Abstract
Net photosynthesis in six different conifers was studies under various natural environmental conditions. Changes in the pattern of photosynthesis on clear days, especially the midday decrease, are apparently primarily controlled by changes in leaf water potential. In noble fir and Scots pine, water potential probably acts mainly through its influence on stomatal movement. In grand fir, Douglas—fir, hemlock ,and Sitka spruce, however, some other mechanism, probably mesophyll resistance to CO2 diffusion, seems to play a more important role. Both mechanisms probably operate concurrently in all species. Daily variations in leaf water potential seem to occur primarily in response to changes in atmospheric moisture, or, more precisely, vapor pressure gradient from leaf to atmosphere. Variation in carbohydrate content, through its influence on solute concentration, may also influence leaf water potential.
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