Increased water availability at various timescales has different effects on stomatal closure point in isohydric piñon pine and anisohydric juniper

Abstract

Stomatal Closure Point (SCP) has commonly been used to describe drought response strategies in plants, with isohydric species maintaining relatively high, constant SCP compared to anisohydric species that can lower SCP with increasing drought severity. However, there is evidence that, within these groups, SCP may respond dynamically to environmental conditions. Here, we explored how increasing water availability affects SCP in classically isohydric piñon pine and anisohydric one-seed or Utah juniper at various spatial- (i.e., from branch, to tree, to ecosystem) and temporal- (i.e., hours to decades) scales. Our results show that short-term increases in water availability decreased SCP in isohydric piñon pine, making it more anisohydric, while short-term rehydration had no effect on SCP in anisohydric juniper. Increasing mean annual precipitation, on the other hand, increased SCP in both species. Our findings are consistent with documented differences in the use of ABA to control stomata in iso- and aniso-hydric species on short timescales, and with structural acclimation in both species at long timescales. These results illustrate that the local environment plays a large role in determining SCP.

Keywords: Juniperus monosperma; Pinus edulis; Anisohydric; Drought; Isohydric; Water availability.

Fig. 1

Piñon SCP declines with increased branch hydration. Piñon SCP significantly decreased by more than 1.5 MPa (p = 0.03) with “Overnight Rehydration” relative to “No Overnight Rehydration” in the “Initial Rehydration” treatment (A). Juniper SCP did not change with rehydration (B). Error bars are standard errors. Lower case letters indicate significant differences at p = 0.05.

Fig. 2

Piñon Ψ_min declines after months of regular watering. Pre-dawn (Ψ_pd) and mid-day (Ψ_md, Ψ_min) leaf water potentials of five potted mature piñon pine trees irrigated daily for over one year and measured weekly after acclimatizing for a three month (14 week) period (A). After 5 months of regular watering (“Zone 1”), piñon Ψ_min declined by over 1.2 MPa (p < 2.2e⁻¹⁶), and this decrease was maintained for the duration of the study (8 months; “Zone 2”; B). Vertical black dashed line in panel (A) indicates the point after which pre-dawn (orange dashed line) and mid-day (red dashed line) Ψ_leaf differed significantly for more than two consecutive weeks and Ψ_md remained significantly lower than Ψ_pd. Error bars are standard deviation. Lower case letters indicate significant differences at p = 0.05.

Fig. 3

Piñon and juniper SCP increased with water availability across the landscape. Juniper (black) and piñon (gray) SCP both increased with increasing mean annual precipitation (MAP). Slopes did not differ significantly between species (p = 0.2, Table S1) and SCP increased, on average, by 0.35 MPa with a 100 mm increase in MAP (p = 0.05, Table 1).

Fig. 4

Hypothetical variation in SCP with water availability at various timescales. (A) On short timescales in piñon pine (solid line), increased water availability decreases SCP, possibly by shifting Ψ_ABA (blue arrows), the trigger point for ABA production. (B) On short timescales (solid red line) under decreased leaf water potentials (Ψ_leaf), ABA increases in ‘R type’ species, like piñon pine (left panel), while ABA briefly increases, peaks and then declines in ‘P type’ species like juniper (right panel). Under rehydration (dashed blue lines), a more negative Ψ_leaf is required to trigger the same amount of ABA production in ‘R type’ species while ABA remains at low values in ‘P type’ species. (C) Piñon pine is unable to adjust turgor loss point (Ψ_TLP) (left panel), however, Ψ_TLP increases with increasing water availability in juniper (right panel). (D) In both piñon pine and juniper, structural acclimation may occur on both short and long timescales, increasing SCP in both species (A, dashed line).

Fig. 5

Site locations used in the ecosystem-scale analysis. 1) Tooele, UT; 2) Birdseye, UT; 3) Zion, UT; 4) Pajarito Plateau, NM; 5) Grand Canyon, AZ; 6) Pinedale, AZ; 7) Los Pinos Mountains, NM; 8) Blue, AZ. See Table 2 for more site information.