Relationships between stomatal behavior,xylem vulnerability to cavitation and leaf water relations in two cultivars of Vitis vinifera

Current understanding of physiological mechanisms governing stomatal behavior under water stress conditions is still incomplete and controversial. It has been proposed that coordination of stomatal kinetics with xylem vulnerability to cavitation [vulnerability curve (VC)] leads to different levels of isohydry/anisohydry in different plant species/cultivars. In this study, this hypothesis is tested in Vitis vinifera cultivars displaying contrasting stomatal behavior under drought stress. The cv Montepulciano (MP, near‐isohydric) and Sangiovese (SG, anisohydric) were compared in terms of stomatal response to leaf and stem water potential, as possibly correlated to different petiole hydraulic conductivity (k_petiole) and VC, as well as to leaf water relations parameters. MP leaves showed almost complete stomatal closure at higher leaf and stem water potentials than SG leaves. Moreover, MP petioles had higher maximum k_petiole and were more vulnerable to cavitation than SG. Water potential at the turgor loss point was higher in MP than in SG. In SG, the percentage reduction of stomatal conductance (PLg_s) under water stress was almost linearly correlated with corresponding percentage loss of k_petiole (PLC), while in MP PLg_s was less influenced by PLC. Our results suggest that V. vinifera near‐isohydric and anisohydric genotypes differ in terms of xylem vulnerability to cavitation as well as in terms of k_petiole, and that the coordination of these traits leads to their different stomatal responses under water stress conditions.     

Different combinations of morpho‐physiological traits are responsible for tolerance to drought in wild tomatoes Solanum chilense and Solanum peruvianum

Herbaceous species can modify leaf structure during the growing season in response to drought stress and water loss. Evolution can select combinations of traits in plants for efficient water use in restricted environments. We investigated plant traits that mediate adaptation and acclimation to water stress in two herbaceous drought‐tolerant species. Anatomical, morphological and physiological traits related to stems and leaves were examined under optimal watering (OW) and a long period of restricted watering (RW) in 11 accessions from three Solanaceae species (Solanum chilense, S. peruvianum and S. lycopersicum). The relationships between these traits were tested using linear regression and PCA. There were significant differences in anatomical traits between the species under both OW and RW, where leaf area correlated with stem diameter. Proline and total carbohydrates accumulated highly in S. chilense and S. peruvianum, respectively, and these osmolytes were strongly correlated with increased osmotic potential. Stomatal density varied between species but not between acclimation treatments, while stomatal rate was significantly higher in wild tomatoes. There was a strong positive relationship between stem growth rate and a group of traits together expressed as total stomatal number. Total stomata is described by integration of leaf area, stomatal density, height and internode length. It is proposed that constitutive adaptations and modifications through acclimation that mediate RW play an important role in tolerance to drought stress in herbaceous plants. The capacity for growth under drought stress was not associated with any single combination of traits in wild tomatoes, since the two species differed in relative levels of expression of various phenotypic traits.     

干旱胁迫与复水对块根紫金牛生理特性的影响

以岩溶特有药用植物块根紫金牛为试材,研究土壤水分胁迫及复水条件下其叶片光合参数、相对含水量、质膜透性、渗透调节物质含量的变化特性。结果表明:水分胁迫下,块根紫金牛的叶片净光合速率、气孔导度和蒸腾速率均几乎接近零点,但胞间CO2浓度上升,即非气孔因素限制是光合速率下降的主要原因。水分胁迫不影响块根紫金牛单位面积的总叶绿素和类胡萝卜素含量,但干旱处理的Chl a/b和Car/Chl分别显著低于和高于对照。水分胁迫下,块根紫金牛的叶片相对含水量、相对电导率和丙二醛含量显著增大,即膜系统受到一定的伤害;块根紫金牛叶片脯氨酸含量显著降低,可溶性蛋白含量无显著变化,可溶性糖含量显著增大,但增大幅度不大,说明其在干旱胁迫下的渗透调节能力较弱。复水处理后,块根紫金牛全部指标均能恢复到对照水平,说明其对干旱胁迫较为敏感,主要采取避旱策略。     

Physiological and anatomical changes induced by drought in two olive cultivars (cv Zalmati and Chemlali)

Photosynthetic gas exchange, vegetative growth, water relations and fluorescence parameters as well as leaf anatomical characteristics were investigated on young plants of two Olea europaea L. cultivars (Chemlali and Zalmati), submitted to contrasting water availability regimes. Two-year-old olive trees, grown in pots in greenhouse, were not watered for 2 months. Relative growth rate (RGR), leaf water potential (Ψ_LW) and the leaf relative water content (LWC) of the two cultivars decreased with increasing water stress. Zalmati showed higher values of RGR and LWC and lower decreased values of Ψ_LW than Chemlali, in response to water deficit, particularly during severe drought stress. Water stress also caused a marked decline on photosynthetic capacity and chlorophyll fluorescence. The net photosynthetic rate, stomatal conductance, transpiration rate, the maximal photochemical efficiency of PSII (F _v/F _m) and the intrinsic efficiency of open PSII reaction centres (F′ _v/F′ _m) decreased as drought stress developed. In addition, drought conditions, reduced leaf chlorophyll and carotenoids contents especially at severe water stress. However, Zalmati plants were the less affected when compared with Chemlali. In both cultivars, stomatal control was the major factor affecting photosynthesis under moderate drought stress. At severe drought-stress levels, the non-stomatal component of photosynthesis is inhibited and inactivation of the photosystem II occurs. Leaf anatomical parameters show that drought stress resulted in an increase of the upper epidermis and palisade mesophyll thickness as well as an increase of the stomata and trichomes density. These changes were more characteristic in cv. ‘Zalmati’. Zalmati leaves also revealed lower specific leaf area and had higher density of foliar tissue. From the behaviour of Zalmati plants, with a smaller reduction in relative growth rate, net assimilation rate and chlorophyll fluorescence parameters, and with a thicker palisade parenchyma, and a higher stomatal and trichome density, we consider this cultivar more drought-tolerant than cv. Chemlali and therefore, very promising for cultivation in arid areas.     

Aquaporin regulation in roots controls plant hydraulic conductance,stomatal conductance,and leaf water potential in Pinus radiata under water stress

Stomatal regulation is crucial for forest species performance and survival on drought‐prone sites. We investigated the regulation of root and shoot hydraulics in three Pinus radiata clones exposed to drought stress and its coordination with stomatal conductance (g_s) and leaf water potential (Ψ_leaf). All clones experienced a substantial decrease in root‐specific root hydraulic conductance (K_root‐r) in response to the water stress, but leaf‐specific shoot hydraulic conductance (K_shoot‐l) did not change in any of the clones. The reduction in K_root‐r caused a decrease in leaf‐specific whole‐plant hydraulic conductance (K_plant‐l). Among clones, the larger the decrease in K_plant‐l, the more stomata closed in response to drought. Rewatering resulted in a quick recovery of K_root‐r and g_s. Our results demonstrated that the reduction in K_plant‐l, attributed to a down regulation of aquaporin activity in roots, was linked to the isohydric stomatal behaviour, resulting in a nearly constant Ψ_leaf as water stress started. We concluded that higher K_plant‐l is associated with water stress resistance by sustaining a less negative Ψ_leaf and delaying stomatal closure.     

Gas-exchange characteristics,leaf water potential and chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence in oil palm (<Emphasis Type="Italic">Elaeis guineensis</Emphasis> Jacq.) seedlings under water stress and recovery

The gas-exchange characteristics, leaf water potential and chlorophyll (Chl) a fluorescence of oil palm (Elaeis guineensis Jacq.) seedlings subjected to water stress and recovery were investigated in a greenhouse experiment. At 24 days after imposition of stress, leaf water potential in water-stressed seedlings was doubled compared to that of control and there was a drastic decline in gas-exchange parameters viz. photosynthesis, transpiration, and stomatal conductance. Water stress did not irreversibly affect gas-exchange parameters and quantum efficiency of photosystem II, as seedlings exhibited total recovery of photosynthetic apparatus by 12^th day of rehydration. These findings indicate that oil palm exhibits physiological plasticity to water stress during the seedling stage.     

Growth and physiological responses to successional water deficit and recovery in four warm‐temperate woody species

Plant responses to drought and their subsequent rehydration can provide evidence for forest dynamics within the context of climate change. In this study, the seedlings of two native species (Vitex negundo var. heterophylla, Quercus acutissima) and two exotic species (Robinia pseudoacacia, Amorpha fruticosa) to China were selected in a greenhouse experiment. The gas exchange, stem hydraulic parameters, plant osmoprotectant contents and antioxidant activities of the seedlings that were subjected to sustained drought and rehydration (test group) as well as those of well‐irrigated seedlings (control group) were measured. The two native species exhibited a greater degree of isohydry with drought because they limited the stomatal opening timely from the onset of the drought. However, the two exotic species showed a more ‘water spender’‐like strategy with R. pseudoacacia showing anisohydric responses and A. fruticosa showing isohydrodynamic responses to drought. Severe drought significantly decreased the leaf gas exchange rates and hydraulic properties, whereas the instantaneous water use efficiency and osmoprotectant contents increased markedly. Most of the physiological parameters recovered rapidly after mild drought rehydration, but the water potential and/or supply of nonstructural carbohydrates did not recover after severe drought rehydration. The results demonstrate that the xylem hydraulic conductivity and shoot water potential jointly play a crucial role in the drought recovery of woody plants. In brief, the native species may play a dominant role in the future in warm‐temperate forests because they employ a better balance between carbon gain and water loss than the alien species under extreme drought conditions.     

盐旱复合胁迫对小麦幼苗生长和水分吸收的影响

为明确盐害、干旱及盐旱复合胁迫对小麦幼苗生长和水分吸收的影响,从而为盐害和干旱胁迫下栽培调控提供理论依据。以2个抗旱性不同的小麦品种(扬麦16和耐旱型洛旱7号)为材料,采用水培试验,以NaCl和PEG模拟盐旱复合胁迫,研究了盐旱复合胁迫下小麦幼苗生长、根系形态、光合特性及水分吸收特性的变化。结果表明,盐、旱及复合胁迫下小麦幼苗的生物量、叶面积、总根长与根系表面积、叶绿素荧光和净光合速率均显著下降,但是复合胁迫处理的降幅却显著低于单一胁迫。盐旱复合胁迫下根系水导速率和根系伤流液强度显著大于单一胁迫,从而提高了小麦幼苗叶片水势和相对含水量。盐胁迫下小麦幼苗Na~+/K~+显著大于复合胁迫,但复合胁迫下ABA含量却显著小于单一的盐害和干旱胁迫。因此,盐旱复合胁迫可以通过增强根系水分吸收及降低根叶中ABA含量以维持较高光合能力,这是盐旱复合胁迫提高小麦适应性的重要原因。洛旱7号和扬麦16对盐及盐旱复合胁迫的响应基本一致,但在干旱胁迫下洛旱7号表现出明显的耐性。     

Effects of dew on eco-physiological traits and leaf structures of Leymus chinensis and Agropyron cristatum grown under drought stress北大核心CSCD

Aims: To investigate the effects of dew on plants, we conducted the experiment to determine the physiological characteristics and leaf structures of Leymus chinensis and Agropyron cristatum in response to increasing dew under drought stress. Methods: Four treatments (no dew, three times dew and five times dew per week under drought stress, and well-watering) were designed to examine leaf relative water content, water potential, net photosynthetic rate, water use efficiency, biomass, and leaf structures of L. chinensis and A. cristatum. Important findings: There was a significant increase in the relative water content and water potential by simulated dew increase for two plants species under drought stress (p < 0.05). For A. cristatum, simulated dew increase significantly enhanced the net photosynthetic rate, stomatal conductance, and transpiration rate of plants under drought stress (p < 0.05). On the other hand, there was no significant difference in the stomatal conductance and transpiration rate for L. chinensis among treatments. Simulated dew increase improved the aboveground biomass and root biomass of two species. The ratio of yellow leaves to the total leaves was decreased by simulated dew increase for two species. Dew increase also protected leaf structures against the drought stress, suggesting that the dew increase can slow down the death process of leaves resulted from drought stress. Therefore, the study demonstrated that dew increased the available water for the leaves of L. chinensis and A. cristatum grown in the drought stress and thus had positive effects on the photosynthesis, water physiology and plant development.     

Photosynthetic Responses of a Temperate Liana to Xylella fastidiosa Infection and Water Stress

Xylella fastidiosa is a xylem‐limited bacterial plant pathogen that causes bacterial leaf scorch in its hosts. Our previous work showed that water stress enhances leaf scorch symptom severity and progression along the stem of a liana, Parthenocissus quinquefolia, infected by X. fastidiosa. This paper explores the photosynthetic gas exchange responses of P. quinquefolia, with the aim to elucidate mechanisms behind disease expression and its interaction with water stress. We used a 2 × 2‐complete factorial design, repeated over two growing seasons, with high and low soil moisture levels and infected and non‐infected plants. In both years, low soil moisture levels reduced leaf water potentials, net photosynthesis and stomatal conductance at all leaf positions, while X. fastidiosa‐infection reduced these parameters at basally located leaves only. Intercellular CO₂ concentrations were reduced in apical leaves, but increased at the most basal leaf location, implicating a non‐stomatal reduction of photosynthesis in leaves showing the greatest disease development. This result was supported by measured reductions in photosynthetic rates of basal leaves at high CO₂ concentrations, where stomatal limitation was eliminated. Repeated measurements over the summer of 2000 showed that the effects of water stress and infection were progressive over time, reaching their greatest extent in September. By reducing stomatal conductances at moderate levels of water stress, P. quinquefolia maintained relatively high leaf water potentials and delayed the onset of photosynthetic damage due to pathogen and drought‐induced water stress. In addition, chlorophyll fluorescence measurements showed that P. quinquefolia has an efficient means of dissipating excess light energy that protects the photosynthetic machinery of leaves from irreversible photoinhibitory damage that may occur during stress‐induced stomatal limitation of photosynthesis. However, severe stress induced by disease and drought eventually led to non‐stomatal decreases in photosynthesis associated with leaf senescence.     

Morphophysiological differences in leaves of <Emphasis Type="Italic">Lavoisiera campos</Emphasis>-<Emphasis Type="Italic">portoana</Emphasis> (Melastomataceae) enhance higher drought tolerance in water shortage events

Lavoisiera campos-portoana Barreto (Melastomataceae) has two kinds of leaves, pubescent and glabrous, and branches may have one or both types of leaves at the same moment. The plant is endemic to high altitude rocky fields in Brazil where rainfall is very seasonal. We predicted that these two leaf types are adaptations to different regimes of water availability. In experimental conditions of drought stress, we measured relative water content (RWC), photosynthetic pigments, chlorophyll a fluorescence and osmotic potential, and we counted stomates and measured stomatal conductance on both sides of leaves and compared these between the two leaf types. Stomatal conductance and electron transport rate at a given photosynthetic photon flux were greater in pubescent leaves than in glabrous leaves, and both declined during drought stress. Excessive photon flux density in glabrous leaves was greater during stress and after rehydration. Photosynthetic pigment content and RWC did not change between leaves, and values reduced during the stress period. Both types of leaves showed osmotic adjustment capacity, which occurred earlier in glabrous ones. These morphophysiological differences highlight the adaptation strategies of this plant to withstand drought, since the glabrous portion of the plant presents a preventive behavior, while the pubescent portion only shows the same responses in more advanced stages of drought stress.     

Influence of Exogenous Glycine Betaine and Abscisic Acid on Papaya in Responses to Water-deficit Stress

The effects of exogenous foliar glycine betaine (GB) and abscisic acid (ABA) on papaya responses to water stress were investigated under distinct water regimes. Papaya seedlings (Carica papaya L. cultivar “BH-65”) were pretreated with GB or ABA and subsequently subjected to consecutive periods of drought, rehydration, and a second period of drought conditions. Results indicated that water stress induced ABA, jasmonic acid (JA), and proline accumulation but did not modify malondialdehyde (MDA) concentration. In addition, water deprivation reduced photosynthetic rate, stomatal conductance, relative water content (RWC), leaf fresh weight, and increased leaf abscission. GB applied prior to drought imposition decreased the impact of water stress on ABA, JA, proline accumulation, leaf water status, growth, and photosynthetic performance. However, ABA-pretreated plants did not show alteration of most of these parameters under water stress conditions when compared with non-pretreated plants except a clear induction of JA accumulation. Taken together, the data suggest that GB may modulate ABA, JA, and proline accumulation through the control of stomatal movement and the high availability of compatible solutes, leading to improvement of leaf water status, growth, and photosynthetic machinery function. In contrast, exogenous ABA did not stimulate papaya physiological responses under drought, but interestingly ABA in combination with drought could induce progressive JA synthesis, unlike drought alone, which induces a transitory JA increase and may trigger endogenous ABA accumulation. The data also suggest that irrespective of the pretreatments, papaya did not suffer oxidative damage.     

Stomatal response functions to environmental stress of dominant species in the tropical Andean páramo

ABSTRACT

Background: Stomatal response functions of dominant plant species can provide insights into the behaviour of ecosystems under environmental stress, and provide tools for modelling their response to climate change. However, they remain little studied in tropical Alpine ecosystems.

Aims: Our objective was to formulate and compare stomatal response functions for two dominant páramo species with different adaptive strategies to drought, the stress-tolerant shrub Hypericum laricifolium, and the stress avoiding giant rosette Espeletia schultzii and thus enable making projections as to their future fitness in a changing climate.

Methods: A reanalysis of data found in the literature and new ecophysiological and micrometeorological measurements were used to fit and test new stomatal response functions to environmental variables for these two species.

Results: The response functions of vapour pressure differences between leaf and air showed an exponential decrease for both species, while for photosynthetically active radiation (PAR), peak-form response functions provided the best fit. The response function for leaf water potential was linear for the drought-tolerant shrub and decreased exponentially for the stress avoiding giant rosette. Several thresholds prior to stomatal closure were also included in the functions.

Conclusions: Although stress-avoiding and stress-tolerant strategies are both successful in the Andean páramo, the response functions suggest that the tolerant shrub could be more resistant to more intense drought.     

Morphological and physiological responses of <Emphasis Type="Italic">Vitex negundo</Emphasis> L. var. <Emphasis Type="Italic">heterophylla</Emphasis> (Franch.) Rehd. to drought stress

Water is a main factor limiting plant growth. Integrative responses of leaf traits and whole plant growth to drought will provide implications to vegetation restoration. This study investigated the drought responses of Vitex negundo L. var. heterophylla (Franch.) Rehd. with a focus on leaf morphology and physiology, seedling growth and biomass partitioning. Potted 1-year-old seedlings were subjected to four water supply regimes [75, 55, 35 and 15% field capacity (FC)], served as control, mild water stress, moderate water stress and severe water stress. Leaf morphological traits varied to reduce the distance of water transfer under water stress and leaflets were dispersed with drought. Net photosynthetic rate decreased significantly under water stress: stomatal closure was the dominant limitation at mild and moderate drought, while metabolic impairment was dominant at severe drought. The physiological impairment at severe drought could also be detected from the relative lower water use efficiency and non-photochemical quenching to moderate water stress. Total biomass of well-watered plants was more than twice that at moderate water deficit and nearly ten times that at severe water deficit. In summary, V. negundo var. heterophylla had adaptation mechanism to water deficit even in the most serious condition, but different strategies were adopted. Seedlings invested more photosynthate to roots at mild and moderate drought while more photosynthate to leaves at severe drought. A nearly stagnant seedling growth and a sharp decline of total biomass were the survival strategy at severe water stress, which was not favorable to vegetation restoration. Water supply above 15% FC is recommended for the seedlings to vegetation restoration.     

Effects of water stress and differential hardening treatments on photosynthetic characteristics of a xeromorphic shrub,Eucalyptus socialis,F. Muell.

Summary The influence of water stress on photosynthesis of drought hardened, and non-hardened,Eucalyptus socialis plants was examined. Particular attention was given to the effects of low leaf water potential on stomatal and intracellular resistance to CO₂ transport and on the CO₂ compensation point. Though the hardening treatment had a pronounced influence on leaf morphology, there was no apparent difference in the photosynthetic response to drought stress between hardened and non-hardened treatments, or with repeated drought cycles. These results suggest a high degree of genetic preconditioning to drought in this species.     

Ecophysiological studies of Sonoran Desert plants

Summary The gas exchange and water relations of two Sonoran Desert plants are compared during contrasting periods of water and heat stress. Photosynthesis of Acacia greggii, a winter deciduous shrub, and Cercidium microphyllum, a chlorophyllous stemmed tree, show a moderate correlation with dawn plant water potential. For both species a relationship between stomatal conductance and dawn plant water potential was not apparent, although A. greggii demonstrated a greater overall stomatal conductance. This affected a greater daytime decrease in plant water potential at all levels of water stress and suggests A. greggii is less sensitive to water stress. Our results suggest the lower limit for gross photosynthesis occurs when dawn plant water potentials are less than -44 and -31 bars for the shrub and tree species, respectively. During periods of extreme water and heat stress the photosynthetic capacity of both species is regulated more by mesophyll than stomatal conductance. However, partial stomatal closure causes plant water potential to increase during the day and exceed dawn values. During periods of minimal water and heat stress the daily course of photosynthesis parallels the change in stomatal conductance and irradiance. Maximum gross photosynthesis rates are nearly three-fold higher than the rates observed during periods of stress, with those of A. greggii generally greater than the rates observed in plants of C. microphyllum.     

Comparative ecophysiological responses to drought of two shrub and four tree species from karst habitats of southwestern China

Drought stress is one of the most important factors in limiting the survival and growth of plants in the harsh karst habitats of southwestern China, especially at the seedling establishment stage. The ecophysiological response to drought stress of native plants with different growth forms is useful for re-vegetation programs. Two shrub and four tree species were studied, including Pyracantha fortuneana (evergreen shrub), Rosa cymosa (deciduous shrub), Cinnamomum bodinieri (evergreen tree), and other three deciduous trees, Broussonetia papyrifera, Platycarya longipes, and Pteroceltis tatarinowii. The seedlings were randomly assigned to four drought treatments, i.e., well-watered, mild drought stress, moderate drought stress, and severe drought stress. Leaf water relations, gas exchange, chlorophyll fluorescence, and growth of the seedlings were investigated. Under severe drought stress, the two shrubs with low leaf area ratio (LAR) maintained higher water status, higher photosynthetic capacity, and larger percent biomass increase than the most of the trees. The two shrubs also had lower specific leaf area, greater intrinsic water use efficiency, and thermal dissipation than the trees. This suggested that the two shrubs had high tolerance to severe drought and were suitable for re-vegetation in harsh habitats. The evergreen C. bodinieri exhibited higher leaf mass ratio (LMR) and LAR than the deciduous species under mild and moderate stress. However, the low maximum quantum efficiency of PSII photochemistry (F _v/F _m) and net assimilation rate, and the sharp decreases of water potential, LMR, LAR, and biomass under severe stress indicated C. bodinieri’s weak tolerance to severe drought. In response to drought stress, the three deciduous trees revealed sharp reductions of biomass due to the large drought-induced decreases of gas exchange, LAR, and LMR. Under drought conditions, the deciduous trees minimized water loss by stomatal closure and by reducing transpiration leaf area and light harvesting through shedding leaves. This suggested that the three deciduous trees were more sensitive to water availability than the shrubs and used avoidance strategies against drought stress. However, the better growth performance of the deciduous trees than that of the shrubs under favorable conditions suggested that deciduous trees could be suitable for habitats with mild and temporary drought stress.     

Water Potential Components, Stomatal Function, and Liquid Phase Water Transport Resistances of Four Arctic and Alpine Species in Relation to Moisture Stress

Transpiration measurements of two alpine tundra species, Deschampsia caespitosa and Geum rossii, and two arctic tundra species, Dupontia fischeri and Carex aquatilis, were conducted under varying atmospheric and soil moisture stress regimes to determine if the stomatal response to water stress may play a role in the local distributions of these species. Under low soil moisture stress, stomata of the species restricted typically to wet meadow areas, Deschampsia and Dupontia, did not exhibit closure until leaf water potential declined. However, when soil moisture stress was low and atmospheric stress increased, Geum and particularly Carex exhibited partial stomatal closure before leaf water potential dropped, suggesting a direct response of the stomata to the vapor pressure gradient between the leaf and the atmosphere. Lower liquid phase water transport resistance from the soil to the leaves may also reduce the development of leaf moisture stress in Geum. Furthermore, Geum and possibly Carex appeared to undergo less of a loss of leaf turgor when leaf water potential decreased. This response may serve to maintain leaf cell turgor and to abate the reduction in leaf enlargement.     

Comparison of proline accumulation in two mediterranean shrubs subjected to natural and experimental water deficit

The effect of water stress on proline accumulation was tested in two contrasted species of Mediterranean scrub: Halimium halimifolium (L.) Willk and Pistacia lentiscus L. Leaf water potential, stomatal resistance and proline content have been measured both in experimental and in natural water stress conditions. Both species accumulated proline in their leaves when leaf water potential dropped below a threshold value of –3.0 MPa, under natural as well as under experimental conditions. In the field, however, a time-lag between decrease of leaf water potential and proline accumulation could be observed. In Halimium halimifolium, proline accumulation appeared to be associated with severe stress conditions as most plants with high proline contents suffered irreversible wilting, especially in the greenhouse. P. lentiscus showed a different pattern, accumulating proline at two different times of the year, as a response to cold or to drought. The results of our study indicated that the role of proline in this species, rather than an osmotic agent, seems to be more related to a protective action in cases of severe stress conditions.     

Drought responses,phenotypic plasticity and survival of Mediterranean species in two different microclimatic sites

• Climate models predict a further drying of the Mediterranean summer. One way for plant species to persist during such climate changes is through acclimation. Here, we determine the extent to which trait plasticity in response to drought differs between species and between sites, and address the question whether there is a trade‐off between drought survival and phenotypic plasticity.
• Throughout the summer we measured physiological traits (photosynthesis – A_max, stomatal conductance – g_s, transpiration – E, leaf water potential – ψl) and structural traits (specific leaf area – SLA, leaf density – LD, leaf dry matter content – LDMC, leaf relative water content – LRWC) of leaves of eight woody species in two sites with slightly different microclimate (north‐ versus south‐facing slopes) in southern Spain. Plant recovery and survival was estimated after the summer drought period.
• We found high trait variability between species. In most variables, phenotypic plasticity was lower in the drier site. Phenotypic plasticity of SLA and LDMC correlated negatively with drought survival, which suggests a trade‐off between them. On the other hand, high phenotypic plasticity of SLA and LDMC was positively related to traits associated with rapid recovery and growth after the drought period.
• Although phenotypic plasticity is generally seen as favourable during stress conditions, here it seemed beneficial for favourable conditions. We propose that in environments with fluctuating drought periods there can be a trade‐off between drought survival and growth during favourable conditions. When climate become drier, species with high drought survival but low phenotypic plasticity might be selected for.