Stomatal Responses To Soil And Air Humidity And Models Of Stomatal Conductance

Terrestrial plants inhale CO₂through stomata on leaves in photosynthesis, andmeanwhile water dissipate out of stomata as stomatal transpiration. Water consumed bytranspiration is supplied by the root and stem. The driving force for the water supplyderives from transpiration, i.e. water supply is controlled by water demand. On the otherhand, transpiration relies on water supply that is usually restricted. Plants regulate stomatalconductance to control transpiration. Soil humidity and air humidity are two importantenvironmental factors relative to stomatal regulation.Stomata respond to transpiration rate rather than air humidity per se. The model ofBuckley et al. and Dewar’s model both can model the general stomatal response to watervapor deficit at leaf surface （D_s）: stomatal conductance decreases linearly as transpirationrate increases because of rising D_s. The absolute value of the slope increases with theeffective hydraulic resistance from soil to leaf epidermis in the model of Buckley et al. InDewar’s model, however, the absolute value of the slope is determined by the hydraulicresistance from epidermal cells to guard cells instead. Several researches support the modelof Buckley et al. in this regard.The most sensitive response of plants to drought is stomatal closing. As regards themechanism of this response, there are two contrasting theories: the theory of root-sourcedchemical signal and that of hydraulic signal. Both of them have convincing evidence whileneither of them is complete. They should be integrated. The model of Buckley et al. offersa basis for an integrated model of stomatal conductance.The core hypothesis of the model of Buckley et al. is “hydro-active local feedback”:guard cell osmotic pressure is actively regulated in response to water status in theimmediate vicinity of guard cells in the epidermis. Evidence had indicated that guard cellsof Vicia faba respond actively to osmotic stress. However, Grantz and Schwartz found thatguard cells of Commelina communis L. did not respond actively to osmotic stress in detached epidermis. They had incubated detached epidermis in relatively highconcentration of KCl in ambient air. We incubated detached epidermis in40mM KCl inCO₂-free air. Mannitol was added by solution replacement without interrupting CO₂-freeair. Solute accumulation in guard cells was restricted in response to osmotic stress. ABAgreatly enhanced this active response. Furthermore, ABA stimulated active response ofguard cells of initially open stomata to osmotic stress: guard cell osmotic pressure wasreduced actively. Our results support “hydro-active local feedback” hypothesis, andsuggest that ABA should enhance guard cell sensitivity to epidermal water potential ifABA is to be introduced into the model.The model of Buckley et al. can accommodate physiological properties concerningstomatal regulation, such as ABA synthesis in the shoot and the root, the interaction ofABA and hydraulic signal, some potential chemical signals, the effect of pH on themovement of ABA, changes in the hydraulic conductance of plants including xylemembolism, and water storage in plants. We introduced root-sourced ABA and theinteraction between ABA and hydraulic signal into the model, and successfully modeledisohydric behavior and stomatal response to D_sin Regime C. After xylem embolism wasintroduced in addition, stomatal response to D_sin Regime B was modeled. Simulation alsoshowed that moderate xylem embolism enhanced stomatal closing and the homeostasis ofleaf water potential.