Effects Of Exogenous Spermidine Treatment On The Growth And Photosynthetic Characteristic Of Lettuce Under Root-Zone And Air High Temperatures

Lettuce(Lactuca sativa L.)is one of the most commonly cultivated vegetables used in hydroponic systems.In the summer,high air temperature,especially in the greenhouse,will cause high root-zone temperature,which hampers the root function seriously.Spermidine(Spd)is a kind of polyamines(PAs),which is closely related to plant stress.This study attempts to explore the effects of exogenous Spd treatment on the growth and photosynthetic characteristics of lettuce under high root-zone temperature and the effect of root-zone cooling treatment(RZC)and Spd root pre-treatment(SRP)on the growth,photosynthetic charateristics and chlorophyll flurescence characteristics during the high temperature season.1.The effects of different root zone temperatures on the growth and photosynthetic characteristics of Lactuca sativa L.were studied by hydroponic cultivation.Lettuce plants were cultivated in nutrient solution with different temperatures,i.e.,20?,25?,30?,35?and natural temperature(CK).Under 25 ? root-zone temperature the lettuce plant fresh and dry weight,the net photosynthetic(PN)and the sotmatal conductance(Gs)were significantly elevated.The apparent quantum yield(AQY)and the maximum photosynthetic rate were increased both under saturation of light intensity and under saturation of CO2 concentration;The carboxylate efficiency(CE)had no significant difference.Meanwhile,under 35 ? root-zone temperature,the PN,Gs,AQY,CE and the maximum photosynthesis rate under saturation of light intensity and under saturation of CO2 concentration were all decreased.This indicated that under 25 ? root-zone temperature the lettuce yield and photosynthesis were increased by improving the Gs,while under 35? root-zone temperature the lettuce yield and photosynthesis were decreased because the heat stress of roots induced the inhibition of photosynthetic system and the relative enzymes.2.The effects of spermidine(Spd)root-pretreatment on the growth and photosynthetic characteristics of Lactuca sativa L.under root-zone heat stress were studied by hydroponic cultivation.The experiment was carried out at the nutrient solution temperature of 35?.The main results showed that with the root-pretreating of 0.1 mmol· L-1 Spd,the fresh weight and dry weight of lettuce plants were significantly higher than those of the control.PN and Gs were significantly elevated.The AQY,CE,and the maximum photosynthetic rate were increased both under saturation of light intensity and under saturation of CO2 concentration,resulting in the improvement of the electron transport efficiency and carbon assimilation efficiency.By Spd root-pretreatment under root-zone heat stress conditions,the photosynthetic system and the relative enzymes were protected and the photosynthesis of lettuce was improved by increasing the efficiency of the carbon assimilation and protecting electron transport chain and carbon assimilation.At the same time,the improvement of Spd root-pretreatment on lettuce yield and photosynthesis was much better than the Spd spraying treatment.3.The effects of spermidine(Spd)spraying on the growth and photosynthetic characteristics of Lactuca sativa L.under root-zone heat stress were studied by hydroponic cultivation.The experiment was carried out at the nutrient solution temperature of 35?.The main results showed that with the spraying of 0.5 mmol· L-1 Spd,the fresh weight and dry weight of lettuce plants were significantly higher than those of the control.PN and Gs were significantly elevated.However,the value of Gs with the Spd leaf spraying treatment was higher than that with the Spd root-pretreating.AQY,CE,and the maximum photosynthetic rate were increased both under saturation of light intensity and under saturation of CO2 concentration,resulting in the improvement of the electron transport efficiency and carbon assimilation efficiency.By Spd spraying under root-zone heat stress conditions,the net photosynthesis and yield of the lettuce plants were improved via increasing the Gs and protecting electron transport chain and carbon assimilation.But the improvement of Spd root-pretreatment on lettuce yield and photosynthesis was much better than the Spd spraying treatment.4.The effects of root zone cooling(at 25 ?)and exogenous Spd root-pretreatment(0.1 mmol·L-1)on the growth,leaf photosynthetic parameters,and chlorophyll fluorescence characteristics were examined in hydroponic lettuce grown in a high-temperature season(average temperature>30 ?).Both treatments significantly promoted plant growth and photosynthesis in the high-temperature season,but the mechanisms of photosynthesis improvement were different.The former improved plant photosynthesis by increasing Gs to enhance CO2 supply,thus promoting photosynthetic electron transport activity and phosphorylation,which improved the level of the photochemical efficiency of photosystem?(PS ?),rather than enhancing CO2 assimilation efficiency.The latter improved plant photosynthesis by enhancing CO2 assimilation efficiency,rather than stomatal regulation.Combination of root zone cooling and Spd root-pretreatment significantly improved PN of lettuce plants in a high-temperature season by both improvement of Gs to enhance CO2 supply and enhancement of CO2 assimilation.The enhancement of photosynthetic efficiency in both treatments was independent of altering light-harvesting or excessive energy dissipation.