Drought-Tolerance Mechanism And Manipulative Models Of Root-Signal ABA In Alfalfa(Medicago Sativa L.)under Water Stress

Available water for agriculture is becoming scare and there is an urgent necessity to adopt effective strategies of irrigation management. Deficit irrigation is considered as water- saving irrigation strategy. The effect of water deficit on morphological, physiological, some biochemical parameters and yield related to water deficit tolerance was studied in alfalfa(Medicago sativa L).The experiment was conducted in greenhouse of the Chinese Academy of Agricultural Sciences(CAAS), Beijing, China from September 2014 to May 2015. Three alfalfa varieties(Aohan, Zhongmu No.1 and Suntory) were exposed to four irrigation levels of water-holding capacity viz 100%(w1)(control); 85%(w2); 70%(w3) and 55%(w4), which were considered as deficit irrigation treatments. The results showed that morphological, physiological, some biochemical parameters and forage yield of alfalfa varieties were significantly affected by deficit irrigation treatments. The main results summarized as follows: 1- Increasing irrigation deficiency exhibited a reduction in morphological and forage yield of alfalfa. w4 obtained the maximum decreased in plant height, number of branches, stem diameter and shoot dry biomass as 45.95%, 25.41%, 34.38% and 67.92% respectively compared with the control. Moreover, the results showed that plant height, stem diameter, days to flowering and shoot dry biomass was differently affected among different varieties of alfalfa. Zhongmu No.1 was significantly taller(48.16 cm), highest stem diameter(2.29 mm), early flowering(198 days) and out yields the other two varieties(20.41g/plant). 2- The results showed that ABA content in roots of alfalfa was significantly affected by deficit irrigation treatments during different plant growth stages. With the exception of 90 and 105 days after transplanting, ABA increased after transplanting up to harvest in all treatments. Increasing deficit irrigation showed significant increase in ABA content in roots of alfalfa. The minimum content of ABA was obtained from w1(44 ng/g.FW) and then increased to 56.6, 64.6 and 94.4 ng/g.FW for w2, w3 and w4 respectively on 105 days after transplanting. Moreover, the results showed that ABA content was differently affected among different varieties of alfalfa under different water stress levels. The minimum ABA content was 49.9, 61.7 and 83.2ng/g.FW obtained with Zhongmu No.1, Suntory and Aohan varieties respectively, at 105 days after transplanting. Highest water stress w4(55%) reduced root length by 20.92%, lateral roots by 20.71%, root fresh biomass by 43.79% and root dry biomass by 37.96%. The root to shoot ratio was 1.9 times higher in water stressed plants compared with the control, indicating that water stress in alfalfa is affected more shoot growth than root one. 3- The result showed that, ABA, MDA, SOD and Sugar contents in leaves of alfalfa were significantly affected by deficit irrigation treatments during different times of plant growth stages. With exception to 90 and 105 days after transplanting, ABA in leaves of alfalfa was increased after transplanting up to harvest in all treatments. Increasing deficit irrigation showed significant increased in ABA content. The minimum ABA content was 133.2ng/g.FW obtained under w1 and then increased to 138.1, 141.7 and 145.9 ng/g.FW for w2, w3 and w4 at 105 days after transplanting respectively. Moreover the result showed that, ABA was affected among different varieties of alfalfa under water stress. The minimum ABA content was 135.3, 138.8 and 145.1 ng/g.FW which was obtained by Zhongmu No.1, Suntory and Aohan at 105 days after transplanting respectively. The result showed that activities of MDA and SOD were affected by deficit irrigation at different plant growth stages. Increasing deficit irrigation showed a significant increase in MDA and SOD enzyme. The period for high activity of MDA enzymes was 105 and 120 days after transplanting while for SOD activity was 105 days after transplanting. Moreover, soluble sugar content increased with increasing deficit irrigation. In period of 105 days after transplanting soluble sugar content had a lower value. The results showed that, MDA, SOD and soluble sugar content was affected among different varieties of alfalfa under water stress. Zhongmu No.1obtaind the lowest MDA, SOD and soluble sugar content, while Aohan was the heighst one.Finally, we concluded that, deficit irrigation was decreased in morphological and forage yield of alfalfa. Water stress in alfalfa is affected more shoot growth than root one.The minimum ABA content was obtained at 105 days after transplanting. Moreover, the maximum activity of MDA enzyme was obtained at105 and 120 days after transplanting while the maximum activity of SOD enzyme was obtained at 105 days after transplanting.Thus, the period from 105 to 120 days after transplanting is considered critical period of irrigation of alfalfa. Aohan observed heighest ABA content in roots and shoots, heighest SOD, MDA and soluble sugar content. Moreover, it was late flowering, there for Aohan was more tolerant variety to drought stress than Zhongmu No.1 and Suntory.