., 2009).
Nematodes
The nematodes also are responsible for major problems in some warm season legume crops. In Nigeria, nematode attack in cowpea is very severe in the dry season when plant-ing with irrigation. Several resistance sources have been identified for nematodes (Singh, 1998), of which IT89KD-288 was found to be resistant to four strains of Meloidogyne incog-nita in the USA (Ehlers et al., 2000); this geno-type was found to be very effective against nematodes, and showed high yielding poten-tial in trials conducted in areas highly prone to nematode attack in Nigeria (Singh et al.
2002). IT89KD-288 was taken by one farmer in 1994 and, through farmer to farmer diffu-sion, it has become a popular variety because of its nematode resistance and high yield in the dry season. Roberts et al. (1996) identi-fied the IT84S-2049 cowpea line from IITA as being completely resistant to diverse popula-tions of the root-knot nematodes M. incognita and Meloidogyne javanica. Systematic genetic studies have indicated that resistance in IT84S-2049 was conferred by a single domi-nant gene, which was allelic to either the Rk gene or another gene very closely linked to Rk; therefore, the symbol Rk2 was pro-posed to designate this new resistance factor.
Rodriguez et al. (1996) screened nine cowpea varieties for resistance to the root-knot nema-tode M. incognita; they observed that IITA-3, Habana 82, Incarita-1, IT86D-364, IT87D-1463-8, Vinales 144, P902 and IITA-7 were highly resistant, whereas the local variety Cancharro was highly susceptible.
Breeding of Warm Season Food Legumes 73
Seed quality traits
Warm season food legumes are well known for their high seed protein and oil content. The most important limiting amino acids in food legumes, such as the sulfur-containing amino acids (methionine and cystine), are impor-tance targets in protein quality improvement programmes. Efforts to increase cystine and methionine levels in soy proteins have been primarily aimed at increasing the concentra-tion of protein subunits, which are known to have higher levels of these two amino acids.
Increasing the protein and oil content is also an important target in warm season food leg-ume crops for improving seed quality along with yield. However, a negative correlation between yield and protein content or between yield and oil content is well documented in these crops (Dahiya et al., 1977; Wilcox and Shibles, 2001). Increasing both protein and oil concentration in seeds is an important breed-ing goal in soybean, but these are negatively correlated (Brim and Burton, 1979). It has been reported that soybean oil content is governed by additive gene effects, additive × additive epistatic interaction and complementary epistasis (Rahangdale and Raut, 2002), and therefore use of recurrent selection schemes could be the most effective means of increas-ing oil content (Burton and Brim, 1981).
Protein content is governed by consider-able non-additive gene action in mung bean, thus making it a complex trait to transfer (Chandra and Tickoo, 1998). Rotundo et al.
(2009) suggested that this negative associa-tion could be overcome by increasing the sup-ply of assimilates per seed without sacrificing reproductive efficiency. In India, Naik et al.
(2002) developed a local pureline, BSN 1 from Nagpuri, having a high yield and 27.8% seed protein. In urd bean, a positive association has been observed between protein content, seed yield, 100-seed weight and pods/plant (Kole et al., 2002). Urd bean seeds contain 25%
protein, but only limited efforts have so far been made to study the extent of genotypic variation for protein content in relation to other yield components (Kole et al., 2002).
Dark green colour, shiny and bold seeds are important quality factors for mung bean con-sumers in India; however, in Bangladesh and
adjoining regions, yellow and small grains are commonly consumed.
High phytic acid (PA) levels in soybean seeds cause mineral malnutrition in humans, and to investigate this problem systematic studies have been conducted. Recently, it has been observed that total phosphorus (P) and phytate P (PhyP) are controlled by dominant recessive epistasis, which may be of assist-ance in developing low-phytate varieties (Sompong et al., 2010). The quality of soybean oil is also determined on the basis of the ratios of polysaturated fatty acids, saturated fatty acids and mono-unsaturated fatty acids, and essential fatty acids such as linoleic/linolenic.
High linolenic acid levels in soybean oil have poor oxidative stability (Patil et al., 2004).
Isoflavon in soybean oil is another impor-tant target for improvement in oil quality.
For this trait, epistatic interactions have been observed, apart from malonyldiadzin (MDZ).
To obtain the largest selection gains for this trait, priority should be given to exploiting either the additive genetic variances in super-ior lines or the cytoplasmic effect and the epistatic interactions between cytoplasmic and nuclear genes (Chiari et al., 2006). Lutein is a major carotenoid in soybean seed, and is beneficial for maintenance of eye health; this component is positively correlated with oleic acid and negatively correlated with linoleic and linolenic acid (Lee et al., 2009).
Agronomic traits
In mung bean, yield is correlated with leaf area index (LAI), number of branches per plant, pods per plant and seeds per pod (Makeen et al., 2007). Multiple leaflet traits give a greater leaf area, thus intercepting more sunlight to help increase yield. This trait is controlled by single recessive gene (Sripisut and Srinives, 1986), whereas leaflet number is controlled by two loci (Soehendi et al., 2007).
A recent study suggests that leaflet size is more important than leaflet number in relation to seed yield (Sriphadet et al., 2010). Determinate growth habit and compact plant type are also preferred traits for the development of varieties suitable for intercropping in mung
bean (Tickoo et al., 2006). Large differences of 68–140 days exist for maturity time in urd bean; however, due to the intensification of multiple cropping systems, early varieties are required to suit this system (Sinha, 1988;
Chadha et al., 2009).
Lodging resistance is the main tar-get characteristic for soybean cultivars. An erect growth habit, which reduces mechani-cal harvesting loss and allows maximum light penetration through the plant canopy, is a target trait in the USA in soybean for improved plant type. Soybean breeders have
used several other traits with mixed results, including narrow leaflets, brachytic stems (short internode), stem termination to alter height, and more fibrous rooting (Wells et al., 1993). Change in the length of the reproduc-tive period has been focused in soybean on adaptation to specific environments.
However, in practice, lengthening the pod-filling period and/or changing the rate of dry matter accumulation in pods have allowed minor improvement in yield, with a positive correlation having been between these two traits (Smith and Nelson, 1986).
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