The water content of foliage and moisture stress of twigs associated with stomatal closure were determined for 3-year-old seedlings of ponderosa pine, lodgepole pine, grand fir, Douglas-fir, and Engelmann spruce. Stomatal behavior during increasing soil drought was interpreted from transpiration trends of potted seedlings kept under controlled conditions, and relative degree of stomatal opening was determined with a pressure infiltration technique. In grand fir and Douglas-fir, a decline in transpiration rate in response to drought was associated with a relatively small reduction in size of stomatal aperture as compared with the pines and spruce, indicating that mesophyll resistance may have been a significant factor limiting transpiration in the firs. At stomatal closure, or end of the transpiration decline, needle water contents ranged from 122.5% to 162.5% and were lowest in Engelmann spruce and Douglas-fir, intermediate in the pines, and highest in grand fir. Needle relative turgidities ranged from 81.6% to 86.1% with little difference between species, and twig moisture stress ranged from 14.6 to 25.1 atm. The pines and spruce had the lowest stress, indicating that the stomata of these species were more sensitive to increasing leaf moisture stress than those of the firs. Transpiration-decline curves obtained with excised twigs indicated that stomatal closure also occurred more rapidly in the pines and spruce than in the firs.
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W. Lopushinsky (1969) studied this question.