Diurnal Variation in Gas Exchange: The Balance between Carbon Fixation and Water Loss

Abstract

Stomatal control of transpiration is critical for maintaining important processes, such as plant water status, leaf temperature, as well as permitting sufficient CO 2 diffusion into the leaf to maintain photosynthetic rates (A). Stomatal conductance often closely correlates with A and is thought to control the balance between water loss and carbon gain. It has been suggested that a mesophyll-driven signal coordinates A and stomatal conductance responses to maintain this relationship; however, the signal has yet to be fully elucidated. Despite this correlation under stable environmental conditions, the responses of both parameters vary spatially and temporally and are dependent on species, environment, and plant water status. Most current models neglect these aspects of gas exchange, although it is clear that they play a vital role in the balance of carbon fixation and water loss. Future efforts should consider the dynamic nature of whole-plant gas exchange and how it represents much more than the sum of its individual leaf-level components, and they should take into consideration the long-term effect on gas exchange over time.As the waxy surface of most leaves makes them virtually impermeable to CO 2 and water, nearly all CO 2 absorbed by the plant and water lost pass through the stomatal pores (Cowan and Troughton, 1971;Caird et al., 2007;Jones, 2013). Although these pores represent only a small fraction of the leaf surface, stomatal behavior has major consequences for photosynthetic CO 2 fixation and water loss from leaf to canopy levels, influencing carbon and hydrological cycles at global scales (Hetherington and Woodward, 2003;Keenan et al., 2013). Guard cells that surround the stomatal pore open and close in response to environmental stimuli, controlling the flux of gas between the leaf interior and the bulk atmosphere. Stomatal conductance (g s ) appears to be closely linked with mesophyll demands for CO 2 , and a strong correlation between photosynthetic rate (A) and g s is often observed (Wong et al., 1979;Farquhar and Sharkey, 1982;Mansfield et al., 1990;Buckley and Mott, 2013), and although conserved, it is not always constant (Lawson and Morison, 2004;Bonan et al., 2014). However, the A and properties of each leaf may not be identical and depend on acclimation to the surrounding microclimatic conditions; therefore, each leaf could be considered unique (Niinemets, 2016).In order to maintain an appropriate water status, plants must balance water loss between leaves with different properties depending on the availability of soil water, which raises the question about the regulation of g s at the whole-plant level. Early experiments by Meinzer and Grantz (1990) showed that the balance between water loss and water transport capacity enables the maintenance of a constant leaf water status over a wide range of plant sizes and growing conditions. Therefore, plants regulate the transpiration of each leaf independently in response to variations in microclimate by constantly adjusting stomatal aperture. The sto...

Forests, atmospheric water and an uncertain future: the new biology of the global water cycle

“…Leaves and other plant parts are typically waxy and relatively impermeable, with most gas exchange and water loss occurring through stomatal pores. Stomata control the entry of carbon dioxide into the leaf and the emission of water through transpiration (simultaneously regulating leaf temperature, Matthews et al 2017). These adjustable pores evolved around 400 million years ago and have influenced our planet's water-cycle ever since (Berry et al 2010;Brodribb and McAdam 2017).…”

“…The control of these pores determines the exchange of gases, including water vapour and carbon dioxide, between the plants' intercellular space and the external atmosphere. The behaviour of stomatal pores is assumed to optimize carbon gain (growth), depending on environmental conditions including access to water and the risks that arise from drought, but our understanding of these relationships remains a "work in progress" (Klein 2014;Martin-StPaul et al 2017;Matthews et al 2017;Meinzer et al 2017). Furthermore, trees capture nutrients by drawing in soil water, thus increased transpiration rates can be a response to low nutrient environments (Matimati et al 2013;Huang et al 2017).…”

Acclimation to Fluctuating Light Impacts the Rapidity of Response and Diurnal Rhythm of Stomatal Conductance

“…The use of steady state 110 models under fluctuating environmental conditions can lead to inaccurate predictions of the 111 diurnal response of g s , as these models do not really take into account the slow temporal 112 response of stomata (Vialet-Chabrand et al, 2013Matthews et al, 2017). Moreover,…”

Small Pores with a Big Impact

“…Several reviews and articles in this Focus Issue center around stomata as a target for genetic engineering to improve water use efficiency and their importance for crop survival. The Update reviews of Vialet-Chabrand et al (2017b) and Matthews et al (2017) address the potential for improved water use efficiencies by enhancing the kinetics of guard cell responses to environmental cues relevant for photosynthesis. Hughes et al (2017) demonstrate the efficacy of manipulating stomatal densities to enhance water use efficiency and drought tolerance, while Hochberg et al (2017) highlight the interplay between stomatal function, water delivery to the leaf, and leaf shedding in the field.…”