ICARDA’s main research station and its related testing sites in the Near East are located in the heart of the Fertile Crescent. The research sites are also representative of the harsh Mediterranean dryland climate, which is characterized by extremes of temperature and moisture availability. This means that ICARDA is ideally placed to conduct breeding for drought tolerance. Together with the International Maize and Wheat Improvement Center (CIMMYT) and its national partners, ICARDA is making great strides in improving durum wheat. More improvement is promised, thanks to the use of emerging technologies and the region’s age-old dryland agrobiodiversity.

Durum (Triticum turgidum L. var. durum) wheat is grown mainly in Central and West Asia and North Africa (CWANA) under variable environmental conditions. These environmental variations are the main cause of yield reduction and fluctuation. In the Mediterranean drylands, abiotic stresses, such as drought, cold and heat, are the most important constraints. Biotic stresses, including diseases and insects, also take their toll. In fact, CWANA has the highest number of damaging insect biotypes and some of the most virulent diseases that cause damage to agriculture.
     This combination of abiotic and biotic stresses makes plant breeding in the Mediterranean dryland areas complex and very challenging. In 1977, ICARDA initiated a dryland durum-breeding program in partnership with CIMMYT. The program’s main objective is to develop genotypes and genetic stocks combining yield potential with resistance to drought and other abiotic and biotic stresses, and with improved grain quality. During the last 10 years, stress-tolerant and high-yielding durum genotypes developed at ICARDA have resulted in spectacular gains in Syria, which has seen annual production increase from less than 1 million tonnes in the 1980s to 5.4 million tonnes today.

Drought resistance breeding

Because the Mediterranean drylands are characterized by a high year-to-year variability, breeding cultivars that combine drought resistance, yielding ability, and yield stability is the objective of the ICARDA breeding strategy. In our selection approach, all early segregating populations are subjected in representative selection environments to the stresses encountered in the Mediterranean drylands. Landraces and wheat wild relatives are the main sources of drought resistance. Stress-physiology-traits tools and molecular-marker techniques are also used to efficiently select drought-resistant durum genotypes. The methodology allows identification, in the early stages, of the populations that combine drought resistance, productivity, stability, and resistance to biotic and abiotic stresses

Use of genetic wheat diversity
Mediterranean durum landraces were found to possess desirable traits for resistance to drought, early growth vigor, long peduncle, and high fertile tillering capacity. Use of the landraces in the hybridization program has shown substantial progress can be achieved. Selection for drought resistance has been performed in populations from crosses between improved durum lines, landraces, and durum wild relatives. Wild relative are used to widen the genetic base of durum and improve its resistance to abiotic and biotic stresses. Genotypes generated from hybridization with Triticum wild relatives have been found to produce high grain yield under favorable and dry conditions. Under drought, the best crosses were those with Triticum carthlicum, T. dicoccoides, T. monococcum, T. polonicum, T. dicoccum and Aegilops species.

Use of stress physiology
The ICARDA durum program has found that earliness, fertile tillering, spike fertility, peduncle length, and early plant vigor are associated with higher grain yield under drought conditions. Fertile tillering is, however, by far the most potent predictor of durum wheat grain yield under moisture-stress conditions. Other physiological measurements were made. Comparison between the Near East landraces and the ICARDA-improved durum genotypes showed that significant genetic gain in grain yield is associated with physiological traits such as photosynthesis and osmotic adjustment.

Use of molecular markers
The program is using molecular markers to identify drought resistance traits difficult to select for, using conventional field or laboratory methods. Drought resistance in durum was found to be associated with some molecular markers and morpho-physiological traits. The quantitative trait loci (QTL) analysis was conducted on mapping Jennah Khetifa/Cham 1 (JKC) and Omrabi5/T. dicoccoides// Omrabi5 (MDM) populations. Markers linked to drought resistance and grain quality are being identified. In JKC and MDM, QTLs for grain quality were determined — for gluten strength, protein content, yellow pigment, alphalipoxygenase, flour extraction, and test weight. As for drought resistance, the carbon isotope discrimination (CID), which we have shown to correlate positively with grain yield and its components, was found to be located in the JKC & MDM on different chromosomes (4Bs and 3Bl).
     The combination of different research tools in the improvement of durum cultivars has generated promising genetic material for the Mediterranean drylands. Research is ongoing to combine genes for drought resistance in durum for further enhancement of yield and yield stability.

Dr Miloudi M. Nachit (M.Nachit@cgiar.org) is CIMMYT/ ICARDA Durum Wheat Breeder, based at ICARDA.