Osmotic and elastic adjustments in cold desert shrubs differing in rooting depth: coping with drought and subzero temperatures

Physiological adjustments to enhance tolerance or avoidance of summer drought and winter freezing were studied in shallow- to deep-rooted Patagonian cold desert shrubs. We measured leaf water potential (Ψ_L), osmotic potential, tissue elasticity, stem hydraulic characteristics, and stomatal conductance (g _S) across species throughout the year, and assessed tissue damage by subzero temperatures during winter. Species behavior was highly dependent on rooting depth. Substantial osmotic adjustment (up to 1.2?MPa) was observed in deep-rooted species exhibiting relatively small seasonal variations in Ψ_L and with access to a more stable water source, but having a large difference between predawn and midday Ψ_L. On the other hand, shallow-rooted species exposed to large seasonal changes in Ψ_L showed limited osmotic adjustment and incomplete stomatal closure, resulting in turgor loss during periods of drought. The bulk leaf tissue elastic modulus (ε) was lower in species with relatively shallow roots. Daily variation in g _S was larger in shallow-rooted species (more than 50?% of its maximum) and was negatively associated with the difference between Ψ_L at the turgor loss point and minimum Ψ_L (safety margin for turgor maintenance). All species increased ε by about 10?MPa during winter. Species with rigid tissue walls exhibited low leaf tissue damage at ?20?°C. Our results suggest that osmotic adjustment was the main water relationship adaptation to cope with drought during summer and spring, particularly in deep-rooted plants, and that adjustments in cell wall rigidity during the winter helped to enhance freezing tolerance.     

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.     

Two different strategies of Mediterranean macchia plants to avoid photoinhibitory damage by excessive radiation levels during summer drought

The adaptive strategies to high radiation and water stress of the drought tolerant evergreen sclerophylls Quercus coccifera and Arbutus unedo are compared to those of the semi-deciduous Cistus spp. (C. albidus and C. monspeliensis). Cistus spp. partially avoided drought by a marked reduction of their transpirational surface through leaf abscission during summer, when predawn water potential declined below -5.5 MPa. Chlorophyll fluorescence measurements revealed a reversible diurnal decrease of maximum photochemical efficiency of PSII (F_v/F_m), which became more accentuated during summer drought in all species. An important strategy to avoid damage by excessive radiation levels in Cistus spp. was the structural regulation of light interception through leaf angle changes, from a more horizontal orientation in spring (< 35°) to a more vertical orientation in summer (> 70°). Horizontal orientated leaves were highly susceptible to photoinhibition, and excessive radiation often resulted in irreversible photodamage followed by leaf abscission during summer, whereas vertical leaf orientation appeared to protect the leaf from severe photoinhibition. Still, these mechanisms were not fully successful in avoiding chronic photoinhibition, and predawn F_v/F_m values remained low in Cistus spp. during summer (only exhibiting a partial overnight recovery). Evergreen sclerophylls were less susceptible to photoinhibition, and the diurnal decline in F_v/F_m remained fully reversible during drought. Structural regulation of light interception was not found to be an important strategy in these species, and only small, though significant changes in leaf angle occurred. The ecological importance of the adaptive strategies of each functional group is discussed.     

Change in water loss regulation after canopy clearcut of a dominant shrub in Sahelian agrosystems, Guiera senegalensis J. F. Gmel

This paper analyzes the effect of the canopy age of Guiera senegalensis J.F. Gmel on water regulation processes and adaptative strategy to drought over a period of 2 years. The species is widespread in the agricultural Sahel. Before sowing, farmers cut back the shrubs to limit competition with crops. The stumps resprout after the millet harvest. Leaf water potential and stomatal conductance were measured in two fallows and in the two adjacent cultivated fields. Leaf transpiration rate and soil-to-leaf hydraulic conductance were deduced. The decrease in both stomatal and plant hydraulic conductance caused by seasonal drought was greater in mature shrubs than in current year resprouts. The decrease in predawn and midday leaf water potentials in response to seasonal drought was isohydrodynamic, and it was greater in mature shrubs, suggesting that current year resprouts are under less stress. In resprouts, the leaf transpiration rate stopped increasing beyond a hydraulic conductance threshold of 0.05 mol. m^?2 s^?1 MPa^?1. Vulnerability to cavitation was determined on segments of stems in the laboratory. The leaf water potential value at which stomatal closure occurred was ?2.99 ± 0.68 MPa, which corresponded to a 30 % loss in xylem conductivity. Thanks to its positive safety margin of 0.6 MPa, G. senegalensis can survive above this value. The observed strategy places G. senegalensis among the non-extreme xeric plants, leading us to suppose that this species will be vulnerable to the expected increase in regional drought.     

Tissue water relations of four co-occurring chaparral shrubs

Summary Chaparral shrubs of California have a suite of morphological and physiological adaptations to withstand the prolonged summer droughts of a mediterranean climate. Not all species of chaparral have the same rooting depth and there is some evidence that those with shallow roots have tissue that is most tolerant to water stress. We tested this notion by comparing the tissue water relations of four co-occurring chaparral shrubs: Quercus durata, Heteromeles arbutifolia, Adenostoma fasciculatum, and Rhamnus californica. We used a pressure-volume technique and a dew-point hygrometer to metsure seasonal changes in osmotic potential when plant tissue was fully hydrated and osmotic potential at predawn, midday, and the turgor loss point. We also calculated seasonal changes in the minimum daily turgor potential, saturated weight/dry weight ratio of leaf tissue, and the bulk modulus of elasticity. We had information on the seasonal water use patterns and apparent rooting depths of these same four shrubs from a previous study (Davis and Mooney 1986). All evidence indicated that Rhamnus had shallow roots and Quercus deep roots. Our results indicated that the tissue water relations of our four co-occurring chaparral shrubs were not alike. Even though Rhamnus had shallow roots, it had the least xerophytic tissue. Seasonal osmotic potential and saturated weight/dry weight ratios were relatively high and bulk modulus of elasticity and minimum daily turgor potentials were low. Furthermore, even though Quercus had deep roots and experienced no seasonal water stress at our study site, its tissue water relations indicated relatively high tolerance to water stress. We conclude that seasonal drought tolerance of stem and leaf tissue of co-occurring chaparral shrubs does not necessarily correspond to rooting depth, to soil moisture resources available to the shrub, or to the degree of seasonal water stress experienced by the shrub.     

In Situ Investigation of Leaf Water Status by Portable Unilateral Nuclear Magnetic Resonance

A portable unilateral nuclear magnetic resonance (NMR) instrument was used to detect in field conditions the water status of leaves of herbaceous crops (Zea mays, Phaseolus vulgaris), mesophyllous trees (Populus nigra), and natural Mediterranean vegetation characterized by water-spending shrubs (Cistus incanus) and water-saving sclerophyllous trees (Quercus ilex). A good relationship was observed between NMR signal, leaf relative water content, and leaf transpiration in herbaceous leaves undergoing fast dehydration or slowly developing a drought stress. A relationship was also observed between NMR signal and water potential of Populus leaves during the development of a water stress and when leaves recovered from the stress. In the natural vegetation, the relationship between NMR signal and water status was found in Cistus, the species characterized by high transpiration rates, when measured during a drought stress period and after a rainfall. In the case of the sclerophyllous Quercus, the NMR signal, the relative water content, and the transpiration rate did not change at different leaf water status, possibly because a large amount of water is compartmentalized in cellular structures and macromolecules. The good association between NMR signal and relative water content was lost in leaves exposed for 24 h to dehydration or to an osmotic stress caused by polyethylene glycol feeding. At this time, the transverse relaxation time became longer than in leaves maintained under optimal water conditions, and two indicators of membrane damage, the ion leakage and the emission of products of membrane lipoxygenation [(Z)-3-hexenal, (Z)-3-hexenol, and (E)-2-hexenol], increased. These results taken all together give information on the physiological state of a leaf under a developing stress and show the usefulness of the NMR instrumentation for screening vegetation health and fitness in natural and cultivated conditions. It is concluded that the portable unilateral NMR instrument may be usefully employed in field conditions to monitor nondestructively the water status of plants and to assist agricultural practices, such as irrigation scheduling, to minimize stomatal closure and the consequent limitation to plant production.     

Comparative Water Relations of Phaseolus vulgaris L. and Phaseolus acutifolius Gray

Leaf area expansion, dry weight, and water relations of Phaseolus vulgaris L. and P. acutifolius Gray were compared during a drying cycle in the greenhouse to understand the characteristics which contribute to the superior drought tolerance of P. acutifolius. Stomates of P. acutifolius closed at a much higher water potential than those of P. vulgaris, delaying dehydration of leaf tissue. P. acutifolius had a more deeply penetrating root system, which also contributes to its drought tolerance. Root-shoot ratios did not differ between the two species either under well watered or water stressed conditions. Leaf osmotic potential was also similar in the two species, with no apparent osmotic adjustment during water stress. These results indicate that P. acutifolius postpones dehydration and suggest that sensitive stomates and a deeply penetrating root system are characteristics which, if incorporated into cultivated beans, might increase their drought tolerance.     

Seasonal patterns of leaf water relations in four co-occurring forest tree species: Parameters from pressure-volume curves

Summary Leaf water relationships were studied in four widespread forest tree species (Ilex opaca Ait., Cornus florida L., Acer rubrum L., and Liriodendron tulipifera L.). The individuals studied all occurred on the same site and were selected to represent a range of growth forms and water relationships in some of the principal tree species of the region. The water relations of the species were analyzed using the concept of the water potential-water content relationship. The pressure-volume method was used to measure this relationship using leaf material sampled from naturally occurring plants in the field. Water potential components (turgor, osmotic, and matric) were obtained by analysis of the pressure-volume curves.Initial osmotic potentials (the value of the osmotic component at full turgidity) were highest (least negative) at the start of the growing season. They decreased (becoming progressively more negative) as the season progressed through a drought period. Following a period of precipitation at the end of the drought period, initial osmotic potentials increased toward the values measured earlier in the season.Seasonal osmotic adjustments were sufficient in all species to allow maintenance of leaf turgor through the season, with one exception: Acer appeared to undergo some midday turgor loss during the height of the July drought period.In addition to environmental influences, tissue stage of development played a role; young Ilex leaves had higher early season initial osmotic potentials than overwintering leaves from the same tree.The seasonal pattern of initial osmotic potential in Liriodendron and the observed pattern of leaf mortality suggested a possible role of osmotic potentials in the resistance of those leaves to drought conditions. The fraction of total leaf water which is available to affect osmotic potentials, called the osmotic water fraction in this study, was greatest in young tissue early in the season and declined as the season progressed.The results of this study showed that the water potential-water content relationship represents a dynamic mechanism by which plant internal water relations may vary in response to a changing external water-availability regime. The measured water relationships confirmed the relative positions of the species along a water-availability gradient, with Cornus at the wettest end and Ilex at the driest end of the gradient. Acer and Liriodendron were intermediate in their water relations. The spread of these species along a water-availability gradient on the same site suggested that coexistence is partially based on differential water use patterns.     

Differential effects of two species of arbuscular mycorrhiza on the growth and water relations of Spartium junceum and Anthyllis cytisoides

Anthyllis cytisoides and Spartium junceum are two leguminous shrubs native of semiarid mediterranean areas, often used in revegetation strategies. Mycorrhization of both shrubs with Glomus intraradices BEG 72 enhanced both plants growth and water relations under drought stress. Root colonization achieved by Glomus mosseae was lower than the level achieved by G. intraradices in both plants studied, and the effects of the inoculation with G. mosseae BEG 116 were less positive than those observed for G. intraradices. Before the onset of the drought stress period the specific leaf weight (SLW) of S. junceum plants inoculated with G. mosseae was lower than the SLW of control and G. intraradices plants. At the end of the stress period, after 15 days of withholding water, the relative water content of S. junceum twigs was lower for G. mosseae inoculated plants and higher for G. intraradices inoculated plants, compared to control, non-inoculated plants. At the end of the recovery period, 15 days after the reestablishment of watering, there were no differences between inoculation treatments on the parameters related to the plants water status. Anthyllis cytisoides plants inoculated with G. intraradices had lower leaf osmotic potential, more leaves, and higher chlorophyll content (measured as SPAD values). Anthyllis cytisoides plants responded to drought defoliating, but defoliation was lower for the plants inoculated with G. intraradices. At the end of the drought, the leaf osmotic potential was lowest for G. intraradices plants as was the relative water content (RWC) whilst Glomus mosseae inoculated plants had the highest RWC, SLW and osmotic potential values. At the end of the recovery period, all plants recuperated the osmotic potential values measured at the pre-stress period. In our experiments, G. intraradices BEG 72 was found to be superior to G. mosseae BEG 116, this difference could be attributed to the origin of the fungus, native from a Mediterranean area, compared to G. mosseae (BEG116) isolated from the UK.     

The evolutionary rate of leaf osmotic strength drives diversification of Primulina species in karst regions

Leaf water storage capacity and osmotic strength are important traits enabling species to adapt to environments that are often moisture limited. However, whether these drought tolerance traits are correlated with the species diversification rate (DR) of plant lineages is yet to be determined. In this study, we selected a species-rich genus (Primulina) of plants widely distributed in karst regions in which species frequently experience variable periods of drought. We measured water storage capacity-related traits (including leaf thickness and water content per mass) and saturated osmotic strength in the leaves of 58 Primulina species growing in a common garden. Subsequently, using phylogenetic methods, we examined the relationships between the rate of species diversification and the drought tolerance traits and between the species DR and evolutionary rates of these traits. We found that neither water storage capacity nor saturated osmotic strength showed significant relationships with the rate of species diversification. However, the evolutionary rate of saturated osmotic strength showed a significant correlation with the species DR, although no comparable significant relationship was detected regarding the evolutionary rate of water storage capacity. Our study indicates that the diversification among Primulina species has typically been accompanied by an extensive divergence of leaf osmotic strength but not a divergence in leaf water storage capacity. These findings will enhance our current understanding of how drought tolerance influences the diversification of plant species in karst regions.     

内蒙古高原锦鸡儿属植物的形态和生理生态适应性

比较研究内蒙古高原锦鸡儿属(Caragana)中生种,旱生种和强旱生种的叶片形态结构、渗透调节、气孔调节和保护酶,目的是揭示锦鸡儿属不同类型植物的生态适应策略。中生种叶片平展,被稀疏绿色短柔毛;旱生种叶片平展或呈瓦状,被灰色柔毛;强旱生种叶片呈瓦状或卷成筒状,被直立或伏帖绢毛。叶片厚度强旱生种>旱生种>中生种,叶片面积、生物量和比叶面积(SLA)强旱生种<旱生种<中生种。叶片长宽比,强旱生种和旱生种大于中生种。这些形态结构导致保水能力强旱生种>旱生种>中生种,光能利用能力中生种>旱生种>强旱生种。渗透调节物质含量、细胞质离子浓度和细胞渗透势强旱生种>旱生种>中生种。渗透调节物质含量的差异主要表现在强旱生种可溶性糖和无机离子含量远高于旱生种,后者又远高于中生种。叶含水量、自由水含量、叶水势和气孔导度中生种>旱生种>强旱生种,束缚水含量、束缚水/自由水比值、POD和SOD活性正好相反,CAT活性旱生种>中生种>强旱生种。这些生理特性导致抗旱能力强旱生种>旱生种>中生种,但代谢速率正好相反。旱生种和中生种表现出较少的日水分亏缺,强旱生种水分亏缺从清晨到傍晚持续大幅增加。细胞膜相对透性和MDA含量强旱生种>旱生种>中生种。自由基含量表现为旱生种>中生种>强旱生种。这些数据说明虽然旱生种和强旱生种形成了多种特点来适应干旱环境,但仍然是不充分的。结论:(1)分布于半湿润至半干旱区的锦鸡儿属中生种依靠活跃的代谢、大量的水分消耗和快速生长使其在生物环境中取得竞争优势;生活在干旱地区和强干旱地区的旱生种和强旱生种依靠低代谢、节水和高抗旱性来抵抗苛刻的非生物环境。(2) 旱生种和强旱生种主要通过可溶性糖和无机离子的积累,调节细胞质渗透势,保持水分平衡,这是一种相对节省能量的适应对策。     

Quercitol plays a key role in stress tolerance of Eucalyptus leptophylla (F. Muell) in naturally occurring saline conditions

Understanding trees adaptation to arid, saline conditions is a major challenge for catchment revegetation in Australia. The accumulation of low molecular weight solutes is an established response of trees to the effects of salt and/or drought stress. Recent studies have shown that quercitol – a cyclitol – contributes significantly towards the adjustment of osmotic potential in some species of Eucalyptus. The present study investigated the role of quercitol in leaf tissues of Eucalyptus leptophylla (F. Muell) under fluctuating environmental stresses. Analysis of leaf tissues from trees growing at distances between 0 and 125 m from hyper-saline lakes suggests that quercitol contributes significantly to the adjustment of osmotic potential induced by drought in E. leptophylla. The presence of substantial concentrations of quercitol in xylem sap suggests that quercitol plays additional roles in signalling amelioration of osmotic stress in myrtaceous species. Quercitol concentrations fluctuate in both xylem and leaf tissues on a seasonal basis, suggesting a form of environmental regulation of solutes. The capacity of soil profiles to store rainwater, rather than proximity to hyper-saline groundwater largely determined osmotic stress in studied trees. Understanding such avoidance/tolerance mechanisms will be crucial to advance tree selection and breeding for stress tolerance.     

Photosynthesis in Encelia farinosa Gray in Response to Decreasing Leaf Water Potential

Photosynthetic responses of intact leaves of the desert shrub Encelia farinosa were measured during a long term drought cycle in order to understand the responses of stomatal and nonstomatal components to water stress. Photosynthetic rate at high irradiance and leaf conductance to water vapor both decreased linearly with declining leaf water potential. The intercellular CO₂ concentration (c_i) remained fairly constant as a function of leaf water potential in plants subjected to a slow drought cycle of 25 days, but decreased in plants exposed to a 12-day drought cycle. With increasing water stress, the slope of the dependence of photosynthesis on c_i (carboxylation efficiency) decreased, the maximum photosynthetic rates at high c_i became saturated at lower values, and water use efficiency increased. Both the carboxylation efficiency and photosynthetic rates were positively correlated with leaf nitrogen content. Associated with lower leaf conductances, the calculated stomatal limitation to photosynthesis increased with water stress. However, because of simultaneous changes in the dependence of photosynthesis on c_i with water stress, increased leaf conductance alone in water-stressed leaves would not result in an increase in photosynthetic rates to prestressed levels. Both active osmotic adjustment and changes in specific leaf mass occurred during the drought cycle. In response to increased water stress, leaf specific mass increased. However, the increases in specific leaf mass were associated with the production of a reflective pubescence and there were no changes in specific mass of the photosynthetic tissues. The significance of these responses for carbon gain and water loss under arid conditions are discussed.     

Effect of drought on pigments, osmotic adjustment and antioxidant enzymes in six woody plant species in karst habitats of southwestern China

Drought stress is one of the most important factors limiting the survival and growth of plants in the harsh karst habitats of southwestern China. Detailed knowledge about the ecophysiological responses of native plants with different growth forms to drought stress could contribute to the success of re-vegetation programs. Two shrubs, Pyracantha fortuneana and Rosa cymosa, and four trees, Broussonetia papyrifera, Cinnamomum bodinieri, Platycarya longipes and Pteroceltis tatarinowii, were randomly assigned to four drought treatments, i.e. well-watered, mild drought stress, moderate drought stress, and severe drought stress. Midday water potential, the maximum quantum efficiency of PSII photochemistry (F_v/F_m), pigments, osmotic solutes (soluble sugars and proline), cellular damages, and antioxidant enzymes (superoxide dismutase, catalase and peroxidase) were investigated. Drought stress significantly decreased pigments content, but increased the ratio of carotenoids to total chlorophylls in the studied species. After prolonged severe drought stress, the two shrubs exhibited higher F_v/F_m, less reductions of midday water potential, and lower increases of malondialdehyde content and ion leakage than the four trees. Prolonged severe drought stress largely decreased accumulations of osmotic solutes and activities of antioxidant enzymes in the four trees, but significantly increased proline content and superoxide dismutase activity in the two shrubs and peroxidase activity in P. fortuneana. The positive relationships were observed among activities of antioxidant enzymes, and between contents of osmotic solutes and activities of antioxidant enzymes. These findings suggested that the two shrubs had higher tolerance to severe drought stress than the four trees due to higher capacities of osmotic adjustment and antioxidant protection.     

Seasonal physiological plasticity and recovery capacity after summer stress in Mediterranean scrub communities

Under natural conditions the overlapping of multiple stressors may initiate coordinated ecophysiological responses in Mediterranean species. Seasonal plasticity may enable plants to better cope with adverse environmental conditions and/or resource variability. In this article, we study the seasonal responses of 12 woody species in two sites of differing water availability, in a Mediterranean-type climate. Plants were measured for water potential, photochemical efficiency, photosynthetic pigments and leaf proline content throughout the year. The results revealed that species presented different ecophysiological strategies, even when sharing the same area. In the xerophytic site, some species suffered severe water stress (−12 MPa and F _v/F _m lower than 0.3), while others exhibited optimal values of F _v/F _m and only moderate water stress. All the plants recovered after the first autumn rains. In the hygrophytic site, some sclerophyll species did not exhibit signs of water stress, but did suffer photoinhibition in clear winter days. A plasticity index was calculated to provide an integrated value of species plasticity. In summer, plasticity was higher in the xerophytic site, while in winter it was higher in the hygrophytic site. Ordination analysis of the physiological traits supports the traditional gradient of Mediterranean strategies from drought semideciduous to evergreen sclerophyll species, although spiny legume species formed an independent functional group. The functional responses of species clearly differ among plant communities according to the prevailing site stressors, but no unique pattern emerges. Species combine traits in broader strategies according to previous evolutionary story exhibiting a certain amount of trade among traits, each contributing to alleviate a part of the plant stress.     

Morphological,physiological and biochemical responses to drought stress of Stone pine (<Emphasis Type="Italic">Pinus pinea</Emphasis> L.) seedlings

In this study, the effect of irrigation intervals (drought stress) on growth, predawn xylem water potential (Ψ _w), the osmotic potential at full turgor (Ψπ ₁₀₀), the osmotic potential at the turgor loss point (Ψπ _TLP), osmotic adjustment and osmotic solutes (soluble sugars and proline) of Pinus pinea L. seedlings were examined. An experiment was carried out under greenhouse conditions using four watering treatments (control, 7-, 14- and 21-day irrigation intervals) in the first growth season; from mid-July to early November. Results showed that irrigation interval had significant effect on growth characteristics, Ψ _w, water relation parameters, and osmotic solutes. The increasing irrigation interval significantly decreased the seedling height, root collar diameter, root, stem and needle dry weight, number of lateral branches, root percentage, root:shoot ratio and diameter:height ratio. Ψ _w and total soluble sugars decreased while proline content increased with the increase of drought stress. The Ψπ ₁₀₀ and Ψπ _TLP significantly decreased in drought-stressed seedlings compared to control (no stress) seedlings. The results suggest that the impact of drought stress increased with the increase of irrigation interval. Therefore, in the drought-stressed P. pinea seedlings were indicated osmotic adjustment by increasing the proline content and decreasing Ψπ ₁₀₀ and Ψπ _TLP during drought stress. Growth decreased under drought stress conditions in P. pinea seedlings.     

Differential Activity of Autochthonous Bacteria in Controlling Drought Stress in Native Lavandula and Salvia Plants Species Under Drought Conditions in Natural Arid Soil

The effectiveness of autochthonous plant growth-promoting rhizobacteria was studied in Lavandula dentata and Salvia officinalis growing in a natural arid Mediterranean soil under drought conditions. These bacteria identified as Bacillus megaterium (Bm), Enterobacter sp. (E), Bacillus thuringiensis (Bt), and Bacillus sp. (Bsp). Each bacteria has different potential to meliorate water limitation and alleviating drought stress in these two plant species. B. thuringiensis promoted growth and drought avoidance in Lavandula by increasing K content, by depressing stomatal conductance, and it controlled shoot proline accumulation. This bacterial effect on increasing drought tolerance was related to the decrease of glutathione reductase (GR) and ascorbate peroxidase (APX) that resulted sensitive indexes of lower cellular oxidative damage involved in the adaptative drought response in B. thuringiensis-inoculated Lavandula plants. In contrast, in Salvia, having intrinsic lower shoot/root ratio, higher stomatal conductance and lower APX and GR activities than Lavandula, the bacterial effects on nutritional, physiological and antioxidant enzymatic systems were lower. The benefit of bacteria depended on intrinsic stress tolerance of plant involved. Lavadula demonstrated a greater benefit than Salvia to control drought stress when inoculated with B. thuringiensis. The bacterial drought tolerance assessed as survival, proline, and indolacetic acid production showed the potential of this bacteria to help plants to grow under drought conditions. B. thuringiensis may be used for Lavandula plant establishment in arid environments. Particular characteristic of the plant species as low shoot/root ratio and high stomatal conductance are important factors controlling the bacterial effectiveness improving nutritional, physiological, and metabolic plant activities.     

An overview of Cistus ectomycorrhizal fungi

The genus Cistus comprises a group of about 20 shrub species found in wide areas throughout the whole Mediterranean region to the Caucasus. Being one of the main constituents of the Mediterranean-type maquis, this plant genus is peculiar in that it has developed a range of specific adaptations to resist summer drought and frequent disturbance events, such as fire and grazing. In addition, it can form both ectomycorrhizas and arbuscular mycorrhizas. In this paper, we review the information available on the ectomycorrhizal fungi of Cistus across its entire geographic range, as gathered and critically sifted from both published literature sources and personal observations. Although the resulting data matrix was based primarily on accounts of sporocarp inventories in the field, existing knowledge on the features of Cistus natural and synthesized ectomycorrhizas was also included and discussed. In total, more than 200 fungal species belonging to 40 genera have been reported so far to be associated with Cistus. An analysis of the pattern of ectomycorrhizal diversity and host specificity revealed that members of the Cortinariaceae and Russulaceae make the most of both Cistus-aspecific and Cistus-specific mycobionts. Further studies are needed to expand our preliminary knowledge of the mycorrhizal ecology and biology of Cistus and its fungal associates, focusing on topics such as mycobiont diversity, host specificity, fungal succession, mycorrhizal influence on stress tolerance, and impact of disturbances, while comparing the findings with those from other ecosystems.     

Components of water potential estimated from xylem pressure measurements in five tree species

Summary Pressure volume curves were measured with a pressure bomb in leaves collected in the field from Ilex opaca, Acer rubrum, Liquidambar styraciflua, Liriodendron tulipifera and Cornus florida. Water potential components were calculated from the curves. The species differed in the relationships measured. In all species the trends from summer to fall were toward lower (more negative) osmotic potentials, lower matric potentials more rapid loss of turgor with increasing leaf water deficit, and the occurrence of incipient plasmolysis at lower values of leaf water deficit. Initial osmotic potentials ranged from-14.8 to-19.8 bars, similar to values reported in the literature for other mesophytic plants. These values, however, were much higher than those reported for halophytes and xerophytes. The fraction of leaf water which contributes to the osmotic potential ranged from 0.74 to 0.98 in this study. Values reported for other mesophytes and for halophytes and xerophytes all fall well within this range. Patterns of component water potentials are discussed in relation to potential growth rates and water flow in the total plant system.     

Plant functional types broadly describe water use strategies in the Caatinga,a seasonally dry tropical forest in northeast Brazil

1. In seasonally dry tropical forests, plant functional type can be classified as deciduous low wood density, deciduous high wood density, or evergreen high wood density species. While deciduousness is often associated with drought‐avoidance and low wood density is often associated with tissue water storage, the degree to which these functional types may correspond to diverging and unique water use strategies has not been extensively tested.
2. We examined (a) tolerance to water stress, measured by predawn and mid‐day leaf water potential; (b) water use efficiency, measured via foliar δ¹³C; and (c) access to soil water, measured via stem water δ¹⁸O.
3. We found that deciduous low wood density species maintain high leaf water potential and low water use efficiency. Deciduous high wood density species have lower leaf water potential and variable water use efficiency. Both groups rely on shallow soil water. Evergreen high wood density species have low leaf water potential, higher water use efficiency, and access alternative water sources. These findings indicate that deciduous low wood density species are drought avoiders, with a specialized strategy for storing root and stem water. Deciduous high wood density species are moderately drought tolerant, and evergreen high wood density species are the most drought tolerant group.
4. Synthesis. Our results broadly support the plant functional type framework as a way to understand water use strategies, but also highlight species‐level differences.