Ethylene-mediated stomatal responses to dehydration and rehydration in seed plants.

Ethylene, a plant hormone that significantly influences both plant growth and response to stress, plays a well-established role in stress signaling. However, its impact on stomatal opening and closure during dehydration and rehydration remains relatively unexplored and is still debated. Exogenous ethylene has been proven to induce stomatal closure through a series of signaling pathways, including the accumulation of reactive oxygen species (ROS), subsequent synthesis of nitric oxide (NO) and hydrogen sulfide (H2S), and SLOW ANION CHANNEL-ASSOCIATED 1 (SLAC1) activation. Thus, it has been suggested that ethylene might function to induce stomatal closure synergistically with abscisic acid (ABA). Furthermore, it has also been shown that increased ethylene can inhibit ABA- and jasmonic acid (JA)-induced stomatal closure, thus hindering drought-induced closure during dehydration. Simultaneously, other stresses, such as chilling, ozone pollution and K+ deficiency, inhibit drought and ABA-induced stomatal closure through an ethylene synthesis dependent way. However, ethylene has been shown to take on an opposing role during rehydration, preventing stomatal opening in the absence of ABA through its own signaling pathway. These findings offer novel insights into the function of ethylene in stomatal regulation during dehydration and rehydration, gaining a better understanding of the mechanisms underlying ethylene-induced stomatal movement in seed plants.

Rapid drought-recovery of gas exchange in Caragana species adapted to low mean annual precipitation.

While variation in mean annual precipitation (MAP) of the native habitat of a species has been shown to determine the ability of a species to resist a hydraulic decrease during drought, it remains unknown whether these variations in MAP also influence the ability of a species to recover and survive drought. Leaf hydraulic and gas exchange recovery following drought and the underlying mechanisms of these responses in six Caragana species from habitats along a large precipitation gradient were investigated during rehydration in a common garden. The gas exchange of species from arid habitats recovered more rapidly during rehydration after mild, moderate and severe drought stress treatments than species from humid habitats. The recovery of gas exchange was not associated with foliar abscisic acid concentration, but tightly related to the recovery of leaf hydraulic conductance (Kleaf ). The recovery of Kleaf was associated with the loss of Kleaf during dehydration under mild and moderate drought stress, and to leaf xylem embolism formation under severe drought stress. Results pointed to the different ability to recover in gas exchange in six Caragana species post-drought is associated with the MAP of the species in its native habitat.

Polyamines inhibit abscisic acid-induced stomatal closure by scavenging hydrogen peroxide.

Stomatal closure is regulated by plant hormones and some small molecules to reduce water loss under stress conditions. Both abscisic acid (ABA) and polyamines alone induce stomatal closure; however, whether the physiological functions of ABA and polyamines are synergistic or antagonistic with respect to inducing stomatal closure is still unknown. Here, stomatal movement in response to ABA and/or polyamines was tested in Vicia faba and Arabidopsis thaliana, and the change in the signaling components under stomatal closure was analyzed. We found that both polyamines and ABA could induce stomatal closure through similar signaling components, including the synthesis of hydrogen peroxide (H2 O2 ) and nitric oxide (NO) and the accumulation of Ca2+ . However, polyamines partially inhibited ABA-induced stomatal closure both in epidermal peels and in planta by activating antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), to eliminate the ABA-induced increase in H2 O2 . These results strongly indicate that polyamines inhibit abscisic acid-induced stomatal closure, suggesting that polyamines could be used as potential plant growth regulators to increase photosynthesis under mild drought stress.

Ethylene constrains stomatal reopening in Fraxinus chinensis post moderate drought.

Clarifying the mechanisms underlying the recovery of gas exchange following drought is the key to providing insights into plant drought adaptation and habitat distribution. However, the mechanisms are still largely unknown. Targeting processes known to inhibit gas exchange during drought recovery, we measured leaf water potential, the leaf hydraulic conductance, stomatal reopening, abscisic acid (ABA) and the ethylene emission rate (EER) following moderate drought stress in seedlings of the globally pervasive woody tree Fraxinus chinensis. We found strong evidence that the slow stomatal reopening after rehydration is regulated by a slow decrease in EER, rather than changes in leaf hydraulics or foliar ABA levels. This was supported by evidence of rapid gas exchange recovery in plants after treatment with the ethylene antagonist 1-methylcyclopropene. These findings provide evidence to rigorously support ethylene as a key factor constraining stomatal reopening from moderate drought directly, thereby potentially opening new windows for understanding species drought adaptation.

Divergent stem hydraulic strategies of Caragana korshinskii resprouts following a disturbance.

Resprouting plants are distributed in many vegetation communities worldwide. With increasing resprout age post-severe-disturbance, new stems grow rapidly at their early age, and decrease in their growth with gradually decreasing water status thereafter. However, there is little knowledge about how stem hydraulic strategies and anatomical traits vary post-disturbance. In this study, the stem water potential (Ψstem), maximum stem hydraulic conductivity (Kstem-max), water potential at 50% loss of hydraulic conductivity (Kstem  P50) and anatomical traits of Caragana korshinkii resprouts were measured during a 1- to 13-year post-disturbance period. We found that the Kstem-max decreased with resprout age from 1-year-old resprouts (84.2 mol m-1 s-1 MPa-1) to 13-year-old resprouts (54.2 mol m-1 s-1 MPa-1) as a result of decreases in the aperture fraction (Fap) and the sum of aperture area on per unit intervessel wall area (Aap). The Kstem  P50 of the resprouts decreased from 1-year-old resprouts (-1.8 MPa) to 13-year-old resprouts (-2.9 MPa) as a result of increases in vessel implosion resistance (t/b)2, wood density (WD), vessel grouping index (GI) and decreases in Fap and Aap. These shifts in hydraulic structure and function resulted in an age-based divergence in hydraulic strategies i.e., a change from an acquisitive strategy to a conservative strategy, with increasing resprout age post-disturbance.

A clear trade-off between leaf hydraulic efficiency and safety in an aridland shrub during regrowth.

It has been suggested that a trade-off between hydraulic efficiency and safety is related to drought adaptation across species. However, whether leaf hydraulic efficiency is sacrificed for safety during woody resprout regrowth after crown removal is not well understood. We measured leaf water potential (ψleaf ) at predawn (ψpd ) and midday (ψmid ), leaf maximum hydraulic conductance (Kleaf-max ), ψleaf at induction 50% loss of Kleaf-max (Kleaf P50 ), leaf area-specific whole-plant hydraulic conductance (LSC), leaf vein structure and turgor loss point (πtlp ) in 1- to 13-year-old resprouts of the aridland shrub (Caragana korshinskii). ψpd was similar, ψmid and Kleaf P50 became more negative, and Kleaf-max decreased in resprouts with the increasing age; thus, leaf hydraulic efficiency clearly traded off against safety. The difference between ψmid and Kleaf P50 , leaf hydraulic safety margin, increased gradually with increasing resprout age. More negative ψmid and Kleaf P50 were closely related to decreasing LSC and more negative πtlp , respectively, and the decreasing Kleaf-max arose from the lower minor vein density and the narrower midrib xylem vessels. Our results showed that a clear trade-off between leaf hydraulic efficiency and safety helps C. korshinskii resprouts adapt to increasing water stress as they approach final size.

Dew absorption by leaf trichomes in Caragana korshinskii: An alternative water acquisition strategy for withstanding drought in arid environments.

Investigating plant morphological traits can provide insights into plant drought tolerance. To date, many papers have focused on plant hydraulic responses to drought during dehydration, but atmospheric water absorption by trichomes to mitigate drought stress by influencing leaf hydraulics in plant species that inhabit arid environments has been largely ignored. The experiment in this study was designed to assess how dew absorbed by leaf trichomes helps Caragana korshinskii withstand drought. The results showed that under a drought stress and dew (DS & D) treatment, C. korshinskii displayed a strong capacity to absorb dew with trichomes; exhibited slow decreases in leaf water potential (Ψleaf ), leaf hydraulic conductivity (Kleaf ), and gas exchange; experienced 50% Kleaf and gas exchange losses at lower relative soil water content levels than plants treated with drought stress and no dew (DS & ND); and experienced 50% Kleaf loss (Kleaf P50 ) at similar Ψleaf levels as DS & ND plants. Its congener C. sinica, which does not have leaf trichomes, displayed little ability to absorb dew under drought stress and did not show any remarkable improvement in the above parameters under the DS & D treatment. Our results indicated that leaf trichomes are important epidermal dew-uptake structures that assist in partially sustaining the leaf hydraulic assimilation system, mitigate the adverse effects of drought stress and contribute to the distribution of C. korshinskii in arid environments.

Combined high leaf hydraulic safety and efficiency provides drought tolerance in Caragana species adapted to low mean annual precipitation.

Clarifying the coordination of leaf hydraulic traits with gas exchange across closely-related species adapted to varying rainfall can provide insights into plant habitat distribution and drought adaptation. The leaf hydraulic conductance (Kleaf ), stomatal conductance (gs ), net assimilation (A), vein embolism and abscisic acid (ABA) concentration during dehydration were quantified, as well as pressure-volume curve traits and vein anatomy in 10 Caragana species adapted to a range of mean annual precipitation (MAP) conditions and growing in a common garden. We found a positive correlation between Ψleaf at 50% loss of Kleaf (Kleaf P50 ) and maximum Kleaf (Kleaf-max ) across species. Species from low-MAP environments exhibited more negative Kleaf P50 and turgor loss point, and higher Kleaf-max and leaf-specific capacity at full turgor, along with higher vein density and midrib xylem per leaf area, and a higher ratio of Kleaf-max : maximum gs . Tighter stomatal control mediated by higher ABA accumulation during dehydration in these species resulted in an increase in hydraulic safety and intrinsic water use efficiency (WUEi ) during drought. Our results suggest that high hydraulic safety and efficiency combined with greater stomatal sensitivity triggered by ABA production and leading to greater WUEi provides drought tolerance in Caragana species adapted to low-MAP environments.

Ethylene, not ABA, is closely linked to the recovery of gas exchange after drought in four Caragana species.

Drought is a cyclical phenomenon in natural environments. During dehydration, stomatal closure is mainly regulated by abscisic acid (ABA) dynamics that limit transpiration in seed plants, but following rehydration, the mechanism of gas exchange recovery is still not clear. In this study, leaf water potential (ψleaf ), stomatal conductance (gs ), leaf hydraulic conductance (Kleaf ), foliar ABA level, ethylene emission rate in response to dehydration and rehydration were investigated in four Caragana species with isohydric (Caragana spinosa and C. pruinosa) and anisohydric (C. intermedia and C. microphylla) traits. Two isohydric species with ABA-induced stomatal closure exhibited more sensitive gs and Kleaf to decreasing ψleaf than two anisohydric species which exhibited a switch from ABA to water potential-driven stomatal closure during dehydration. Following rehydration, the recovery of gas exchange was not associated with a decrease in ABA level but was strongly limited by the degradation of the ethylene emission rate in all species. Furthermore, two anisohydric species with low drought-induced ethylene production exhibited more rapid recovery in gas exchange upon rehydration. Our results indicated that ethylene is a key factor regulating the drought-recovery ability in terms of gas exchange, which may shape species adaptation to drought and potential species distribution.