Gas exchange and water‐use efficiency in plant canopies

Abstract

In this review, I first address the basics of gas exchange, water-use efficiency and carbon isotope discrimination in C 3 plant canopies. I then present a case study of wateruse efficiency in northern Australian tree species. In general, C 3 plants face a trade-off whereby increasing stomatal conductance for a given set of conditions will result in a higher CO 2 assimilation rate, but a lower photosynthetic water-use efficiency. A common garden experiment suggested that tree species which are able to establish and grow in drier parts of northern Australia have a capacity to use water rapidly when it is available through high stomatal conductance, but that they do so at the expense of low water-use efficiency. This may explain why community-level carbon isotope discrimination does not decrease as steeply with decreasing rainfall on the North Australian Tropical Transect as has been observed on some other precipitation gradients. Next, I discuss changes in water-use efficiency that take place during leaf expansion in C 3 plant leaves. Leaf phenology has recently been recognised as a significant driver of canopy gas exchange in evergreen forest canopies, and leaf expansion involves changes in both photosynthetic capacity and water-use efficiency. Following this, I discuss the role of woody tissue respiration in canopy gas exchange and how photosynthetic refixation of respired CO 2 can increase whole-plant water-use efficiency. Finally, I discuss the role of water-use efficiency in driving terrestrial plant responses to global change, especially the rising concentration of atmospheric CO 2 . In coming decades, increases in plant water-use efficiency caused by rising CO 2 are likely to partially mitigate impacts on plants of drought stress caused by global warming.

Water-Use Efficiency: Advances and Challenges in a Changing Climate

“…There continue to be advances in our understanding of plant response to a changing CO 2 environment, one of these responses is a change in the stomatal density as observed by Caine et al (2019) in rice and the evolution toward changes in stomatal density from C 3 to C 4 plants (Way et al, 2014). The use of more advanced techniques, e.g., carbon isotope discrimination (Cernusak, 2018; Gao et al, 2018a) or molecular genetics (Avramova et al, 2018) promise to offer new insights into understanding these linkages.…”

Elevated CO<sub>2</sub>, increased leaf-level productivity and water-use efficiency during the early Miocene

“…Globally, plants draw down ~120 Gt of atmospheric C per year through photosynthesis, representing the largest annual C flux on Earth (Beer et al, 2010). Total plant biomass is believed to be determined in large part by atmospheric carbon dioxide concentrations (Ca), and it is predicted that future increases in Ca will have a three-pronged effect on the terrestrial biosphere: 1) increased global temperatures will shift the boundaries of climate zones and thereby the potential forest expanse (Rubel and Kottek, 2010); 2) productivity will increase because global photosynthesis is C limited and increased Ca will make more C available to the terrestrial biosphere (Zhu et al, 2016); and 3) elevated Ca will increase plant water-use efficiency and reduce the threshold for physiological drought (Cernusak, 2020), making more land area available for biosphere expansion (Zhou et al, 2017). Plant fossils record the effect of past changes in climate, including CO2 enrichment, and thus fossil floras provide insight into changes in the carbon cycle and their effects on the terrestrial biosphere from a natural, whole-ecosystem perspective.…”

Elevated CO<sub>2</sub>, increased leaf-level productivity, and water-use efficiency during the early Miocene

“…T. Reichgelt et al: Elevated CO 2 , increased leaf-level productivity, and water-use efficiency increased C a will have a fertilization effect on the terrestrial biosphere (Zhu et al, 2016); and (3) elevated C a will increase plant water-use efficiency and reduce the threshold for physiological drought (Cernusak, 2020), making more land area available for biosphere expansion (Zhou et al, 2017). Plant fossils record the effect of past changes in climate, including CO 2 enrichment, and thus fossil floras provide insight into changes in the carbon cycle and their effects on the terrestrial biosphere from a natural, whole-ecosystem perspective.…”