Study Of Physiological And Biochemical Response Mechanism Of Cotton Seedlings Under Low Temperature Stress

Cotton(Gossypium hirsutum) is one of the most important economic crops in China and the world, cotton production occupies a very important position in our national economy. Due to the special geographical and climatic environment in Xinjiang, spring climate changed frequently, as quite often happened of "coldness in the late spring" phenomenon, resulted in a significant delay in growth period, then budding, flowering and maturity were postponed too, which greatly affect the final yield and quality.Xinluzao33(cold sensitive variety) and Zhongmiansuo50(high cold resistant variety) were selected as test materials in this study. The physiological and biochemical indexes, including photosynthesis, photoinhibition of photosynthetic system Ⅱ (PSII), activity of antioxidant enzymes, content of osmotic adjustment substance, were determined under different stress-temperature gradient and time gradient of low temperature stress. The purpose of this study was to elaborate the response mechanism of physiological and biochemistry changes under low temperature stress, so as to learn more about the cold resistance mechanism of cotton seedlings, this is helpful to cultivate the cold-resistant cotton varieties, to provide a theoretical basis for discovery and promotion of chilling tolerant germplasm, breeding of new chilling-tolerant varieties and evaluation on chilling tolerance of a large number of cotton varieties. The main conclusions of this experiment were as follows:1. Low temperature stress leaded to that the balance of synthesis and degradation of chlorophyll was broken, the change of content related to the dominant of synthesis or degradation. Low temperature stress affected the net photosynthetic rate (Pn), transpiration rate (Tr), non-photochemical quenching coefficient (NPQ) significantly; when the stress degree was lighter, stomatal conductance (Gs), stomatal limitation value (Ls), intercellular CO2concentration (Ci), the initial fluorescence (Fo), PSII maximum photochemical efficiency(Fv/Fm), PSII actual photochemical efficiency (ΦPSII), photochemical quenching coefficient (qP) could be recovered to approach the level before chilling stress; as the stress density increased, tissues and photochemical reaction centers of leaves were damaged severely, pathways of photosynthetic electron transport were inhibited, photosynthetic capacity of mesophyll cell were decreased, more excitation energy was consumed as the form of heat dissipation, which could contribute to reduction of photoinhibition.2. In the low temperature environment, cotton seedlings could adjust the antioxidant enzyme activity and soluble substance content to maintain metabolism balance and protect the activity of macromolecules, lighten the damage of peroxidation caused by low temperature stress. The damage caused by15℃and10℃treatment could be partially mitigated and restore. Under5℃and0℃treatment, it could improve more the content of soluble sugar (SS) and soluble protein (SP), the activity of superoxide dismutase (SOD) and peroxidase (POD), to strengthen the cold tolerance of cotton seedlings, but the damage was too serious to recover.3. High cold resistant varieties could adjust the chlorophyll content (Chl), Tr and NPQ more sensitively, start the response mechanism of scavenging free radicals and osmotic regulation more rapidly, to protect the photosynthetic institutions, maintaining high water, nutrition absorption and transport power and CO2using ability, and keep high photosynthetic conversion efficiency and photochemical electronic transfer activity, which would enhance chilling adaptability and restore ability.