Effects of light availability versus hydraulic constraints on stomatal responses within a crown of silver birch

Abstract

Responses of leaf conductance (gL) to variation in photosynthetic photon flux density (QP), leaf-to-air vapour pressure difference (VPD), bulk leaf water potential (Psi(x)), and total hydraulic conductance (GT) were examined in silver birch (Betula pendula Roth) with respect to leaf position in the crown. To reduce limitations caused by insufficient water supply or low light availability, experiments were also performed with branchlets cut from two different canopy layers. The intact upper-canopy leaves demonstrated 1.8-2.0 times higher (P<0.001) daily maxima of gL compared with the lower-canopy leaves growing in the shadow of upper branches. In the morning, gL in the shade foliage was primarily constrained by low light availability, in the afternoon, by limited water supply. Leaf conductance decreased when Psi(x) fell below certain values around midday, while the sun foliage experienced greater negative water potentials than the shade foliage. Midday stomatal openness was controlled by leaf water status and temperature, rather than by transpiration rate (E) via the feedforward mechanism. Mean GT was 1.7 times higher (P<0.001) for the upper-canopy foliage compared to that of the lower canopy. At least 34-39% of the total resistance to the water flow from soil up to the shade foliage, and 54% up to the sun foliage, resided in 30-cm distal parts of the branches. Artificial reduction of hydraulic constraints raised Psi(x) and made gL less sensitive to changes in both atmospheric and plant factors. Improved water supply increased gL and E in the lower-canopy foliage, but not in the upper-canopy foliage. The results support the idea that leaves in the lower canopy are hydraulically more constrained than in the upper canopy.

Soil Temperature and Phosphorus Supply Interactively Affect Physiological Responses of White Birch to CO&lt;sub&gt;2&lt;/sub&gt; Elevation

“…The lack of significant responses in stomatal conductance to the CO 2 elevation under the adverse soil temperature (i.e., 7˚C) could be interpreted as that the stomatal conductance at the low soil temperature was already at such a low level that it could not go down any further. However, it is also possible that the low soil temperature reduced root growth and thus water intake and transport, which indirectly affected the stomatal conductance and photosynthesis through its impact on leaf water relations [56].…”

Increases in atmospheric CO2 have little influence on transpiration of a temperate forest canopy

“…4c). However, hydraulic conductivity is not independent of light intensity, that is, lower light levels can reduce hydraulic conductivity (Sellin & Kupper, 2005) Taken together, these analyses suggest little, if any, direct [CO 2 ] effect on G S for either species, and appreciable indirect [CO 2 ]-induced G S reduction in Pinus taeda and Liquidambar styraciflua, with the canopy response falling close to that of P. taeda (Fig. 4c) due to its dominance of stand E C (Fig.…”

Interactive effects of defoliation and climate change on compensatory growth of silver birch seedlings

“…In the built environments, the PFD did not fully correspond to that prevailing at 62°N; therefore it may have affected the photoperiod of the seedlings. However, the PFD was in the range of saturating irradiance for leaf water conductance in silver birch, which varies between 100 and 800 µmol m -2 s -1 and is dependent on foliage position on the crown (Sellin and Kupper 2005). Since leaf water transport affects stomata and thus the diffusion of CO 2 , it has an effect on photosynthesis (Sellin and Kupper 2005).…”

“…However, the PFD was in the range of saturating irradiance for leaf water conductance in silver birch, which varies between 100 and 800 µmol m -2 s -1 and is dependent on foliage position on the crown (Sellin and Kupper 2005). Since leaf water transport affects stomata and thus the diffusion of CO 2 , it has an effect on photosynthesis (Sellin and Kupper 2005). Moreover, saturation point for the photosynthetic PFD in silver birch is around 900 µmol m -2 s -1 ; exceeding this point causes no increase in photosynthesis (Wang et al 1995).…”