Drought tolerance traits in Medicago species: A review

This review provides a comprehensive overview of the effects of drought on the growth of medic plants; then, it explores some drought tolerance traits including the morpho-physiological and biochemical parameters. In addition, medic plants were compared to other legumes and to cereals. At the molecular level, the response of Medicago plants to drought was also highlighted. Results of several independent experiments showed that the reduction of plant biomass production of medic plants ranged between 12 and 73% under water deficit stress. Cereals are more tolerant to drought than legumes. Yield reduction in cereals ranged between 25 and 40% versus 20 and 80% in legumes. For cereal species, wheat exhibited the lowest (25%) yield reduction as compared to barley and maize (34 and 40%), respectively. In legumes, reductions were about 20, 40, and 60% in lentils, chickpea, and common bean. Medic drought tolerance was found to be associated with traits like the ability to maintain the photosynthetic activity, the optimization of root development, the water use efficiency, the osmoregulation capacity via the decrease of osmotic potential, accumulation of mineral (K+) or organic solutes (proline), and the modification of carbohydrate metabolism toward the accumulation of soluble sugars. Water deficit stress induces a lipid and protein metabolism adaptation via the accumulation of some amino acids and the decrease in malondialdehyde concentrations. The target traits suggested in medic for breeding and for genetic engineering are related to water relations, such as relative water content, water use efficiency as well as root development.