Impact of Stomatal Density and Morphology on Water-Use Efficiency in a Changing World

Abstract

Global warming and associated precipitation changes will negatively impact on many agricultural ecosystems. Major food production areas are expected to experience reduced water availability and increased frequency of drought over the coming decades. In affected areas, this is expected to reduce the production of important food crops including wheat, rice, and maize. The development of crop varieties able to sustain or improve yields with less water input is, therefore, a priority for crop research. Almost all water used for plant growth is lost to the atmosphere by transpiration through stomatal pores on the leaf epidermis. By altering stomatal pore apertures, plants are able to optimize their CO₂ uptake for photosynthesis while minimizing water loss. Over longer periods, stomatal development may also be adjusted, with stomatal size and density being adapted to suit the prevailing conditions. Several approaches to improve drought tolerance and water-use efficiency through the modification of stomatal traits have been tested in the model plant Arabidopsis thaliana. However, there is surprisingly little known about the stomata of crop species. Here, we review the current understanding of how stomatal number and morphology are involved in regulating water-use efficiency. Moreover, we discuss the potential and limitations of manipulating stomatal development to increase drought tolerance and to reduce water loss in crops as the climate changes.

Keywords: crops; drought response; stomatal conductance; stomatal density and size; water-use efficiency.

Figure 1

Stomatal traits vary between species. The eudicots (A) Arabidopsis thaliana and (B) Phaseolus vulgaris display kidney-shaped guard cells (colored in green). The grasses (C) Oryza sativa and (D) Triticum aestivum show dumbbell-shaped guard cells (solid green) and specialized subsidiary cells (light green gradient). Clear differences in stomatal size and stomatal density can be observed. Scale bars 10 μM.

Figure 2

OsEPF1oe rice plants with reduced stomatal density and size are able to maintain high rates of gas exchange under heat stress conditions by opening their stomatal pores (adapted from Caine et al., 2019). Epidermis of (A) non-transgenic control and (B) OsEPF1oe plants grown at 30°C, bars = 25 μM. Stomata of control plants grown at (C) 30°C and (D) 40°C. Control plants show increases in stomatal density and in maximum leaf stomatal conductance average values under high temperature conditions. OsEPF1oe plants grown at (E) 30°C and (F) 40°C. Transgenic line shows an increase in stomatal aperture at 40°C, reaching similar g_s levels as control plants, despite lower maximum stomatal conductance. Units: g_smax and g_s = mol m⁻² s⁻¹, SD = mm⁻².