Stomatal conductance

5.7 Stomatal conductance

Zinnert J., Estiarte M., Johnson D. M. (2019)

In : CLIMEX HANDBOOK –

https://climexhandbook.w.uib.no/2019/11/03/stomatal-conductance/

Stomatal conductance is a calculation of the influence of stomatal opening on rate of diffusion of CO₂ entering or water vapour exiting through the stomata of a leaf. Stomata are an important regulatory point for water movement through the soil–plant–air-continuum (van den Honert, 1948) through varying the diffusion resistance. The conductance of water vapour (i.e. inverse of the diffusion resistance) expresses the regulatory control exerted by stomata through the degree of stomatal opening (Pearcy et al., 1989). Environmental signals such as incident light intensity, CO₂ concentration, water availability, vapour pressure deficit (VPD), and leaf temperature affect stomatal aperture (Farquhar et al., 1980). Under favourable conditions of low evaporative demand and high light, maximum stomatal conductance determines the upper limit of CO₂ assimilation in a leaf. The ability of a plant to regulate stomatal opening in response to environmental conditions enables it to modulate the rate of transpiration while maintaining carbon uptake (Cowan 1977, Farquhar et al. 1980). Stomatal closure is considered to be the earliest response to drought (Flexas & Medrano, 2002) and other environmental stressors, and is a limitation to photosynthesis. However, dependence of leaf temperature on stomatal conductance occurs through leaf transpiration. Plants must balance lowering stomatal conductance to conserve water with preventing extreme leaf temperatures which affect metabolic rates and physiological processes (De Boeck et al., 2016). As such, stomatal conductance is an important parameter especially in climate-change studies and in process-based models that can be used to predict species responses to climate change (Sperry & Love, 2015; Tai et al., 2018). Stomatal conductance influences both photosynthesis and transpiration, exerting major control on carbon and water cycling, and energy exchange from leaf to landscape level as well as between the Earth’s surface and atmosphere. Because stomata control the rate of water loss in vegetated areas, they affect atmospheric moisture levels and surface temperature. Stomatal conductance is a clearly defined variable that is easily measured for model parameterisation and has substantial biological relevance (Buckley & Mott, 2013). Maximum stomatal conductance is also a function of stomatal density and size (Drake et al., 2013), which vary due to genetic factors or environmental conditions during growth (Bertolino et al., 2019). Stomatal density measurements may, thus, complement conductance measurements to compare species and when treatments can affect leaf development.

5.7.1 What and how to measure?