Stomatal responses to humidity in selected conifers.

Stomatal response to changes in leaf-to-air water vapor pressure difference (D) was studied in needles of the current year's shoot of three-year-old seedlings of Picea sitchensis (Bong.) Carr., Pinus contorta Dougl. ex Loud., Larix x eurolepis Henry, and Pinus sylvestris L. Both eight-week- and ten-month-old shoots of P. sylvestris were studied. Stomata of all the species responded by closing to some degree as D was increased over the range 0.4-2.0 kPa. Ten-month-old shoots of P. sylvestris showed the smallest reduction (7.5%), and shoots of P. sitchensis the largest reduction (64.6%) in stomatal conductance. However, in no species was stomatal closure sufficient to cause a reduction in transpiration (E) as D increased. Net photosynthesis (A) declined linearly as D was increased and as a result the ratio of E/A increased linearly in all species. Only the stomata of P. contorta and L. x eurolepis behaved in an 'optimal' way, i.e., estimated values of dEdA were approximately constant as D increased. For P. sylvestris, shoots of both ages, dE/dA increased markedly with D, whereas in P. sitchensis it declined. Explanation of these data does not require a mechanism of stomatal closure involving a site that senses the vapor pressure deficit outside the leaf. However, it is unlikely that a simple 'feedback' response involving bulk leaf water potential can explain the responses measured because changes in needle water potential were less than 0.1 MPa during an experiment.