Water Use Efficiency and Stomatal Conductance
The great influence of increased [CO2] on stomatal conductance is documented and presented in an overview table. It is also shown that in a dense stand of deciduous tree saplings conductance is clearly reduced at elevated [CO2].
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Water Use Efficiency and Stomatal Conductance
Abstract The great influence of increased [CO2] on stomatal conductance is documented and presented in an overview table. It is also shown that in a dense stand of deciduous tree saplings conductance is clearly reduced at elevated [CO2]. Keywords Instantaneous water use efficiency • Guard cell • Air to leaf water vapor pressure deficit • Water potential • Ozon
5.1
Water Use Efficiency
Very early on, it was well documented that the instantaneous water use efficiency (WUE), i.e., the ratio of CO2 net assimilation [An(leaf)] to water loss via transpiration (E), increases with increasing atmospheric [CO2] in woody plants (Gaudille`re and Mousseau 1989). This effect on WUE might be one of the most distinct consequences of e[CO2]. It has been calculated for food crops that transpiration will be reduced by 34 % and An(leaf) will increase by 50 % on average if current ambient [CO2] is approximately doubled (Kimball 1983a, b). Measurements on Fagus sylvatica leaves showed a less pronounced effect on transpiration but a reduction of 18 % at 700 μmol CO2 mol1 was still noted, together with an increase in An(leaf) of ~30 % (Overdieck and Forstreuter 1994). One typical course of the relationship between [CO2] around the leaf (Ca) and instantaneous WUE is shown in Fig. 5.1. The effect of e[CO2] on WUE can presumably last over long time periods. Wu et al. (2015) used δ13C analyses to reconstruct several persistent warm periods within the lifespan of the studied Picea schrenkiana and calculated an increase in intrinsic WUE (CO2 uptake/stomatal conductance, iWUE) of 28 % over 160 years.
5.2
Stomatal Conductance
Guard cells most likely “sense CO2” by responding to Ci (intercellular [CO2]) rather than to [CO2] at the leaf surface (Vavasseur and Raghavendra 2005). Both C3- and C4-plants show a consistent and similar decrease in stomatal conductance (gs) at e [CO2] (Ainsworth and Long 2005). It is well known that ions and organic solute © Springer Science+Business Media Singapore 2016 D. Overdieck, CO2, Temperature, and Trees, Ecological Research Monographs, DOI 10.1007/978-981-10-1860-2_5
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5 Water Use Efficiency and Stomatal Conductance
instantaneous WUE (µmol CO2 mmol H2O-1 )
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Ginkgo biloba
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C a (µmol CO2 mol-1 ) Fig. 5.1 Mean instantaneous water use efficiency (WUE; An/E) of Ginkgo biloba leaves in response to increasing [CO2] around the leaves (Ca). n ¼ 15 leaves, measured at light saturation, 27 C, and 67 % rel. air humidity; recalculated data from Overdieck and Strassemeyer (2005); SD: standard deviations
concentrations regulate the turgor pressure in guard cells, which determines the aperture of the stomata. Stomatal closure requires guard cell membrane potentials to be less negative (Assmann 1993, 1999). It has also been shown that e[CO2] increases the activity of outward-rectifying K+ channels, decreases the activity of inward-rectifying K+ channels, enhances anion channel activity, stimulat
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