Effects Of Exogenous Melatonin On Carbohydrate And Nitrate Metabolism In Cucumber Seedling Leaves Under High Temperature Stress

When the outside temperature rises5~10℃above the optimum temperature for plantgrowth, it will produce a high temperature stress. High temperature stress can cause a seriesof changes in external morphology, physiological and biochemial characteristics to plants,which will affect plant growth and development, then impact the economic efficiency of theplants. Cucumber is a thermophilic crop, its optimum growth temperature is25~30℃, whenthe temperature is over35℃it will be harmed. With the exacerbating of greenhouse effect inrecent years, whether it is in southern open field culture or northern summer and facilitycultivation, often threated by high temperature stress, the high temperature stress has becomea major obstacle factor that affect cucumber production.In the present study, hot-tolerant cultivar 'American Baoer' and hot-sensitive cucumbercultivar 'Jinyan No.4' were chosen as experimented materials, and by the method of foliarspraying,this paper studied the effects of exogenous melatonin (MT) on the growth,carbohydrate metabolism and nitrogen metabolism system in cucumber seedlings under hightemperature stress. The results showed that:1. The increases of seedling height, stem diameter and leaf width of cucumber undernormal and high temperature were significantly promoted by melatonin treament. Aftermelatonin treatment, the fresh and dry weight of cucumber seedlings were enhanced.Meanwhile the healthy index and dry matter content were increased. Compared with thehot-tolerant cultivar 'American Baoer', exogenous melatonin was more effective in promotingdevelopment for hot-sensitive cucumber cultivar 'Jinyan No.4'.2. Time course changes of carbohydrate contents and activities of the four enzymes inthe seedling leaves were investigated after the treatment. The results showed that compared tothe control seedlings, the seedlings of both tested genotypes under high temperature havesignificantly higher contents of all measured soluble carbohydrates. Under the stress of hightemperature, the activity of sucrose synthase (SS) in both genotypes and the activity of acidinvertase (AI) in hot-sensitive cultivar increased, but the activities of sucrose phosphatesynthase (SPS) and neutral invertase (NI) in both genotypes and the activity of AI inhot-tolerant cultivar decreased. Sucrose and fructose accumulated in both cultivars. Exogenous MT applied stimulated accumulation of soluble sugars and increase of SS activity,kept the stability of other enzymes. The results indicated that exogenous MT can alleviate thedamage of high temperature stress to cucumber seedlings, regulate carbohydrate metabolismof cucumber seedlings under high temperature stress, promote the formation of new metabolicbalance and then improve the ability of cucumber seedlings against high temperature.3. The results showed that under the stress of high temperature, the nitrate content inleaves of both cucumber cultivars increased firstly and then declined later. The total nitrogenor ammonium content in the leaves of 'Jinyou No.4' declined firstly but increased constantlyin 'American Baoer'. The activity of nitrate reductase (NR) increased firstly but thendecreased later in the leaves of both cucumber cultivars, but the activities of glutamatesynthase (GS), glutamine synthetase (GOGAT) and glutamate dehydrogenase (GDH)decreased constantly. All of the four enzyme activities in 'American Baoer' decreased slowerthan those in 'Jinyou No.4'. The exogenous MT could significantly increase the activities ofNR, GS, GOGAT and GDH, increased the nitrate content and restrained the accumulation ofammonium content under high temperature stress. These indicate that exogenous MT couldenhance the nitrogen metabolism of cucumber, reduce the damage of high temperature to theplants, and improve the resistance of cucumber seedlings against high temperature stress.Exogenous MT could promote the growth of cucumber seedlings under high temperaturestress, improve the carbohydrates and nitrogen contents, enhance the activity of enzymes incucumbers to strengthen the ability of cucumbers to coordinate material metabolism, alleviatethe adverse of the stress on the cucumbers and enchance the stress resistance.