Cuticular Conductance and the Humidity Response of Stomata

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Abstract
Meidner, H. 1986. Cuticular conductance and the humidity response of stomata.—J. exp. Bot. 37: 517–525. Detailed measurements of cuticular vapour loss from leaves of several species showed that cuticular conductance declined from an early morning maximum of 0·02 cm s−1 to between 0·004 and 0.005 cm scm s−1 even in the absence of stomatal transpiration. Re-establishment of the maximum conductance occurred only in a humid atmosphere and when the xylem system was under pressure (simulated mild root pressure) Cuticular vapour loss alone is, therefore, unlikely to be the underlying mechanism of the humidity response of Stomata. Evidence for the existence of a humidity-sensing feed-forward mechanism is discussed and it is shown that when detailed measurements are made the humidity response is found to have two phases. This indicates a perturbation of the fine turgor balance between epidermat and guard cells that exists in a transpiring leaf. It is argued that the humidity response can be accounted for by reference to hydropassive movements which initiate a metabolic adjustment of the guard cells to altered evaporative demand.