Stomatal control of whole-plant photosynthesis and transpiration in conifer seedlings




Pepin, Steeve

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Because the exchange of carbon dioxide and water vapor between plants and the atmosphere is regulated by changes in stomatal conductance (gs), the responses of stomata to fluctuations in environmental variables have major effects on leaf physiological processes such as carbon assimilation. This dissertation focuses on the stomatal control of whole-plant photosynthesis and transpiration in conifer seedlings.Experiments were conducted on well watered one-year-old Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco), western hemlock ( Tsuga heterophylla (Raf.) Sarg.) and western redcedar (Thuja plicata Donn) seedlings to determine the effects of temperature on whole-plant photosynthetic and stomatal responses to short-term fluctuations in irradiance (Q). Following a step change in Q, time constants (τ, the period over which 63% of the total change occurs) for gs and assimilation rate (A) decreased linearly with increasing air temperature (T air). For example, in western redcedar Ta decreased from 30 ± 4 minutes at 5 °C to 10 ± 1 minutes at 25 °C. In all cases, Ta was within 10-15% of Tgs. There was considerable variation in τ among individuals within a given species. Differences between species became more pronounced with decreasing temperature. Multiplicative models that included functions for τ accounted for 99% of the diurnal variability in A and gs for seedlings exposed to varying Tₐᵢᵣ, Q and vapor 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 (≥ 1 h) of shade (200 or 500 μmol 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 τ was excluded from the models. The 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 T 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. Experiments were also conducted on one-year-old western redcedar seedlings to determine the response of illuminated foliage to transient and reversible changes in total photosynthesizing foliage area (LA ). Reductions in LA were brought about by either shading a portion of the foliage or by reducing the ambient CO₂ concentration (ca) of the air surrounding the lower part of the seedling. In the latter case, the vapor pressure was also changed so that transpiration rates (E) could be manipulated independently of photosynthesis rates. It was hypothesized that following such treatments, there would be rapid short-term compensatory changes in gs and A of the remaining foliage. These would be in response to hydraulic signals generated by changes in the water potential gradient rather than changes in the distribution of sources and sinks of carbon within the seedling. When a portion of the foliage was shaded, there was an immediate reduction in whole-seedling E and a concomitant increase in gs, A and E in the remaining illuminated foliage. However, the intercellular CO₂ concentration did not change. These compensatory effects were fully reversed after the shade was removed. When the lower foliage A was reduced to (<0 μmol m⁻² s⁻¹) by shading or lowering cₐ, but E was either unchanged or increased, there was not a significant increase in gs and A in the remaining foliage. I conclude that short-term compensatory responses in illuminated foliage occur only when reductions in La are accompanied by a reduction in whole-plant E. The relation between the reduction in whole-seedling E and the increase in gs or A is highly linear (R² = 0.69) and confirms the hypothesis of the strong regulation of gs by hydraulic signals generated within the seedling. I suggest that the mechanism of the compensatory effects is a combination of both increased CO₂ supply, resulting from increased gs, and a response of the rate of carboxylation, possibly related to the activity of Rubisco.