RATES OF STOMATAL OPENING IN CONIFER SEEDLINGS IN RELATION TO AIR-TEMPERATURE AND DAILY CARBON GAIN

Experiments were conducted an well watered 1-year-old Douglas fir [Pse udotsuga menziesii (Mirb.) Franco], western hemlock [Tsuga heterophyll a (Raf.) Sarg.] and western redcedar (Thuja plicata Donn) seedlings to determine the effects of temperature on whole-plant photosynthetic an d stomatal responses to short-term fluctuations in irradiance (Q). Fol lowing a step change in Q, time constants (tau, the period over which 63% of the total change occurs) for stomatal conductance (g(s)) and as similation rate (A) decreased linearly with increasing air temperature (T-air). For example, in western redcedar tau(A) decreased ham 30 +/- 4 min at 5 degrees C to 10 +/- 1 min at 25 degrees C. In all cases, t au(A) was within 10-15% of tau(gs). There was considerable variation i n tau among individuals within a given species, Differences between sp ecies became more pronounced with decreasing temperature, Multiplicati ve models that included functions for tau accounted for 99% of the diu rnal variability in A and g(s) for seedlings exposed to varying air te mperature, irradiance and vapour pressure deficit, Estimates of daily A were within 2% of those measured. intermittent cloud cover and under story shading were approximated by exposing seedlings to 3-4 episodes (greater than or equal to 1 h) of shade (200 or 500 mu mol m(-2) s(-1) ) or complete darkness during the day. In such cases, daily A was over estimated by up Po 4 and 21%, respectively, if a function for tau was excluded from the models, Our results suggest that there is scope for selecting seedling stock for increased carbon assimilation on the basi s of reduced time constants, For example, in western redcedar, a 40% r eduction in tau could lead to increases in daily carbon gains of almos t 5% depending on the frequency and degree of shading, If these daily gains were translated into increased dry matter production and compoun ded, seasonal gains would be even larger.