Regulation Mechanism Of Vapor Pressure Deficit On Water Transport And Photosynthetic Productivity Of Fruit Vegetables At Ifferent Temperatures

Precise regulation of environmental factors in greenhouses is the main technical measure to achieve efficient cultivation of crops in protected agriculture.Air temperature and relative humidity are the main environmental parameters for plant growth.Vapour pressure deficit（VPD）integrates air temperature and relative humidity,which intuitively reflects the dryness of air moisture and is the main driving force for moisture transport.This paper studied the regulation effects of VPD under different temperatures on the water transport and photosynthetic productivity of vegetables（cucumber,muskmelon,tomato,eggplant and pepper as materials,）which under three temperatures:high temperature,moderate temperature and low temperature,and high and low VPD treatments.The paper explored regulation mechanism of VPD on the water transport power of vegetable crops,the anatomical structure of water transport plants and the photosynthetic productivity system at different temperatures.And it analyzed of the resistance of VPD to regulate the CO₂ transport process in the leaves of vegetables.The paper also explored the effects of tomato growth and fruit quality formation by regulating VPD at different growth stages,and analyzed the water transport process,photosynthetic production capacity,fruit characteristic aroma substances and primary metabolomics of VPD at different stages of tomato.finally found the optimal VPD regulation and management indicators in different growth stages of tomato.The main findings are as follows:（1）It clarified the effect of regulating VPD at different temperatures on the photosynthetic productivity of cucumber and other vegetable crops.By analyzing plant net photosynthetic rate,instantaneous water use efficiency,chlorophyll content change,and plant dry matter accumulation,the results show that reducing VPD under high temperature（35℃）and moderate temperature（28℃）environment can significantly improve the photosynthetic capacity and water content of vegetables.Plant leaf area and dry matter accumulation also increased,but chlorophyll content decreased significantly.Elevating VPD in low temperature（18℃）environment promoted the improvement of photosynthetic capacity and water use efficiency,and the total dry weight of plants increased significantly.（2）It clarified the effect of regulating VPD at different temperatures on the water transport system of vegetables.By analyzing plant water potential distribution,stomatal conductance,hydraulic conductance and other water transport dynamic parameters,as well as root,stem,and leaf water conduction structure parameters,it was found that under high temperature（35℃）environment,reducing VPD significantly improved leaf water potential and stomatal conductance.Hydraulic conductivity of the whole plant were significantly improved,which alleviated the air drought adversity caused by high VPD.At the same time,reducing VPD in a high temperature（35°C）environment significantly increased the thickness of leaf palisade tissue and spongy tissue,and enhanced the water storage capacity of leaves.The transmission resistance optimizes the transpiration rate of plants from the two aspects of water transport mobility and resistance,and achieves efficient water transport.In the moderate temperature（28℃）environment,the regulation mode of VPD on the water transport system of vegetables were the same as that at high temperature,but the regulation range of the water transport force and resistance was smaller than that in the high temperature environment.Elevating VPD in a low temperature（18℃）environment can improve the whole plant hydraulic conductance and leaf thickness of vegetables,and other water transport parameters have no significant regulatory effects.（3）It clarified the regulation effect of VPD on the mesophyll conductance of fruit vegetable,including cucumber,muskmelon and watermelon.By analyzing the relationship between leaf anatomy and mesophyll conductance,it was found that reducing VPD significantly improved the mesophyll conductance of the three vegetables.It is dozens of times of the liquid-phase conductance,indicating that the gas-phase conductance is the main influencing factor of the mesophyll conductance.In addition,the improvement of mesophyll conductance comes from the increase of the intercellular space of the mesophyll tissue and the surface area of the mesophyll cells facing the intercellular space.The larger transmission gap was beneficial to the gas-phase transport process of CO₂.At the same time,under the high VPD environment,the three vegetables responded to the decrease of mesophyll conductance by reducing the transport resistance of the chloroplast cell wall and up-regulating the aquaporin PIP family genes.（4）It clarified the effects of regulating environmental VPD on tomato water productivity and fruit volatile matter in different growth stages of tomato.Under the scheme of reducing VPD at the whole growth stage or at the seedling stage and fruit stage,the water transport efficiency and photosynthetic capacity of tomato were significantly improved,and the single fruit weight was also significantly increased.The content of tomato characteristic aromatic substances such as 3-hexenal,methylheptenone,citral,geranium acetone andβ-ionone increased significantly,which improved the flavor and aroma of tomato fruit.（5）It clarified the effects of regulating environmental VPD on the primary fruit metabolome during different growth stages of tomato.A total of 496 metabolites were detected by metabolomic analysis,of which 327 were up-regulated and 169 were down-regulated by VPD.The number of differential metabolites in tomato fruit under the two VPD regulation schemes of reducing VPD at the whole growth stage or at the seedling stage and fruit stage were significantly higher than other treatments.The metabolism of tomato fruit with reduced VPD during whole growth was mainly concentrated in the linoleic acid metabolism pathway,the glutathione metabolism pathway and the ATP transporter pathway.The metabolism of tomato fruit with reduced VPD at seedling and fruit stages was mainly concentrated in the secondary metabolite pathway and linoleic acid metabolism pathway.In addition,reducing VPD during the whole growth period of tomato accelerates the process of fruit glycolysis-tricarboxylic acid cycle and accelerates the ripening process of tomato fruit.