Physiological traits for ameliorating drought stress

The pattern of physiological responses to drying soil appears to be closely linked to decreases in soil hydraulic conductivity as the soil dries. Hence, physiological amelioration of stress needs to focus on possibilities for either accessing more soil water or slowing the rate of soil water extraction. Deeper rooting appears to be a critical approach to increasing the amount of water available to plants, although the difficulty of documenting differences in rooting depth among genotypes has resulted in little progress in this approach. Water conservation can be achieved by slightly decreasing plant gas exchange, which results in a decreased growth rate, but depending on the environment, can result in long-term increases in mean crop yield. It is speculated slow-wilting genotypes of soybean that appears to have drought tolerance may result from one of these two traits. Finally, symbiotic nitrogen fixation rates decrease with soil drying well in advance of plant gas exchange. Genotypes have been identified that exhibit substantial nitrogen fixation tolerance to drought stress and this trait in improved genetic backgrounds results in yield increase. On-going research has provided exciting possibilities for ameliorating drought stress in soybean, and the future may be even more promising as these traits are combined in superior cultivars. Drought stress is a serious limitation of soybean yields, particularly because of the high sensitivity of symbiotic nitrogen fixation system to soil drying. A new monograph on soybean published by the American Society of Agronomy will soon be released and it contains an excellent review of various options for ameliorating the influences of water deficits on soybean growth and yield (Purcell and Specht, 2004). Therefore, in this brief paper only a few of the key physiological traits are discussed as possible approaches to ameliorating the consequences of drying soil. The traits to be considered are plant gas exchange, rooting, the slow-wilting trait and symbiotic nitrogen fixation.