Linking Turgor with ABA Biosynthesis: Implications for Stomatal Responses to Vapor Pressure Deficit across Land Plants

Abstract

Stomatal responses to changes in vapor pressure deficit (VPD) constitute the predominant form of daytime gas-exchange regulation in plants. Stomatal closure in response to increased VPD is driven by the rapid up-regulation of foliar abscisic acid (ABA) biosynthesis and ABA levels in angiosperms; however, very little is known about the physiological trigger for this increase in ABA biosynthesis at increased VPD Using a novel method of modifying leaf cell turgor by the application of external pressures, we test whether changes in turgor pressure can trigger increases in foliar ABA levels over 20 min, a period of time most relevant to the stomatal response to VPD We found in angiosperm species that the biosynthesis of ABA was triggered by reductions in leaf turgor, and in two species tested, that a higher sensitivity of ABA synthesis to leaf turgor corresponded with a higher stomatal sensitivity to VPD In contrast, representative species from nonflowering plant lineages did not show a rapid turgor-triggered increase in foliar ABA levels, which is consistent with previous studies demonstrating passive stomatal responses to changes in VPD in these lineages. Our method provides a new tool for characterizing the response of stomata to water availability.

Figure 1.

The mean change in foliar ABA level (n = 3, ±95% confidence interval) in two herbaceous angiosperm species (P. sativum wild type [A] and wilty [B]; Arabidopsis [C]) after excised leaves were exposed to external pressures for 20 min (black circles) or 60 min (white circles). Vertical lines indicate Ψ_tlp (Supplemental Fig. S3). The insert in A depicts the mean change in foliar ABA level (n = 4, ±95% confidence interval) in leaves of P. sativum with the abaxial epidermis removed after floating on aqueous solutions of PEG 4000 mixed to particular water potentials for 20 min (black circles) or 60 min (white circles). Stars denote a significant change in foliar ABA level (n.s., not significant; * P < 0.05; ** P < 0.01; *** P < 0.001). Foliar ABA levels presented in terms of dry weight are shown in Supplemental Figure S4.

Figure 2.

A and B, The mean change in foliar ABA level (n = 3, ±95% confidence interval) in two woody angiosperm species (O. europaea and N. cunninghamii) after excised leaves were exposed to external pressures for 20 min (black circles) or 60 min (white circles). Vertical lines indicate Ψ_tlp (Supplemental Fig. S3). Stars denote a significant change in foliar ABA level (n.s., not significant; * P < 0.05; ** P < 0.01; *** P < 0.001). C and D, The mean response of stomatal conductance (n = 3, ±se) to a step change in VPD (depicted by vertical lines) from 0.7 kPa to 1.5 kPa (black circles) or 0.7 kPa to 2.2 kPa (gray circles). Arrows above the regressions in A and B correspond to incipient Ψ_l measured in branches immediately prior to stomatal closure when exposed to a step change in VPD from 0.7 kPa to 1.5 kPa (black arrows) or 0.7 kPa to 2.2 kPa (gray arrows). Foliar ABA levels presented in terms of dry weight are shown in Supplemental Figure S4.

Figure 3.

The response of both stomatal conductance (black) to a step increase in VPD from 0.7 kPa to 1.5 kPa and stomatal resistance (white) to the reversed step change in VPD from 1.5 kPa to 0.7 kPa. Data were collected from the same leaf through a reversible sequence in VPD from 0.7 kPa to 1.5 kPa and returning to 0.7 kPa; in O. europaea a step change of 0.7 kPa to 2.2 kPa and returning to 0.7 kPa was used. Time zero for each trace marks the time of a change in VPD.

Figure 4.

The mean change in foliar ABA level (n = 3, ±95% confidence interval) in three conifer species (A–C), two fern species (D and E), and a lycophyte (F) after excised leaves were exposed to external pressures for 20 min (black circles) or 60 min (white circles). Vertical lines indicate Ψ_tlp (Supplemental Fig. S1). In all species, changes in foliar ABA level were not significant. Foliar ABA levels presented in terms of dry weight are shown in Supplemental Figure S4.