Temperature and humidity effects on branchlet gas-exchange in white spruce: an explanation for the increase in transpiration with branchlet temperature

Abstract

In situ gas-exchange data, for branchlets of white spruce [Picea glauca (Moench) Voss.] in a mature mixed-wood boreal forest in central Canada (53°44´N 105°14´W), were subjected to a multiple regression analysis. Vapor pressure deficit (VPD) and branchlet temperature (t leaf ) were both significant predictors (P<0.0001) of stomatal conductance to water vapor (g sw ) and net photosynthesis (A n ), together explaining 67 and 64% of the variation in g sw and A n , respectively. Since VPD and t leaf were autocorrelated in these field data, but also to further explore the nature of independent effects of temperature and humidity on water and CO 2 exchange in white spruce, steady-state gas-exchange was performed on well-watered greenhouse-grown seedlings of white spruce. Results from laboratory experiments supported the following conclusions: (1) Transpiration (E) increases with VPD to an inflection point that increases linearly with t leaf . This t leaf effect on E could not be explained by trends in VPD, RH, A n or PFD. Rather, our data support a model in which E and g sw are influenced by the balance between 'supply' and 'loss' of water to and from leaf tissue, respectively. The supply of water appears to be in accordance with Darcy's law, where supply of water is proportional to the driving gradient in pressure/ tension, specific permeability (k), and inverse of water viscosity (n -1 ). Approximately half of the increase in E could be explained by the linear increase in n -1 with increasing t leaf . We propose that increases in k explain the remainder of the increase in E with t leaf . (2) VPD and t leaf appear to have independent effects on g sw . In contrast, RH effects on g sw or E were subtle and could be explained by a combination of effects of t leaf and VPD. (3) A n was affected primarily by t leaf , being reduced at low (10°C) and high (40°C) temperatures, and only indirectly by humidity parameters via stomatal conductance, viz. intercellular CO 2 concentrations. Our results have implications for the prediction of water fluxes from plants and canopies in areas where plant temperatures vary diurnally or seasonally.

Stomatal conductance increases with rising temperature

“…7,8 Results of experiments that examined the direct dependence of stomatal conductance on temperature have not been consistent. Previous studies have reported a complete range of responses to increased temperature, including stomatal opening, [9][10][11][12] no significant response, [13][14][15][16] stomatal closure, [17][18][19] peaked response with maximum g s at temperatures optimal for photosynthesis 20 or more complex responses. 21 One possible explanation for these inconsistent results is that to isolate the direct effect of temperature on g s requires a well-controlled environment, particularly with respect to VPD, which is often hard to achieve.…”

Early gene expression in the walnut tree occurring during stimulation of leaf hydraulic conductance by irradiance

“…K leaf depends upon plant species (Tyree et al 1998, Cosmstock 2000, Brodribb et al 2002 and is modulated by environmental factors, such as water stress (Nardini et al 2003, Nardini and, temperature (Fredeen and Sage 1999, Cochard et al 2000, Sack et al 2004) and also by circadian rhythms Holbrook 2004, Lo Gullo et al 2005). K leaf has long been thought to be mainly associated with the regulation of stomatal aperture, suggesting a minor contribution of aquaporins.…”

The relationship between the stable carbon isotope composition of needle bulk material, starch, and tree rings in Picea abies

“…The resulting fractionation is reflected in the stable carbon isotope signature (δ 13 C) of organic components such as sugars, starch, or cellulose, and of whole plant tissues. Light, air humidity, precipitation and temperature are environmental factors known to influence photosynthesis through their effect on stomatal conductance and CO 2 fixation (Nikolov et al 1995;Fredeen and Sage 1999;Bunce 2000). Thus, they indirectly influence the δ 13 C signature of C-compounds with a short (soluble sugars and starch, Brugnoli et al 1988;Brugnoli and Farquhar 2000) or long (plant tissue and tree rings, Leavitt and Long 1991) integration period.…”