Effects Of Vapor Pressure Difference(VPD) And Potassium On Nutrient Absorption And Photosynthetic Physiology Of Tomato At Different Temperatures

Vapour pressure difference（VPD）refers to the difference between the saturated water vapor pressure and the actual water vapor pressure at a certain temperature.It is a reflection of the air water potential and has a significant effect on crop water evaporation.Potassium is the most demanded element in plant growth and development,and participates in many physiological and biochemical processes in plants.This experiment uses tomatoes variety’Jinpeng 14-6’as materials,which were conducted in the greenhouse and the artificial climate room,and set high and low VPD conditions and three potassium levels(2.4,8 mmol-L^-1)to observe and analyze the effects of VPD and potassium on tomato nutrition absorption,dry matter distribution,gas exchange parameters,photosynthetic response,chlorophyll fluorescence and stomatal conductance of tomato under high and low temperature.This paper aims to explore the nutrient absorption mechanism of tomato under different VPD so as to provide theoretical basis for nutrient management and balanced fertilization of tomato cultivation in greenhouse.The main results were as follows:In the greenhouse experiment in summer,compared with high VPD,low VPD significantly increased the total root length,average root diameter,total root volume,total root surface area,number of root tips and number of root forks.The increase of the number of root forks was the largest,increasing by 105.8,172.6 and 37.7%respectively at the potassium levels of 2,4 and 8 mmol/L;meanwhile,reducing VPD significantly increased the proportion of fine roots with a diameter of 0-0.5mm.At medium potassium levels,the ratio of fine root length to total root length was the highest,reaching 78.4%.Low VPD significantly reduced the potassium content of the leaves,but with the increase in root morphology and rapid accumulation of dry matter,to a certain extent,it compensated for the effect of reduced transpiration-driven mass flow on potassium absorption under low VPD.The amount is significantly higher than that of high VPD.VPD×K showed significant interactions on net photosynthetic rate（A）,leaf relative water content（RWC）and leaf water potential(ψ_Leaf);and the A,gs and Ci were significantly higher than that of high VPD,RWC andψ_Leaf also showed a similar trend.Low VPD improves the dry matter distribution rate of the leaves and reduces the dry matter distribution rate of the stems.On the contrary,low VPD promotes the distribution of potassium in the stems and roots,while reducing the distribution of potassium in the leaves.In the high-temperature artificial climate chamber experiment,at high VPD,the accumulation of dry matter and N,P,and K will gradually increase with the increase of potassium level;at low VPD,the medium potassium level is more conducive to the accumulation of dry matter and N,P,and K.N accumulation and leaf water potential were significantly higher than the other five treatments.Compared with high VPD,reducing VPD will have a dilution effect on the N,P,and K content in each organ of the seedling plant,but the accumulation of N,P,and K significantly increases.And plants under low VPD have larger light,CO₂ concentration adaptation interval and high photosynthesis and related parameters,including A,gs,LSP,AQY,R_d,and the CE,etc.,and all reach the maximum at the medium potassium level;at the same time,E,LCP,and R_p were all significantly reduced.It is generally believed that high potassium levels under high VPD and medium potassium levels under low VPD improve tomato photosynthetic performance,N,P,K absorption and dry matter accumulation,which can be used as indicators of high temperature fertilization and environmental management in summer.In the low-temperature artificial climate chamber experiment,low VPD increased the plant’s stomatal conductance（gs）and mesophyll conductance（gm）.The former is mainly related to the significant increase in stomatal density and stomatal size,while the latter is related to VPD and potassium improving the mesophyll anatomy and reducing the diffusion resistance of CO₂.The increased gs and gm were beneficial to the diffusion of CO₂,increased the Ci and Cc,thus enhanced the photosynthesis of the plants.Low VPD significantly increased the potassium content of leaves,but had no significant effect on the potassium content of roots and stems.Similar to the results of the high temperature test,the reduction of VPD also significantly increased the accumulation of potassium in each organ.The photosynthetic performance of the plants was also improved by adding potassium under low VPD,which was mainly related to increased light energy utilization and decreased heat dissipation specifically manifested as photosynthetic pigments,chlorophyll fluorescence parameters(F_v/F_m,F_v’/F_m’,Φ_PSII,ETR and q P)and related antioxidant enzymes（SOD and CAT）increased activity,Car/Chl ratio,active oxygen(H₂O₂ and O^2-)content and NPQ decreased.In conclusion,under low temperature environment,reducing VPD and increasing potassium can alleviate stomatal restriction,improve photosynthetic performance of plants and reduce low temperature stress of tomatoes.