Do stomata respond to relative humidity?

Abstract

The objective of the work reported was to answer the following questions; (1) Do stomata respond to both humidity and temperature? (2) Do these responses interact in sueh a way that relative humidity at the leaf surface is a more appropriate variable than water vapour saturation deficit at the leaf surface and yields a simpler description of the cotnpound response? To answer these questions, we tneasured the response of leaf conductance to humidity under constant leaf temperature, and the response to increasing leaf temperature under constant relative humidity and under constant water vapour saturation deficit. We found that, in Hedera helix subsp, canarietisis (Willd.) Coutinho, there was a reversible response to humidity under constant temperature, and that there was also a response to temperature under constant relative humidity. The relationship between leaf conductance and relative humidity was different when rneasured at the same temperature rather than at diffefent temperatures. An inversely proportional response was consistently obtained when stomatal conductance was expressed in relation to water vapour sa'turation deficit. The interaction between the effects of leaf temperature and water vapour saturation deficit was not compatible with a mechanism of response to humidity and temperature based solely on relative humidity. From these data, we conclude that water vapour saturation deficit is a more appropriate variable for describing stomatal responses to humidity. Key-words: Hedera helix; ivy; gas exehange; stomatal conductance; relative humidity; water vapour saturation deficit; leaf temperature.Abbreviations: A = CO2 flux density (net CO2 assimilation rate); k = proportionality coefficient; h = relative humidity; Ih = relative humidity at the leaf surfaee -assuming that the air at the leaf surfaee is at the same temperature as the leaf; Ti -leaf temperature; x,, = mole fraetion of water vapour in the air outside the boundary layer; x, = mole fraction of water vapour at the leaf surface; x* = x*{Ti) = saturated water vapour molar fraction at T/; D,, = x* -x"; /)., = A' J -xf, gg = stomatal conductance; gi, = boundary layer conductance; C, = CO2 molar fraetion at the leaf surfaee; C, = CO2 molar fraetion in the intercellular air space; / = photon flux density, A = 4t)t)-700nm,

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