Effects Of Water Vapor Pressure Deficit On Photosynthesis And Diffusional Pathway For Carbon Dioxide In Tomato Leaves

The scientific regulation of the greenhouse environment is the major technology to achieve high-quality,efficient and intelligent management in greenhouse.Air humidity is an important factor in the regulation of greenhouse environment.The vapor pressure deficit?VPD?is one of the characterizations of air humidity which reflects the level of atmospheric water deficit.This study focused on the problem of unclear response pathways for photosynthesis to VPD changes in tomato.Different VPD treatments were set up to evaluate the sensitivity of different tomato varieties to VPD changes.It also analyzed the variation of photosynthetic physiological parameters with VPD,quantified the limitation of stomatal conductance and mesophyll conductance on photosynthesis,observed the anatomical features of tomato leaves,stems and roots,and clarified the anatomical features of stomatal conductance and mesophyll conductance,compared the daily variation of gas exchange parameters under different VPD treatments.Finally,it reveals the mechanism of photosynthesis response to VPD changes and provides theoretical basis about optimal regulation and high-yield cultivation in greenhouse.Below are key research findings:?1?Comprehensive evaluation of the sensitivity of different tomato varieties to VPD changes.By analyzing the leaf water retention rate,chlorophyll content,leaf area,total dry weight,root-shoot ratio,specific leaf weight and strong seedling index of 15 tomato varieties,it was found that the variation degree of different indexes was different when VPD was changed,and the variation of leaf water retention rate and root-shoot ratio was the largest.The variation of leaf area was little.In addition,there was a correlation between the different indicators.Principal component analysis method can be used to extract multiple indicators into three mutually opposing comprehensive indicators.Combining membership function method and cluster analysis method,15 tomato varieties were divided into four categories:insensitive varieties?Jinpeng NO.1,Jinpeng Chaoguan and Shenfen No.11?,less sensitive varieties?Dongya Fenguan,Hongdenglong and Hezuo 903?,more sensitive varieties?Maofen802,Dongsheng NO.7,Zhongza No.9,Nongbofen NO.3 and Fenguo 06-2?and sensitive varieties?Jinpeng14-6,Jinfen No.9,Zhongza 105 and Jinfen No.2?.?2?Effects of VPD on photosynthesis.Using the insensitive tomato variety“Jinpeng NO.1”?JP?and the sensitive tomato variety“Zhongza 105”?ZZ?as materials,it was found that there was no significant change in the net assimilation rate of JP under high VPD treatment compared with low VPD treatment.And the net assimilation rate of ZZ significantly dropped by 21%.The decrease of ZZ net assimilation rate under high VPD is mainly due to the decrease of stomatal conductance,followed by the decrease of mesophyll conductance and the limitation of biochemical action.The decrease of stomatal conductance in ZZ under high VPD caused a decrease in intercellular CO₂ concentration,accompanied by a decrease in mesophyll conductance,which further reduced the CO₂ content in the chloroplast.The lack of CO₂ supply in the chloroplast led to reducing photosynthesis.?3?Regulation of stomatal conductance are mediated by stomatal morphology and dynamic.High VPD treatment had no significant effect on the stomatal morphology and stomatal conductance/maximal stomatal conductance of JP,but the stomatal density,stomatal size and stomatal conductance/maximal stomatal conductance of ZZ decreased significantly.48%of the change in stomatal conductance under high VPD treatment can be explained by changes in stomatal density and stomatal size,due to changes in stomatal opening in ZZ.The JP transpiration rate in High VPD treatment increased by 21%,and the xylem diameter,root volume and aquaporin gene expression also increased.It shows that JP under high VPD improves the water supply capacity to chive the high transpiration demand,so that the stomatal conductance is maintained at the level of low VPD treatment.The ZZ leaf vein density decreased significantly under high VPD treatment,and the xylem diameter of the veins decreased,while the morphological structure of stems and roots showed no significant difference,indicating that the ZZ of high VPD decreased the stomatal density and size,but the water supply could not maintain the normal transpiration demand,which resulting to a smaller opening of the stomata and prevents excessive transpiration.?4?Effects of leaf anatomical structure on leaf mesophyll conductance.By analyzing the relationship between the mesophyll conductance and the anatomy of mesophyll,it was found that the change of the anatomical structure of the mesophyll determines the change of the mesophyll conductance.There were no significant changes in the anatomical structure of JP mesophyll under high VPD treatment.Compared with low VPD treatment,there was no difference in the gas phase diffusion conductance of CO₂ in ZZ mesophyll tissue under high VPD treatment,and the liquid phase diffusion conductance decreased,indicating that the decrease of leaf mesophyll conductance was caused by the decrease of liquid phase diffusion conductance.In the liquid phase diffusion pathway,the chloroplast surface area of ZZ facing the intercellular space was significantly reduced under high VPD,and the distance between the chloroplast and the cell membrane was increased.This indicates that the chloroplast area for absorbing CO₂ is reduced,and the diffusion resistance of CO₂ in the cytoplasm is increased,which affects the entry of CO₂ into the chloroplast.?5?Clarification of the limiting factors for greenhouse daily changes in photosynthesis.The diurnal curve of photosynthesis and stomatal conductance of JP is a"double peak"curve under low VPD treatment and a"single peak"curve under high VPD treatment.The intercellular CO₂ concentration is W-shaped under low VPD treatment and V-shaped under high VPD treatment.It is indicated that photosynthesis is mainly restricted by stomatal factors under low VPD treatment,while photosynthetic"noon break"is caused by non-stomatal factors.Photosynthesis under high VPD treatment is mainly limited by non-stomatal factors.The transpiration rate was significantly higher than that of low VPD treatment under high VPD treatment,and the distribution of dry matter of tomato plants to roots increased under high VPD treatment,indicating that the high transpiration of JP in greenhouse high VPD environment was mainly maintained by enhancing root water absorption.In addition,the regulation of VPD should be combined with the adjustment of crop planting density to achieve optimal water use efficiency.