Study On The Response Mechanism Of Physiological And Biochemical Parameters Under Water Stress In Dahlia

Taking Dahlia pinnata'Fenxishi'as test material, using potted approach to treat water stress and rewatering of different levels and times , this paper studied the physiological and biochemical parameters of dalia leaves under water stress , as well as photosynthetic and fluorescence parameters. Through the response mechanism of physiological, biochemical, photosynthetic and fluorescence characteristics under water stress, the paper explored the drought-resistant mechanism and screened appropriate drought-resistant index. The results provided the basic data for the future cultivation and breeding of drought-resistant cultivars. It also provided a theoretical basis for the improvement and innovation of drought-resistant germplasm. The main results of this paper were as follows:With the increasing extent and duration of drought stress, the leaf relative water content, water potential, and chlorophyll content of D. pinnata'Fenxishi'decreased significantly. The content of soluble sugar and proline in leaf also increased significantly with increasing extent and duration of drought stress. Especially for proline content, it increased significantly in the later period of moderate and severe stresses, suggesting its significant response to water stress. The leaf soluble protein content had a trend of down-up-down, suggesting that protein was inhibited by water stress first and then be produced"stressed protein"for responsed to drought stress in the later period.Water stress had a significant effect on the systerm of plasma membrane. Production and remove of free radical was out of balance under water stress, and then the membrane lipid peroxidation happened by the metabolic disorder and plasma membrane was damaged, which led to the leak out of massive ions and the accumulation of MDA. The damage of plasma membrane was most serious under severe stress, and could not recover to the control level after rewatering in the short time. The activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) decreased after an initial increase. There were some differences in the responses of the three enzymes to drought stress and reactive oxygen, exhibiting their coordinating role. The reduction of the activity of antioxidant enzymes made the free radicals accumulated, started directly or indirectly membrane lipid peroxidation, which made the plasma membrane damaged and the accumulation of MDA. Meanwhile, the accumulation of MDA inhibited the activities of SOD and POD, which caused the protective enzyme lost the role of protection and the membrane structure be damaged heavier. CAT might caused the damage effect like POD, and participated in the produce of active oxygen and caused membrane lipid peroxidation.The net photosynthesis rate decreased and the daily curve changed from single zenith to double zenith with the influence of water stress, and appeared "nap" phenomenon. The reasons of Pn in dahlia leaves decreaseing were stomatal and nonstomatal limitation respectively in light and severe water stresses. After rewatering, the photosynthesis characteristics could not recover on the serious stress in the short time which did damage to dahlia leaves earlier than other stresses. Fo of dahlia increased uner water stress and its daily changed curve appeared inverted "V" type. Fm, Fv/Fm andΦPSⅡdecreased and the daily changed curves appeared "V" type. qP and NPQ decreased with the deepening of stress level, and NPQ increased in severe stress but not in significant difference. Water stress caused the photoinhibition of dahlia, and curbed the photochemical activity in PSⅡand reduced the light part used in photochemical reactions and actual photochemical efficiency. The effects on the light transmit conditions in PSⅡantenna pigment of dahlia was not signifficant under the light and moderate stress, but great under severe stress. The severe stress made the electron-transfer quantum yield and photosynthetic electronic transfer ability in PSⅡreduced, and sharpened the degree of photoinhibition and limited the normal operation of the photosynthesis. Photosynthetic institutions in dahlias might have reversible deactivation to adapt to water stress, not sufferring irreversible damage and long-term photoinhibition. Severe stress made photoinhibition aggravating, launched the protective mechanism of photochemical quenches,making dahlia against photoinhibition and protect photosynthetic institutions to meet adversity by heat dissipation. The inhibition of photosynthesis in dahlia under water stress was caused by the result of the change of factors by water deficit.The performance of drought resistance in dahlia was the result of comprehensive effect of physiological factors. Between each index there existed certain relevance, and they could be represented by the few comprehensive indexes of principal components using principal component analysis method. The comprehensive principal components reflected the most information of the original index, and judged drought resistance. Photosynthetic and fluorescence indexes, extracted two principal components respectively.The rest of the physiological and biochemical indexes extracted three principal components, which treated as the drought-resistant comprehensive indexes of dahlia. Indexes which had larger eigenvalue were selected directly as drought-resistant indexes. The appropriate appraisal indexes were relative water content, water potential, MDA content, proline content, soluble sugar content, Pn, Fo, Fm, Fv/Fm andΦPSⅡ.Each index had recovered in different degrees after rewatering, while not in severe stress in the short time. The growth and physiological function of dahlia had been seriously affected by severe stress, so the appropriate relative water content in soil for dahlia was more than 30% of maximal water content in field.