Mechanisms With Leaf Traits In Alfalfa Responding To Drought Stress

Drought has long been a global problem. In agriculture, the reduction of crop yield caused by drought even exceeds the total loss resulted from other natural stresses. In China, shortage of water resources is also a serious problem and the arid and semi-arid regions approximately compose 50% of the land area. In northern China, especially in northwestern regions, drought has been getting even worse and the reduction or even total loss of crop yield caused by drought often occurs. Alfalfa (Medicago sativa L.) is a high quality forage that is widely planted in the arid and semi-arid regions of China. It plays an important role in ecology construction and the production of agriculture and livestock in these regions. It has strong drought resistance capability, however it requires relatively more water for growth and development. So, drought stress is still one of the main adverse factors limiting cultivation of alfalfa, which severely affects the productivity. Therefore it is very important and practical to study further the mechanisms relating to drought resistance in alfalfa.As one of the most important organs linking a plant with the surrounding environment leaf is extremely sensitive to environmental changes. Leaf traits can best embody the effects of external factors on the plants or reflect the adaptation of plants to the outside world. Water is a key factor that affects plant growth and has a pronounced impact on leaf traits. From the point of view of leaf traits, it may further reveal the mechanisms of alfalfa’s tolerance to drought stress.In this study, pot experiments and field trials were performed to investigate the responses of leaf traits of alfalfa to different water treatments at different growth stages. The aim was to test the hypothesis that in alfalfa, leaf traits would change variously at different growth stages responding to water deficit.The main results were as follows:(1) Compared to that in Xifeng Qingyang, Longdong variety in Dingxi where the precipitation is lower showed thicker leaf blade, smaller leaf area (LA), greater leaf length/width ratio (L/W), higher leaf mass per unit area (LMA), leaf dry matter content (LDMC), chlorophyll content (Chi) and leaf nitrogen concentration per unit mass (N,mass).(2) With the increase in soil water deficit, LDMC, Nmass, and leaf thickness (LT) increased gradually. The values of such leaf traits in three varieties were significantly greater under severe water deficit (35% field water capacity, FWC), while, there was no significant difference between those under other water treatments. The LMA was also rising, but different responses could be observed among varieties. The LA showed an decreasing trend, and at the same water level, the LA appeared Xinjiangdaye>Algonquin>Longdong. The Chi, L/W and leaf phosphorus concentration per unit mass (Pmass) changed a lot under different water treatments without solid trends. Longdong variety as a more drought tolerant one among three, had thicker leaf blade, smaller LA, greater LMA but lower LDMC.(3) Both leaf dry mass weight and aboveground biomass of individual plant decreased as soil water deficit turned more serious in three varieties. Longdong variety had a higher productivity under 80% field water capacity(FWC) at 10% flowering stage, While Algonquin and Xinjiangd-aye variety had higher productivity under 80%FWC at 30% flowering stage.(4) The correlations of leaf traits in three varieties of Alfalfa shared something in common, the LMA was positively correlated with LDMC and Nmass, LA was negetively correlated with LMA and LDMC, There was no significant correlation between Chi with other traits. However, different varieties had different impacts upon the correlations. The Nmass was positively correlated with Pmass in Algonquin variety, while in Longdong and Algonquin variety, the Nmass was negatively correlated with LA. Field trials shows that the LMA was positively correlated with LDMC and negatively correlated with LA in Longdong variety.(5) Leaf dry mass weight of individual plant in all varieties was positively correlated with L/W and LA, and negatively correlated with Nmass、LMA and LDMC.Conclusively, the alfalfa can adapt to drought environment through the increases in LT, LDMC, LMA and Nmass or through the decrease in LA. These traits might act coherently and showed variety-specific feature. The LT, LA and LMA were more sensitive to drought, which could be better reasons for alfalfa adaptation to drought. The Chi, L/W and Pmass were easily influenced by envirionmental factors, which were not good traits for explaining how alfalfa adapted to the change in soil water. The LA, L/W, Nmass, LMA, LDMC were good reasons for explaining how alfalfa dry mass weight changed individually.