Effect Of Nodules On Cold Stress Response In Alfalfa (Medicago Sativa L.)

Alfalfa (Medicago sativa L.), the perennial grasses of Legume, has high protein contentand nutritional value, is widely cultivated in the world. It occupies an extremely important positionin the Animal Husbandry Industry. In China, low temperature is the main limiting factor. It issensitive to adversity with rhizobia, wich produce biological nitrogen fixation. Our research is tostudy effect of nodules on cold stress response in alfalfa. We select Ladak+as experimentalmaterial with three different ways of dealing, noduled-activated(NA), noduled-inactivated(NI) andnon-noduled(NN). In low temperature (--6℃), alfalfa is divided into6groups, and treated with0h,2h,4h,6h,8h,10h. We study the survival rate with water and the mechanism changes. Theresults were as follows:1. Treated with8hour, the plant seedling survival rate was only2.5%, which shows thatthe Critical Death Point of the selected alfalfa is8hour((--6℃). With time extend, the survivalrate declined. The cold tolerance showed: NA＞NI＞NI, and cold tolerance of NA were morepronounced. The result shows that Rhizobia symbiosis has an active effect for cold tolerance ofalfalfa.2. With cold stress, the cell membrane permeability increased, the exosmose ofintracellular solute increased, and the conductivity increased. At the early period, theconductivity increased slowly as stress time increased. After phytoremediation balance, theconductivity increased quickly. NA reacted to the stress rapidly, its conductivity increased at alater time, and it had a longer tolerance time. NI、NN took second place. The conductivity increaseof NI was less than NN. To a certain extent, rhizobia symbiosis can enhance sensitivity ofadversity, and quickly responded to it, to reduce or eliminate the possible hazards throughself-regulation.3. In the stress, POD activity of the underground part is higher than the aerial part, andthe underground part respond to adversity firstly. At the beginning, the change of environmenthad a negative effect on POD activity. NA dropped less than others, had stronger adaptation. Aperiod of time, the plant produced coping mechanisms, POD activity increased. As the stressdegree deepened, POD activity increased or reduced. In the stress, change range of NA-PODactivity was smaller, had a longer resistant time. 4. Cold stress had a big influence upon SOD activity of the aerial part. NA-SOD hadthe strongest activity. In the underground part, NA had a longer tolerance time. The change rangeof NI-SOD activity was smaller，NN-SOD activity was quite on the contrary.5. MDA content, both the aerial part and the underground part, increased with theincrease of stress time. The underground part began to increase after a longer stress time, but itchanged relatively quickly in the aerial part. The time that content of the underground part of NAreached peak was later than NI and NN. The level of MDA in the aerial part: NN>NI>NA.NA-MDA had the lowest level, NN-MDA was in the opposite way.6. In the underground part of alfalfa, adversity had small effect on NA-CAT activity,NA showed better tolerance. The level of CAT activity in the aerial part: NA＞NI＞NN. Thenoduled had higher level of CAT activity, and the noduled-activated showed stronger tolerance.7. In a certain intensity of cold stress, the content proline of the underground part andthe aerial part increased gradually. There were no significant difference among NA, NI and NN.Additionally, the content of NN-proline were lower. The noduled showed higher ability to adjustosmotic regulation.8. In the condition of cold stress, the change trend of soluble protein content in theunderground part of NA and NI were basically same, NI and NN were basically same in the aerialpart. NA soluble protein content in the underground part and the aerial part increased with stresslevel increased, but the change was small. The content of NA was less affected by the stress, andhad a smaller change.9. With no stress treatment, content of soluble sugar in NA was the largest. In a certainstress, as the increase of the time, the soluble sugar content increased. When dealing with6h,there were a significant difference (p <0.05) in the underground part among NA, NI and NN,and NA was the highest. The level of soluble sugar in the aerial part: NI> NA> NN. Solublesugar content in the normal growth could be improved with rhizobium. Under the same stresslevel, the noduled had higher soluble sugar. The soluble sugar content of the underground partcould be improved markedly in the noduled-activated.The results can be concluded as followed:(1) In the cold stress of-6℃, the CriticalDeath Point of Ladak+is8hours;(2) In adversity, rhizobia symbiosis is beneficial to growth ofalfalfa;(3) Compared with the non-noduled at the same stress, the noduled has higher osmoticregulation ability, non-enzymatic antioxidant content and antioxidase activity, and has lower leafconductivity change, lipid peroxidation. It has the positive role in the cole tolerance;(4)Compared with the non-noduled, cole tolerance of the noduled-activated is better than thenoduled-inactivated.