Studies On The Effects Of Alternaria Tenuis Nees On Its Host Plant Trifolium Repens L. Under Drought Stress

Trifolium repens L. is one of the most important grazings of the world,and drought is one of the stress conditions that often severely effects its growth,but there's little systematic study about the mechanism of its resistance to drought stress. The pathogen Alternaria tenuis Nees was a parasitic fungus on its host plant Trifolium repens L. in nature. In the long-term co-evolution, they had set up steady relationship. Through field investigations it was found that the the A.tenuis colonization can significantly accelerate the wilt of Trifolium repens L. in times of drought. This paper evaluated the effects of Alternaria tenuis Nees colonization on the physiological and biochemical characteristics of T. repens which are related to its drought resistance.The results showed as following:1 Trifolium repens L. seedlings were treated with the osmotic stress solution PEG 6000(-0.8MPa,- 0.4MPa). The content of MDA, cell membrane electrolyte leakage and several defense enzymes activities were measured at the different times after treat- creased in all days except the first day; SOD activities raised and CAT activities declined, at 1 or 2 days after treatment, but SOD activities decreased and CAT activities increased; ASP activities changed, which given a peak at the first and third day respectively under -0.8MPa stress. All of the alterations were in less extent under -0.4MPa stress than under -0.8MPa correspondingly. This results suggested that the system of the defense enzymes might play a key role by balancing all of the responding enzymes under drought condition in Trifolium repens L..2 This paper evaluated the effects of Alternaria tenuis Nees colonization on drought resistance of Trifolium repens L. in a potted culture experiment under natural water stress. The results showed that the A. tenuis colonization can significantly accelerate the rise of leaves'electric conductivity(EC), MDA content of T. repens and alleviate the accumulation of proline in T.repens under draught stress. Moreover, the contents reduction trends of leaves'relative water content(RWC) and ASA in inoculated plants were apparently quickened compared to those in non- inoculated plants. Besides, activities of SOD and POD in inoculated plants were lower or significantly lower than that in non- inoculated plants from the second day; the CAT activity declined rapidly and lower significantly than that in non- inoculated plants at the third day. The results suggest that under natural water stress, the A. tenuis colonization could depress the osmo-regulatory response of T.repens, lower its protective enzyme activities, and aggravate the accumulation of active oxygen and resulted oxidative damage to membrane lipids. Consequently, the drought resistance of inoculated T. repens was depressed.