Cells, Genes & Molecules

Control of the stomatal response to VPD in sunflower

Stomata close in response to increasing VPD but what is the mechanism responsible for this response?

The stomatal pores on plant leaves allow CO2 to enter and water vapour to leave, thereby controlling plant hydration and photosynthesis. The stomata dynamically respond to a variety of environmental factors however the most pervasive stomatal dynamics are responses to light and leaf–air vapour pressure difference (VPD). Focussing on VPD, the stomata close when VPD increases (or in other words the stomata close if the air surrounding the leaf becomes less humid than the air inside the leaf), moderating the impact of evaporative demand on plant water loss. However, the mechanism responsible for this response in angiosperms remains under debate. It has been hypothesized that it may be controlled by passive changes in stomatal guard cell turgor, changes in ionic fluxes into the guard cells and/or action of the phytohormone abscisic acid.

When exposed to high vapour pressure deficit, sunflower plants rapidly lose turgor, this triggers the biosynthesis of abscisic acid that closes stomata reducing transpiration. Image credit: Amanda A. Cardoso.

In their new study published in AoBP, Cardoso et al. seek to separate hydraulic and hormonal control of the stomatal response to VPD by manipulating the osmotic potential of sunflower (Helianthus annuus) leaves. They also tested this response in an ABA-deficient mutant of sunflower (the so-called wilty mutant). They found that stomatal apertures during VPD transitions were closely linked with foliage ABA levels in sunflower plants with contrasting osmotic potentials. In addition, they observed that the inability to synthesize ABA at high VPD in mutant plants resulted in stomata not responding to increased VPD. These results are consistent with a hormonal, ABA-mediated stomatal response to VPD rather than a hydraulic-driven stomatal response to VPD. Although there are many questions still to be answered regarding this complex mechanism, the authors conclude that foliage ABA levels provide the best metabolic signal to explain the stomatal response to VPD in angiosperms.

Researcher highlight

Amanda A. Cardoso is a Brazilian scientist who has been investigating plant hydraulics since 2016, when she moved to Australia to conduct part of her PhD with Professor Timothy Brodribb at the University of Tasmania. Next, she conducted a postdoc in the laboratory of Professor Scott McAdam at Purdue University in the USA. Amanda is mostly interested in understanding the transport of water within plants and her background goes all the way from xylem to stomata. Besides studying angiosperms, she utilizes non-conventional plants such as lycophytes and ferns to understand the origin and evolution of water transport in plants.