The Biology of Transpiration. From Guard Cells to Globe

Transpiration in higher plants accounts for about three-quarters of the water that is vaporized at the global land surface and one-eighth of that vaporized over the entire globe. The availability of water is one of the major factors restricting terrestrial plant production on a global scale. Since plants do not have membranes that are both permeable to CO₂ and impermeable to water, transpiration is an inevitable consequence of photosynthesis. To control water loss, plants are covered with relatively water-impermeable surfaces that are punctuated by stomatal pores. Almost all of the CO₂ fixed by terrestrial plants and most of the water transpired pass through these stomatal pores. The degree of opening of these pores is modulated by variation in the turgor status of the two surrounding guard cells. The regulation of stomatal aperture determines the compromise between increasing CO₂ fixation and reducing transpiration to prevent desiccation. At the same time, plant transpiration provides evaporative cooling, forming a major component of the leaf energy balance. Transpiration also provides the driving force for transport of water and nutrients from roots to shoots. Consequently, transpiration processes affect the yield and survival of agricultural species, and impact on the global carbon and hydrological cycles. These in turn feed back on climate and have a direct effect on global warming and climate change.

In the past five years, there have been rapid advances at several organizational levels in the understanding the biology of transpiration, many of which have been the direct result of significant advances in the measurement of parameters associated with transpiration. Many of these research areas have developed separately, yet frequently advances in one area have potentially major implications for many other areas. The driving force for this Focus Issue on the Biology of Transpiration was the recent meeting on the same topic held at Snowbird Mountain Resort in Utah. To catalyze the required interactions among scientists working in the diverse areas associated with plant transpiration, all aspects of water transport were covered at levels spanning from gene expression to global modeling.

The regulation of stomatal aperture is dynamic, reversible, and responsive to a number of environmental and intrinsic signals, such as light, CO₂, air humidity, and stress hormones such as abscisic acid. As a consequence, the guard cell has become an important model cell type in the field of plant cell signaling. At present, numerous genetic mutants in Arabidopsis (Arabidopsis thaliana) with alterations in the production, sensing, or response to major plant hormones provide an exciting resource for the study of the regulation of transpiration. In their Update, Nilson and Assmann review the role the model species Arabidopsis currently plays in elucidation of some of these signal transduction pathways. The development of stomata is also providing an exciting focus for the study of integration of genetic and environmental inputs into developmental decisions. Studies of stomatal development also provide insights into past climates. Since the concentration of carbon dioxide in the atmosphere exerts a significant control over stomatal development, stomatal frequency in fossil plants is currently being used as a way of tracking atmospheric CO₂ concentrations over the last 400 million years.

At present, models used to predict weather and climate use empirical functions to approximate the response of stomata to environment. Improving our understanding of stomatal responses and the development of more functional mathematical models of stomatal behavior will help facilitate the development of improved climate models. The regulation of nighttime stomatal conductance is one such example that impacts on estimation of global transpiration, and this is addressed by Caird et al. in their Update article.

Isotopic compositions of CO₂, O₂, and water vapor have also become an important global signal used in climate models. For example, leaf water is generally enriched in ¹⁸O relative to soil water due to a tendency for the heavier molecules to accumulate in leaves during transpiration. Since atmospheric CO₂ undergoes isotopic exchange with leaf water and soil water, the ¹⁸O composition of CO₂ can be used to study spatial and temporal variation in the net exchange of CO₂ in terrestrial ecosystems. These issues, which highlight a need for better understanding of water movement within plants, are addressed in the Update article by Farquhar et al.

We hope that this Focus Issue on the Biology of Transpiration draws particular attention to the importance of interconnecting research across scales ranging from cellular to global levels, and that it will further stimulate cross-disciplinary research in the fields of stomatal function and development, water uptake and transport, and global water exchange processes.

We would like to thank everyone who was involved in the publication of this Focus Issue.