Chickpea (Cicer arietinum L.) is the third most important pulse crop in the world, and first in West Asia and North Africa (WANA). It is mainly grown in spring, taking advantage of conserved soil moisture. Because of large variations in seasonal rainfall, the crop suffers occasionally from drought. Although chickpea is known for its better drought tolerance than most other cool-season food legumes, drought does reduce yields, and can even lead to total crop failure. In 1999/2000, for example, the severe drought in WANA caused up to 75% yield losses in some spring-sown chickpea areas.

What is drought?

Drought is recurring condition of abnormally dry weather leading to moisture stress for plants. Severity depends on a number of factors, including degree of moisture deficiency, its duration, and spatial spread. It can be aggravated by mismanagement of land and water resources.

Agronomic considerations

Selecting appropriate crop varieties, plant density and row spacing to reduce moisture loss by evaporation and transpiration, adjusting planting time to make better use of available water, and adopting supplemental irrigation are the agronomic practices that could help in managing drought.
     Where water is available, irrigation is the main means of combating drought. Research at ICARDA’s Tel Hadya station in Syria (342 mm of average annual rainfall) has shown that 50 mm of supplemental irrigation can increase the yield of spring-sown chickpea by 40%. It can be applied to save the crop in case of unexpected drought, or as a planned practice to supplement the expected total seasonal rainfall in low-rainfall areas.
Crop cultivars differ in their response to a given environment, and this difference can be of help for growers. For example, in Central and West Asia and North Africa (CWANA), and the Caucasus, a transient heat wave that normally occurs around flowering, and the failure of spring rain resulting in terminal drought stress, can cause serious yield losses due to poor seed set. Damage can be reduced by sowing a variety that can better withstand heat and drought or by adjusting planting date to avoid heat and drought damage. Sowing in winter, instead of traditional spring sowing, permits the crop to escape heat and drought. But this can be done only by using varieties specially developed for winter, i.e., having cold tolerance and resistance to blight disease. Trials over a number of years in Syria and Lebanon showed that winter sowing of such improved varieties can almost double yield compared to spring sowing. The winter-sown crop makes more efficient use of soil moisture, escapes heat and drought stress at flowering, and maturing early, it competes less with other crops for labor and machinery at harvest time. As the crop grows taller than the spring-sown crop, it is better suited to mechanical harvesting.
     Most of the spring cultivars grown by farmers in WANA are susceptible to cold and ascochyta blight and cannot be used for winter sowing. Hence, ICARDA, working with national partners, has developed improved varieties for winter sowing. In 1998–2000, when drought was severe in parts of the region, farmers who adopted winter sowing of such improved varieties obtained more than a tonne of chickpea, while those who planted at the usual springtime harvested less than 300 kg/ha. National programs in 20 countries in WANA have selected and released to their farmers 40 improved varieties of chickpea from ICARDA-supplied breeding material.
     Field evaluation in high-altitude areas has also identified chickpea lines that can withstand temperatures as low as –20OC, permitting late winter and early spring sowing in the high-altitude areas in Central Asia and the Caucasus and in Iran. Farmers there have recognized the benefits of early sowing and have started adopting the practice. Government agencies and non-governmental organizations in some countries are now producing improved seed and conducting demonstrations to enhance farmer awareness.

Genetic enhancement for drought tolerance

Drought tolerance refers to the ability of a variety to remain relatively more productive than others under limited water conditions. Drought tolerance/resistance is a very complex trait associated with different attributes. Plants usually adapt to drought stress through three major mechanisms, namely, escape, avoidance and resistance. Although the genetic and physiological bases of these mechanisms have not been established precisely, they have been indirectly exploited by chickpea breeders in developing drought-tolerant cultivars. But, as drought is unpredictable, regular selection of cultivars at a particular site under natural conditions is extremely difficult.

Breeding for drought tolerance

Drought tolerance research in chickpea at ICARDA has addressed various strategic options and found that productivity under drought conditions can be improved using two steps in the breeding process: (1) rejection of lines with low productivity in low-input, drought-prone marginal conditions, and (2) and selecting for high productivity in the evaluation of remaining lines in high-input, drought-prone marginal environments. This approach has been successful. For example, a popular line, Gokce (FLIP 87-8C), has shown its ability to adjust to changing environmental conditions. As there is no single trait—morphological, physiological or biochemical—which alone is responsible for drought tolerance, ICARDA follows a gene pyramiding approach to develop breeding material with high yield and drought tolerance. Bringing genes together in one line (gene pyramiding) for various complementary traits—such as early seedling establishment, early growth vigor, early flowering and maturity—that contribute to escape, dehydration avoidance, and tolerance, results in achieving relatively stable drought tolerance

Different methods of screening for drought tolerance have developed and used at ICARDA.
Using a line-source sprinkler irrigation system, a continuous soil moisture gradient (ranging from none to full irrigation) is created, upon which chickpea genotypes are evaluated for seed yield. Genotypes are scored using drought response index (DRI, the ratio of yield under stress and that without stress), and genotypes with high DRI are selected. Using this technique, it was observed that early flowering is the main component of drought escape in

chickpea, and that it is associated with high harvest index, the number of pods per unit area, and seed yield.      Using this technique, good progress has been made in developing early maturing genotypes, without compromising on yield.
     Crop species have evolved several mechanisms to maintain required water status in their tissues for normal metabolic function under limited soil moisture supply or when atmosphere is excessively desiccating. Dehydration avoidance (DA) is a mechanism that allows the plant to retain a tissue water content during drought stress permitting important plant productive functions to continue. DA is influenced mainly by different root attributes, including root size, morphology, depth, length, density, hydraulic conductance, and others. Since screening for differences in root length and density is very laborious and time consuming, indirect selection through related traits is more practical. Studies of the relationships between different root, shoot and some morpho-physiological parameters have shown that a combination of traits, including earliness to flower, high harvest index and deep rooting, which are responsible for large average yields, should be used as criteria for improvement of DA. Some traits, such as stomata number and size, leaf rolling, leaf movement, and high level of reflectance, which have been reported as good indicators of DA in other crops, can also be explored in chickpea.
     In environments where water deficit can occur at any stage of growth, dehydration tolerance (DT) might play a role in crop survival until soil moisture levels improve with succeeding rains. For such conditions, ICARDA has started using the box screening technique under controlled conditions to evaluate genotypes for DT. Dehydration tolerance relates to the ability of cells to continue metabolizing in times of low moisture. Wooden boxes at least 20 cm deep are filled with a 1:1 mixture of sterilized soil and sand. Seeds of different genotypes are planted in equally spaced rows and at equal depth and the soil-sand mixture is saturated with water. No more water is applied throughout the trial. Evaluation for DT is carried out when the susceptible check line shows wilting. Leaf samples from different genotypes are assessed for cell leakage using an electro-conductivity test. Lines are scored as drought tolerant or susceptible based on number of days they take to show wilting or based on the amount of solute leaching from cells.
     The techniques mentioned above are useful and reliable, but they are also time-consuming, expensive, and cumbersome. ICARDA, jointly with the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), has developed a simple field screening technique for handling large numbers of lines at a time. Material to be screened is planted in late spring (20 March at Tel Hadya) and is evaluated using a scale of 1 to 9 (1 = free of visible drought effects, very good early plant vigor, 100% pod setting; 9 = highly susceptible, lack of early plant vigor, no flowering, no pod setting, no yield, all plants killed). Promising lines (with ratings 1–5) are sown, with and without supplemental irrigation, in replicated trials. Lines producing high yields (more than the mean of the trial) under these late sown (drought) conditions, and those responding well to supplemental irrigation are selected. Many lines selected using this technique have been shared with ICARDA’s national partners through the Center’s Legume International Testing Program as part of the Chickpea International Drought Tolerance Nursery. Several have shown promise in the recipient countries.
     In a search for additional sources of tolerance to drought, ICARDA researchers evaluated different accessions of annual wild Cicer species and observed that C. reticulatum, C. judaicum, and C. bijugum were more drought tolerant than others. C. reticulatum is easily crossed with cultivated chickpea and is being used in ICARDA’s breeding program.
    As drought tolerance is a complex trait, drought occurrence is unpredictable, and screening not very reliable, there is a need to exploit molecular techniques, including identification of QTLs, genes, and molecular markers for marker-assisted selection.

What the future holds

Greater attention has been paid to drought tolerance in the last few years and several evaluation techniques based on morphological, physiological, and adaptive traits have been developed. Studies on measurements of root length, root density, osmoregulation, and most of the other adaptive traits have revealed that these are time- and resource-consuming approaches. Thus, empirical yield-based screening methods will continue to dominate for drought screening unless more simple and refined techniques, such as molecular marker assisted breeding, are identified that might enhance the efficiency of classical breeding.

Dr R.S. Malhotra (R. Malhotra@cgiar.org) is Senior Chickpea Breeder and Dr M.C. Saxena (M.Saxena@cgiar.org) is Assistant Director General (At-Large) at ICARDA.