Response Of Stomatal Conductance And Phytohormones Of Leaves To Vapor Pressure Deficit In Some Species Of Plants

Stomata of leaves play a fundamental role in both acquiring carbon and limiting water loss. Stomatal conductance (gs) is influenced by light, temperature, water supply and the stomatal opening of closing is regulated by phytohormones. Plants have developed advanced strategies and mechanisms through evolution to adapt to local and global environmental changes by compromising photosynthesis and transpiration, and they also affect and promote local and global changes.Stomatal behavior of plant is a hot topic of research on plant physiological ecology. A number of models on stomatal conductance response to different environmental factors have been developed since1970’s. The main stomatal models until now could be divided into two types. One is empirical model, The other type of stomatal model is based on the theory of optimal stomatal behavior. The prevailing pattern regarding the relationship between gs and atmospheric water content is that increasing vapor pressure deficit (VPD) or decreasing relative humidity (RH) lead to reduction of gs. Although different stomatal conductance models are proposed or modified continuously, the essential relationship between gs and VPD (or RH) does not change. Some of present stomatal conductance models had shown poor function compared with measured data. For example, Jarvis’ model and Ball-Woodrow-Berry (BWB) model poorly predated values of gs under high RH condition,and Leuning model may not be appropriate for measured data analysis and ecosystem simulation applications in arid and semiarid zones by comparing the predicted data with measured data of three major species in a semi-arid site. Pseudotsuga menziesii and Selaginella bryopteris et al.showed one top lines’pattern of gs to VPD. All these findings showed diverse and different response patterns of gs to VPD.The response of stomata to environmental and physiological factors is complex. The phytohormone is the important factor to affect stomatal regulation, like abscisic acid (ABA), indole-3-acetic acid (IAA) and Gibberellins (GAS). Increased level of ABA when plant is under stress promotes stomatal closure and/or inhibits stomata opening in order to avoid excess water loss. The other phytohormone, including IAA and GAs are known to antagonize the effects of ABA on stomatal behavior.In this study, the responsive patterns of stomatal conductance to air humidity in three contrasting sites in China:Jinan (N36°35’-36°40’, E116°54’-117°02’), Shandong Province; Turpan (N41°12’-43°40’,E87°16’-91°55’) and Urumqi (N42°45’-45°00’, E86°37’-88°58’) in Xinjiang Uygur Autonomous Region. More than10temperate tree species were measured, including(Fraxinus chinensis Roxb., Populus alba L. var. pyramidalis Bge., Populus tomentosa Carr., Populus euphratica, Cerasus serrulata (Lindl.) G. Don ex London, Prunus serrulata var. lannesiana (Carr.) Rehd., Populus x euramericana ’Neva’, Populus deltoides, Magnolia denudata Desr., Chaenomeles cathayensis (Hemsl.) Schneid., Ginkgo biloba Linn., Platanus x acerifolia(Ait.) Willd., Prunus cerasifera Ehrhar f. atropurpurea (Jacq.) Rehd, Populus alba Linn.). Four tree species (F chinensis, P. alba var., P. tomentosa, P. euphratica) were measured to research the response patterns of stomatal conductance to VPD under very different climate conditions (Jinan, Urumqi, Trupan). Three tree species (F. chinensis, P. alba var., P. tomentosa) were measured to research the response patterns of stomatal conductance to VPD in different seasons (early summer, midsummer, late autumn). Meanwhile, three kinds of phytohormones (abscisic acid, ABA; gibberellic acid, GA3; indole-3-acetic acid, IAA) in leaves of four tree specie (F. chinensis, P. alba var., P. tomentosa, P. euphratica) in three sites were also measured with high performance liquid chromatography. In addition, the two stomatal models (Leuning model and the optimal stomatal model) were tested and compared between the predicted and measured stomatal conductance values in the leaves of the trees.1) Response of stomatal conductance to VPD in four species of trees (F. chinensis, P. alba var., P. tomentosa, P. euphratica) in three different climate zones (Jinan with typical warm humid/semi-humid climate, Urumqi with temperate continental arid climate, Turpan with extreme arid desert climate) were measured. The results showed that:the response of gs to a gradient, of increasing VPD in four tree species in three sites performed peak curves that could be fitted with a Log Normal Model (gs=a·exp(-0.5(ln(D/c)/b)2), D=VPD, R2=0.845-0.996) using measured data in this study. The VPD/RH values corresponding to the maximum of gs (gs-max-VPD/RH) can be calculated using the fitting models for four tree species in three sites. We found that the gs-max-VPD correlated negatively with air relative humidity in three sites during the plant growth season (April-October,2010).The test of the sensitivity of gs to VPD in four tree species in three different climate zones was carried out using Root Mean Square Error (RMSE, σ) of stomatal conductance along the gradient of increasing VPD. Some measured data was chosen to test RMSE of stomatal conductance within the same certain range of VPD in four trees in three sites. The larger a related to the higher sensitivity of gs to VPD. The test demonstrated that the sensitivity in response of gs to VPD showed positively correlation with the concentration of ABA in four trees in three sites, which implied that the ABA level in leaves could be used as one of indicators of sensitivity of stomatal response to VPD.Net photosynthesis rate (An) did not show a significant variation with increasing VPD in four tree species in Jinan, but steadily increased with increasing VPD in Xinjiang. Transpiration rates (E) in four trees remained constantly increasing following an increase in VPD in three sites. P. alba var. in Turpan has the higher An, E and gs than F. chinensis in Trupan and the four trees in the other two sites. The water use efficiency (WUE) in four trees did not show obvious difference under very different climatic conditions, especially F. chinensis could perform good control ability of the stomatal regulation even under extremely arid climate zone.The prevailing empirical model of stomatal conductance to VPD (Leuning stomatal model) and optimal stomatal behavior model could not properly simulate our measurement data with F-test.2) The role of vapor pressure deficit (VPD) in regulating leaf gas exchange of three species of trees (Fraxinus chinensis Roxb., Populus alba L. var. pyramidalis Bge. and Populus tomentosa Carr.) was investigated in Jinan, China. Experiments were performed in early summer (June), midsummer (August) and late autumn (October). Three kinds of phytohormones (GA3, IAA, ABA) in the leaves of the three trees were determined with measured gas exchange. The responses of stomatal conductance (gs) to a gradient of increasing VPD in the leaves of the three trees exhibited a peak curve under different seasons, which differed from the prevailing response pattern of gs to VPD in mostly literature. The peak curves could be fitted with a Log Normal Model (gs=a·exp(-0.5(ln(D/c)/b)2), D=VPD, R2=0.838-0.995). The VPD/RH values of corresponding maximum of gs (gs-max-VPD/RH) could be calculated by fitted models of peak curves of gs to VPD. The RH values, corresponding the gs-max, positively correlated with the mean monthly temperature (R2>0.81) in2010.Two typical stomatal models (Leuning stomatal model and the optimal stomatal model), were used to estimate gs values, and they were poorly performed the prediction of gs in three trees with F-test.The test of the sensitivity of gs to VPD in three tree species was carried out using RMSE, too. Some measured data was chosen to test RMSE of stomatal conductance within the same certain range of VPD in three trees in three seasons. The larger σ related to the higher sensitivity of gs to VPD. The test demonstrated that the concentration of ABA was positively correlated to sensitivity in response of stomatal conductance to VPD in leaves of species of trees under different seasons, which also implied that the ABA level in leaves could be used as one of indicators of sensitivity of stomatal response to VPD.Assimilation continued undiminished in spite of the declined gs at low and high VPD, which demonstrated An was not sensitive to VPD when they growed up in plentiful rainfall area. Transpiration rates (E) in three trees remained constantly increasing following an increase in VPD in three seasons, and the water use efficiency did not show obvious difference under different seasons. These results were same to the result of the first part.However, An of F.chinensis in October decreased sharply. This founding was associated with the senescence in plant leaves. The Chlorophyll concentration of F.chinensis decreased significantly in October, and it’s lower than the other trees’.3) Ten decideous tree species (C. serrulata, P. lannesiana,1-69,1-107, M. denudata, C. cathayensis, G. biloba, P. acerifolia, P. cerasifera, P. alba) were measured to study the response of gs to vapor pressure deficit in April to May,2009-2013. The result showed that the responses of gs to increasing VPD in leaves of ten trees also exhibited a peak curve, which could be fitted with a Log Normal Model (gs=a-exp(-0.5(ln(D/c)/b)2), D=VPD, R2=0.885-0.987). These results proved our findings about the one peak curve in the response of gs to VPD in different climate zones and in different seasons.Taken together, our results reveal that there is an optimal RH/VPD to plant. The stomatal conductance of leaves will decrease when RH/VPD is higher or lower than the optimal RH/VPD. The same results of more than ten tree species in the response of gs to VPD we gotted here, illustrated that this response characteristic of stomatal conductance to RH/VPD could have the universality to plant. However, this result did not conform to the current popular stomatal model of response of gs to VPD. Stomatal conductance models were widely used to simulate the response of gs to environmental factors in trees, predict the vegetation growth of the world terrestrial ecosystem, and predict the impaction on global climatic change. However, this result of the response of gs to VPD showed that the current popular models of stomatal conductance could not predict the global terrestrial vegetation productivity perfectly. The prediction of response of gs to VPD might be incomplete with the two current popular models; the prediction of the plant production in the world and global climatic change might also be inaccurate with current popular model. Therefore, a more perfect gs model which could be able to integrate this founding was needed. Meanwhile, the stomatal response mechanism of one peak curves on gs to VPD could be the focus of further research.