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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 (K leaf ). The recovery of K leaf was associated with the loss of K leaf 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., (© 2023 John Wiley & Sons Ltd.)

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., (© The Author(s) 2021. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

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 (Ψ leaf ), leaf hydraulic conductivity (K leaf ), and gas exchange; experienced 50% K leaf 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% K leaf loss (K leaf P 50 ) at similar Ψ leaf 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., (© 2021 Scandinavian Plant Physiology Society.)

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 (ψ leaf ), stomatal conductance (g s ), leaf hydraulic conductance (K leaf ), 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 g s and K leaf to decreasing ψ leaf 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., (© 2020 John Wiley & Sons Ltd.)

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 (K leaf ), stomatal conductance (g s ), 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 Ψ leaf at 50% loss of K leaf (K leaf P 50 ) and maximum K leaf (K leaf-max ) across species. Species from low-MAP environments exhibited more negative K leaf P 50 and turgor loss point, and higher K leaf-max and leaf-specific capacity at full turgor, along with higher vein density and midrib xylem per leaf area, and a higher ratio of K leaf-max : maximum g s . 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 (WUE i ) during drought. Our results suggest that high hydraulic safety and efficiency combined with greater stomatal sensitivity triggered by ABA production and leading to greater WUE i provides drought tolerance in Caragana species adapted to low-MAP environments., (© 2020 The Authors New Phytologist © 2020 New Phytologist Trust.)

Seed germination behavior is an important factor in the distribution of species. Many studies have shown that germination is controlled by phylogenetic constraints, however, it is not clear whether phylogenetic constraints or environmental cues explain seed germination of a genus from a common ancestor. In this study, seed germination under different temperature- and water-regimes [induced by different osmotic potentials of polyethylene glycol (PEG)] was investigated in the phylogenetically-related Caragana species that thrive in arid, semiarid, semihumid and humid environments. The results showed that the final percentage germination (FPG) decreased from 95% in species from arid habitats to 0% in species from humid habitats, but with no significant phylogenetic signal. Rather, the response of seed germination to temperature and PEG varied greatly with species from arid to humid habitats and was tightly linked to the ecological niche of the species, their seed coat structure and abscisic acid concentration. The findings are not consistent with the hypothesis that within a family or a genus, seed germination strategies can be a stable evolutionary trait, thus constraining interspecific variation, but the results clearly show that seed germination of Caragana species distributed across a range of habitats has adapted to the environment of that habitat.

Predawn leaf water potential (LWP), the LWP between 09:00 and 10:30 h (termed minimum LWP), stem xylem hydraulic conductivity, foliar nitrogen, leaf gas exchange and leaf traits were measured on the same days in adults and 1-year-old to 7-year-old resprouts that had regrown after removing all the aboveground shoots. Height growth and accumulation of aboveground biomass quickly decreased with resprout age and there was no difference between 7-year-old resprouts and the uncut adults. Predawn LWP showed no significant difference between resprouts and adults, but the minimum LWP decreased gradually from -2.0 MPa in 1-year-old resprouts to -3.0 MPa in 7-year-old resprouts. The decrease in minimum LWP was associated with increased hydraulic resistance, as indicated by the gradual decrease in leaf area-specific hydraulic conductivity (KL) and sapwood area-specific hydraulic conductivity (KS) and the associated increase in stem native percentage loss of hydraulic conductivity in older than 2-year-old resprouts. The leaf nitrogen content per unit area (Narea) also decreased steadily from 3.6 g m(-2) in 1-year-old resprouts to 1.7 g m(-2) in 7-year-old resprouts. With the decline in LWP and Narea, the rate of leaf photosynthesis per unit area (Aarea) decreased from 20 μ mol m(-2) s(-1) in 1-year-old resprouts to 11 μ mol m(-2) s(-1) in 7-year-old resprouts. In adults, although KS decreased further compared with 7-year-old resprouts, the minimum LWP, KL, Narea and the rate of photosynthesis increased by 0.3 MPa, 29, 34 and 23%, respectively. The results show that a progressive loss of stem hydraulic conductivity and a steady decrease in foliar nitrogen with age were associated with a decrease in the photosynthetic rate of Caragana korshinskii Kom. resprouts, possibly changing the allocation of photosynthetic assimilates and slowing resprout height growth.

Caragana species are widespread in northwest China. However, species in semiarid areas of Inner Mongolia experience summer and autumn rainfall, whereas morphologically similar close relatives in arid areas in Xinjiang Province experience wet springs, but hot dry summers. We hypothesize that the differences among species in response to soil drought help to explain their distributions. A comparison of the closely related species C. intermedia and C. microphylla from Inner Mongolia (semiarid species) and C. pruinosa and C . spinosa from Xinjiang (arid species) was conducted to examine whether responses to a water deficit in a pot experiment were associated with the environments of their habitat. In the two semiarid species, the stomatal conductance was relatively insensitive to the decrease in predawn leaf water potential (Ψ), and leaves did not abscise or die even when the Ψ decreased below −6.0 MPa, while in seedlings of the two arid species the stomata closed at a relatively high Ψ, and the leaves abscised gradually when Ψ fell below about −3.0 MPa. Furthermore, major vein density, minor vein density and the cubed ratio of the conduit wall thickness to the conduit lumen breadth [(t/b)] of minor veins increased by an average of 60, 150 and 220 %, respectively, in the semiarid species compared with the arid species. The results indicated that semiarid species have typical anisohydric properties, while arid species have typical isohydric properties. We suggest that the divergence in water-use strategies in the genus may be associated with variation in vein architecture, and may possibly play an important role in determining the distribution of these species in the different environments of northwest China. [ABSTRACT FROM AUTHOR]