Seed filling in grain legumes under water deficits, with emphasis on chickpeas

Grain legumes (pulses) are important contributors to world food supplies and nutrition, especially in the developing world, and they are also a major factor in enhancing sustainability through intensification and diversification of agricultural production systems (Byerlee and White, 2000). The current total world production of grain legumes is about 56.5 million tonnes per annum (FAO, 2004). Dry bean (Phaseolus spp.) at 19.0 million tonnes is the primary grain legume produced in the world, followed by field pea (Pisum sativum L.) at 10.3 million tonnes and chickpea (Cicer arietinum L.) with 7.1 million tonnes (FAO, 2004). In terms of utilization, about two-thirds of the world's grain legumes are consumed as human food and a quarter ends up in animal feed. The average yield of grain legumes varies between 780 and 870 kg/ha, depending on the environmental conditions. More than 90% of grain legumes are grown under dryland (rainfed) conditions, and the increasing use of irrigation for cash crops and cereals has further pushed them to more marginal areas in the developing world. Variability in yield and quality are major causes of production/market instability for grain legumes and stability of yield and quality, particularly in rainfed farming systems, deserves greater research attention. In this chapter, the major focus will be on chickpea, although the literature on other grain legumes will also be accessed where relevant. Chickpea, from its center of origin in West Asia, has spread to its present-day range between 20 and 40° latitude (Abbo et al., 2001, 2003). Current cultivation occurs largely in West and Central Asia, South Asia, southern Europe, northern Africa, Latin America, and, more recently, in North America and Australia (FAO, 2004). It is a self-pollinated, annual grain legume that grows in both subtropical and Mediterranean-type climates. In subtropical environments, such as South Asia and northeastern Australia, chickpea is predominantly grown in the post-rainy season on receding stored soil moisture (Siddique et al., 2000). In Mediterranean-type climates, such as West Asia and North Africa, chickpea is traditionally sown in late winter or early spring to minimize damage from the devastating disease ascochyta blight (Ascochyta rabiei) that develops during the cold, wet winters, but then plants have to rely on stored soil moisture from the winter-dominant rainfall or supplemental irrigation (Abbo et al., 2003; Siddique and Bultynck, 2004). In southern Australia, which also has a winter-dominant Mediterranean-type rainfall pattern, chickpea crops are sown at the break of season in autumn and grow on current rainfall (Siddique et al., 2000), but the occurrence of ascochyta blight in the mid-1990s has resulted in a marked decrease in chickpea production in southern Australia and future expansion depends upon the development of resistant cultivars. In both Mediterranean-type and subtropical environments, chickpea is exposed to declining soil moisture, increasing temperatures, and increasing vapor pressure deficits during seed filling (Johansen et al., 1994; Leport et al., 1998, 1999; Sedgley et al., 1990; Singh et al., 1997; Turner, 2003). As chickpea is predominantly used for human consumption, the physical characteristics of the seed are important in terms of consumer acceptance. There are two types of chickpea: (i) the large-seeded kabuli chickpea, which has a light-colored and thin seed coat, that is predominantly grown around the Mediterranean Basin and used as whole seed and in hommus and falafal, and (ii) the small-seeded desi chickpea, which has a darker and thicker seed coat than the kabuli type, that is primarily grown in South Asia, where it is split and used as dahl or flour. The principal quality parameters are seed size, uniformity, color and shape, freedom from external damage and foreign material, and ease of processing (Siddique, 1993). Seed size is an important quality factor since large seeds command a higher price in the market than do small seeds, particularly in kabuli chickpea (Eser et al., 1991). However, even though desi chickpea are often dehulled and split before they are consumed (Siddique, 1993), average seed size and uniformity of seed size are important, since a range of seed sizes makes processing more difficult and decreases splitting yield (Williams and Singh, 1987). Reductions in seed size in indeterminate legume crops grown in environments prone to terminal drought are common (Davies et al., 1999; Dracup and Kirby, 1996a; Leport et al., 1998; Thomson et al., 1997). Genotypes better able to maintain seed size and yield must either avoid terminal drought by completing seed filling prior to the commencement of drought (Subbarao et al., 1995; Turner et al., 2001) or tolerate terminal drought and maintain seed filling during drought (Turner et al., 2001). Reproductive development involves a number of stages, all of which can be affected by drought and ultimately have an influence on seed size and seed yield. © 2005 Elsevier Inc. All rights reserved.