Effects Of High Temperature And CO₂ Coupling On Physiology,Biochemistry,and Metabonomics Of Greenhouse Cucumber In The Fruiting Period

Based on the fact of global warming,the increase of CO₂concentration,and the practical problem of"CO₂deficiency"in summer greenhouse cultivation,the cucumber"Jinyou 35",which has a wide cultivation area in a solar greenhouse in northern China,was selected as the research object.The effects of high temperature,different concentrations of CO₂,high temperature,and suitable concentration of CO₂on the morphological structure,photosynthesis,physiology and biochemistry,yield,and quality of greenhouse cucumbers were designed and studied.The effects of high temperature and a suitable concentration of CO₂on the metabonomics of cucumber leaves and fruits in the greenhouse were studied.The main results are as follows:（1）High temperature affects the morphological,physiological,and biochemical processes of cucumber by hindering photosynthesis,source-sink transport and changing the activities of enzymes and non-enzymes in plants,accelerating plant senescence,shortening the stages of nutrition and reproduction,and reducing the yield and quality of cucumber in the greenhouse.The increased application of CO₂alleviated the heat stress of cucumber caused by high temperature in a concentration-dependent and time-dependent manner and promoted the growth of cucumber.The net photosynthetic rate of cucumber leaves enriched with different concentrations of CO₂increased synchronously with the increase in CO₂concentration.The enrichment of CO₂inhibited the damage to photosynthetic apparatus caused by high-temperature stress,increased the optimum temperature of photosynthesis of cucumber leaves,increased photosynthetic leaf area,and photosynthetic yield,and increased antioxidant enzyme activity and fruit yield.Under long-term high CO₂concentrations,the excessive accumulation of starch and the decrease of the activities of photosynthesis and antioxidant enzymes in cucumber leaves had adverse effects on photosynthesis.When cucumber is planted in the solar greenhouse in summer in the north,a concentration of 1200 umol·mol^-1is suitable for high-temperature enrichment of CO₂.（2）The application of CO₂could significantly improve cucumber’s morphological and physiological indexes under different temperature conditions.The interaction of high temperature and a suitable concentration of CO₂could increase the photosynthetic gas exchange parameters,chlorophyll fluorescence parameters,Rubisco activity,and RCA activity of leaves treated with high temperature and rich CO₂,and the content of soluble sugar and starch in leaves increased,and the activity of antioxidant enzymes increased.The stomatal size of high-temperature enriched CO₂treatment was smaller than that of average temperature enriched CO₂treatment.However,due to higher stomatal density and stomatal opening,it still obtained a higher stomatal opening ratio of 0.4%during high-temperature stress,which was beneficial to cucumber leaves absorbing CO₂through stomata,reducing the limitation of stomatal on photosynthetic carbon assimilation during photosynthetic"lunch break."The coupling effect of high temperature and CO₂can improve the photoprotection ability of cucumber leaves to convert excess light energy into heat energy,accelerate the electron transfer rate of photosystemⅡ,protect the photosynthetic structure and improve photosynthetic activity,which is beneficial to the physiological adaptation of cucumber plants to short-term and long-term high-temperature stress.（3）The number of chloroplasts in the high temperature plus CO₂treatment group was significantly more than in other treatments.The chloroplast structure was intact,the internal thylakoid layers were more,and the structure was compact.Taking chloroplast cells as a unit,the volume of starch grains in greenhouse cucumber leaves treated with high temperature plus CO₂treatment was more significant than that of high-temperature treatment but smaller than that of regular temperature treatment with CO₂treatment.In the CO₂enrichment treatment at average temperature,the accumulation of starch grains in cucumber leaves expanded the chloroplast volume,destroyed the internal structure,and accelerated leaf senescence.（4）The temperature and CO₂concentration increase significantly increased the number of fruits per plant,fruit quality,and yield per plant,thus obtaining the highest harvest in the same picking cycle.High-temperature enrichment of CO₂had no significant effect on cucumber fruit’s soluble sugar and starch content.However,it could significantly increase the ascorbic acid content and effectively reduce the content of nitrite.（5）In the study of the coupling effect of high temperature and a suitable concentration of CO₂on metabonomics of cucumber leaves in the greenhouse,the metabolite ion peaks were extracted by XCMS software,and 6124 positive ion peaks and6652 negative ion peaks were obtained.In the positive ion mode,screened 69 differential metabolites.In the negative ion mode,screened 104 differential metabolites.The number of metabolites in cucumber leaves increased significantly with applying CO₂under high temperatures.Under the coupling effect of high temperature and a suitable concentration of CO₂,the primary metabolites with a significant difference were photosynthate sugars.The changing trend of differential metabolites was similar to that of biochemical indexes in the corresponding treatment.There were differences in the effects of temperature and CO₂on carbohydrate metabolites in different treatments.Sucrose and raffinose were significantly affected by high temperatures,and CO₂mainly affected the concentration of mannose,galactose,raffinose,tagatose,linolenic acid,glyceric acid,and other metabolites.（6）In the study of the coupling effect of high temperature and a suitable concentration of CO2 on the metabonomics of cucumber fruit in the greenhouse,the metabolite ion peaks were extracted by XCMS software,and 2505 positive ion peaks and3136 negative ion peaks were obtained.Seventy-nine differential metabolites were screened in positive ion mode and screened 78 differential metabolites in negative ion mode.High temperature mainly caused the changes of amino acids,sugars,and organic acid metabolites in cucumber fruits.The changes of differential metabolites of sugar and organic acids caused by the change of CO₂concentration were closely related to the transport and transport of starch and soluble sugar in fruit physiological indexes.In the high temperature+CO₂treatment-high temperature treatment group,the up-regulated accumulation of metabolites was closely related to the resistance to high-temperature stress.（7）Compared with the effect of high-temperature stress,the change in CO₂concentration significantly impacted the metabolic pathways involved.The changes in metabolic pathways of differential metabolites in cucumber leaves and fruits are mainly related to the galactose metabolic pathway（map00052）,starch and sucrose metabolic pathway（map00500）,and carbon metabolic pathway（map01200）.The paths of change and enrichment are closely related to the accumulation and transport of photosynthates and the enhancement of plant heat tolerance.