Extreme drought can deactivate ABA biosynthesis in embolism‐resistant species

Abstract

The phytohormone abscisic acid (ABA) is synthesised by plants during drought to close stomata and regulate desiccation tolerance pathways. Conifers and some angiosperms with embolism‐resistant xylem show a peaking‐type (p‐type) response in ABA levels, in which ABA levels increase early in drought then decrease as drought progresses, declining to pre‐stressed levels. The mechanism behind this dynamic remains unknown. Here, we sought to characterise the mechanism driving p‐type ABA dynamics in the conifer Callitris rhomboidea and the highly drought‐resistant angiosperm Umbellularia californica. We measured leaf water potentials (Ψl), stomatal conductance, ABA, conjugates and phaseic acid (PA) levels in potted plants during a prolonged but non‐fatal drought. Both species displayed a p‐type ABA dynamic during prolonged drought. In branches collected before and after the peak in endogenous ABA levels in planta, that were rehydrated overnight and then bench dried, ABA biosynthesis was deactivated beyond leaf turgor loss point. Considerable conversion of ABA to conjugates was found to occur during drought, but not catabolism to PA. The mechanism driving the decline in ABA levels in p‐type species may be conserved across embolism‐resistant seed plants and is mediated by sustained conjugation of ABA and the deactivation of ABA accumulation as Ψl becomes more negative than turgor loss.

Passive stomatal closure under extreme drought in an angiosperm species

“…(A) The relationship between stomatal conductance (grey) and foliage ABA level (black) during drought in U. californica , data are generalized additive models (±SE) taken from Mercado-Reyes et al (2023) from the same plants from which gas exchange data was collected. (B-F) The response of stomatal conductance (black dots) to instantaneous branch rehydration (denoted by the vertical line), the foliage ABA level is shown above each panel, the water potential prior to rehydration is shown parallel to the line denoting rehydration, the water potential at the end of the gas exchange trace is also shown.…”

“…All data collected to date that reports a peaking-type ABA dynamic during drought has found that this phenomenon only occurs at very low water potentials (less than -4 MPa) and consequently a requirement is highly embolism resistant xylem (Brodribb et al 2014; Nolan et al 2017; Yao et al 2021b; Yao et al 2021a; Mercado-Reyes et al 2023). Recent work in U. californica and C. rhomboidea suggests that ABA biosynthesis is deactivated at water potentials lower than turgor loss point and requires sufficient time for the residual ABA to be conjugated for the levels to decline (Mercado-Reyes et al 2023). Modelling suggests that passive stomatal control in angiosperms is possible once epidermal turgor is lost and the mechanical advantage of the epidermis is removed (Buckley 2019).…”

“…In some species of Cupressaceae and Taxaceae (Brodribb et al 2014) and in extremely embolism resistant species of Fabaceae (Nolan et al 2017; Yao et al 2021b; Yao et al 2021a) and Lauraceae (Mercado-Reyes et al 2023) ABA levels increase as soil water potential begins to decline, closing stomata, but then once plants reach a water potential that approximates bulk leaf turgor loss point then ABA levels in the leaf begin to decline. Rapid rehydration experiments in the Cupressaceae conifer Callitris indicates that when ABA levels are low under extreme drought the stomata reopen to maximum apertures as fast as leaf water potential relaxes (Brodribb and McAdam 2013; McAdam and Brodribb 2015).…”

Divergent effects of single and combined stress of drought and salinity on the physiological traits and soil properties of Platycladus orientalis saplings

“…Single drought stress and combined stress of drought and moderate salt stress resulted in significantly higher root ABA content, while root ABA content under combined stress of drought and severe salt stress was not significantly different from the control. Previous studies have shown that Cupressaceae tree species exhibit p-type ABA dynamics under severe drought, i.e., ABA levels increase early in the drought and decrease as the water potential further decreases ( Brodribb et al., 2014 ; Mercado-Reyes et al., 2023 ). Similar reasons may explain the lower root ABA in DSS group compared with DMS group in our study.…”

“…Similar reasons may explain the lower root ABA in DSS group compared with DMS group in our study. Roots are the primary site of ABA synthesis, and the decrease in root ABA content may result from inhibited synthesis under stress ( Mercado-Reyes et al., 2023 ). We observed significantly lower leaf ABA in the DSS group than in the DMS group and hypothesized that p-type ABA dynamics also applied in the leaves of P. orientalis saplings.…”