In situ characterisation of whole‐plant stomatal responses to VPD using leaf optical dendrometry

Abstract

Vapour pressure deficit (VPD) plays a crucial role in regulating plant carbon and water fluxes due to its influence on stomatal behaviour and transpiration. Yet, characterising stomatal responses of the whole plant to VPD remains challenging due to methodological limitations. Here, we develop a novel method for in situ assessment of whole‐plant stomatal responses (gc) to VPD in the herbaceous plant Tanacetum cinerariifolium. To do this, we examine the relationship between daytime VPD and the corresponding soil–stem water potential gradient (ΔΨ) monitored using the optical dendrometry in well‐hydrated plants under nonlimiting light in both glasshouse and field conditions. In glasshouse plants, ΔΨ increased proportionally with the VPD up to a threshold of 1.53 kPa, beyond which the slope decreased, suggesting a two‐phase response in gc. This pattern aligned with corresponding gravimetrically measured gc behaviour, which also showed a decline when VPD exceeded a similar threshold. This response was then compared with that of field plants monitored using the optical dendrometry technique over a growing season under naturally variable VPD conditions and nonlimiting light and water supply. Field plants exhibited a similar threshold‐type response to VPD but were more sensitive than glasshouse individuals with a VPD threshold of 0.74 kPa. The results showed that whole‐plant gc responses to VPD can be characterised optically in T. cinerariifolium, introducing a new tool for the monitoring and characterisation of stomatal behaviour in situ.

Stomatal response to VPD is not triggered by changes in soil–leaf hydraulic conductance in Arabidopsis or Callitris

“…Characterizing g c responses to VPD L has become an important focus in both crop phenotyping and plant‐atmosphere gas exchange modelling, yet remains challenging due to methodological constraints (Gilbert et al ., 2011; Gholipoor et al ., 2013; Shekoofa et al ., 2014; Grossiord et al ., 2020). The constancy of K s‐l with changing water potential and VPD L under well‐hydrated conditions observed in this study supports the idea that stomatal responses to VPD L can be inferred from the relationship between plant water potential and VPD L (De Swaef et al ., 2009; Rodriguez‐Dominguez et al ., 2019; Bourbia et al ., 2022, 2023; Bourbia & Brodribb, 2023). This opens new possibilities for characterizing stomatal behaviour in response to diurnal changes in VPD L , as well as tracking plant hydration and water use dynamics on a fine temporal scale under well‐hydrated conditions in both agricultural and natural systems (Bourbia et al ., 2023).…”

“…The constancy of K s‐l with changing water potential and VPD L under well‐hydrated conditions observed in this study supports the idea that stomatal responses to VPD L can be inferred from the relationship between plant water potential and VPD L (De Swaef et al ., 2009; Rodriguez‐Dominguez et al ., 2019; Bourbia et al ., 2022, 2023; Bourbia & Brodribb, 2023). This opens new possibilities for characterizing stomatal behaviour in response to diurnal changes in VPD L , as well as tracking plant hydration and water use dynamics on a fine temporal scale under well‐hydrated conditions in both agricultural and natural systems (Bourbia et al ., 2023). However, this approach may not be viable under drought conditions that induce significant declines in K s‐l (Blizzard & Boyer, 1980; Rodriguez‐Dominguez & Brodribb, 2020; Bourbia et al ., 2021).…”

“…Notably, in all species studied so far, K s‐l has been shown decline significantly only when reaching levels of water stress causing almost complete stomatal closure (Rodriguez‐Dominguez & Brodribb, 2020; Abdalla et al ., 2021; Bourbia et al ., 2021). Therefore, under relatively hydrated conditions, where most transpiration typically occurs (Ψ soil ≥ −1 MPa), K s‐l may be stable enough to allow monitoring of plant water use by optical Ψ proxy (Bourbia et al ., 2023; Bourbia & Brodribb, 2023).…”