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Rates of stomatal opening in conifer seedlings in relation to air temperature and daily carbon gain

Pepin, S., Livingston, N.J.
Plant, cell and environment 1997 v.20 no.12 pp. 1462-1472
Thuja plicata, stomatal movement, carbon dioxide, interspecific variation, stomatal conductance, artificial shade, seedlings, gas exchange, air temperature, light intensity, photosynthesis, diurnal variation, vapor pressure, Pseudotsuga menziesii, Tsuga heterophylla, stomata, British Columbia
Experiments were conducted on well watered 1-year-old Douglas fir [Pseudotsuga menziesii (Mirb.) Franco], western hemlock [Tsuga heterophylla (Raf.) Sarg.] and western red cedar (Thuja plicata Donn) seedlings to determine the effects of temperature on whole-plant photosynthetic and stomata responses to short-term fluctuations in irradiance (Q). Following a step change in Q, time constants (tau, the period over which 63% of the total change occurs) for stomata conductance (gs) and assimilation rate (A) decreased linearly with increasing air temperature (Tair). For example, in western red cedar tau A decreased from 30 +/- 4 min at 5 degrees C to 10 +/- 1 min at 25 degrees C. In all cases, tau A was within 10-15% of tau gs. There was considerable variation in tau among individuals within a given species. Differences between species became more pronounced with decreasing temperature. Multiplicative models that included functions for tau accounted for 99% of the diurnal variability in A and gs for seedlings exposed to varying air temperature, irradiance and vapour pressure deficit. Estimates of daily A were within 2% of those measured. Intermittent cloud cover and understory shading were approximated by exposing seedlings to 3-4 episodes (greater than or equal to 1 h) of shade (200 or 500 micromoles m-2 s-1) or complete darkness during the day. In such cases, daily A was overestimated by up to 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 basis of reduced time constants. For example, in western redcedar, a 40% reduction in tau could lead to increases in daily carbon gains of almost 5% depending on the frequency and degree of shading. If these daily gains were translated into increased dry matter production and compounded, seasonal gains would be even larger.