Patterns of leaf surface wetness for montane and subalpine plants

The frequency and duration of water on leaf surfaces have important consequences for plant growth and photosynthetic gas exchange. The objective of the present study was to compare the frequency and duration of leaf wetness under natural field conditions among species and to identify variation in structural features of leaves that may reduce surface wetness. During June–September 1992 in the central Rocky Mountains (USA), natural leaf wetting due to rain and dewfall was observed on 79 of 89 nights in open meadow habitats compared to only 29 of 89 nights in the understorey. Dew formation occurred at relative humidities that were often well below 100% because of radiational heat exchange with cold night skies and low wind speeds (< 0.5 m s^?1). A survey of 50 subalpine/montane species showed that structural characteristics associated with the occurrence and duration of leaf surface wetness differed among species and habitats. Both adaxial and abaxial surfaces accumulated moisture during rain and dewfall events. Leaf surfaces of open-meadow species were less wettable (P= 0.008), and had lower droplet retention (P= 0.015) and more stomata P= 0.017) than adjacent understorey species. Also, leaf trichomes reduced the area of leaf surface covered by moisture. Ecophysiological importance is suggested by the high frequency of leaf wetting events in open microsites, influences on growth and gas exchange, and correspondence between leaf surface wettability and habitat.     

Acclimation of leaf hydraulic conductance and stomatal conductance of Pinus taeda (loblolly pine) to long-term growth in elevated CO2 (free-air CO2 enrichment) and N-fertilization

We investigated how leaf hydraulic conductance (K_leaf) of loblolly pine trees is influenced by soil nitrogen amendment (N) in stands subjected to ambient or elevated CO₂ concentrations (CO₂^a and CO₂^e, respectively). We also examined how K_leaf varies with changes in reference leaf water potential (Ψ_leaf‐ref) and stomatal conductance (g_s‐ref) calculated at vapour pressure deficit, D of 1 kPa. We detected significant reductions in K_leaf caused by N and CO₂^e, but neither treatment affected pre‐dawn or midday Ψ_leaf. We also detected a significant CO₂^e‐induced reduction in g_s‐ref and Ψ_leaf‐ref. Among treatments, the sensitivity of K_leaf to Ψ_leaf was directly related to a reference K_leaf (K_leaf‐ref computed at Ψ_leaf‐ref). This liquid‐phase response was reflected in a similar gas‐phase response, with g_s sensitivity to D proportional to g_s‐ref. Because leaves represented a substantial component of the whole‐tree conductance, reduction in K_leaf under CO₂^e affected whole‐tree water use by inducing a decline in g_s‐ref. The consequences of the acclimation of leaves to the treatments were: (1) trees growing under CO₂^e controlled morning leaf water status less than CO₂^a trees resulting in a higher diurnal loss of K_leaf; (2) the effect of CO₂^e on g_s‐ref was manifested only during times of high soil moisture.     

Effects of nitrogen deposition,drought and their interaction,on functional and structural traits of Fraxinus ornus L. and Quercus ilex L.

A controlled experiment was conducted in order to understand how functional and structural traits of species with different leaf habits (Fraxinus ornus and Quercus ilex) shift as a consequence of nitrogen (N) addition (30 kg ha yr^?1) and to explore the effect that N has on the water stress response. The experiment was divided in two stages: stage I, N addition under well-watered condition; stage II, N addition under drought. Functionality of the photosynthetic machinery, growth and biomass partitioning were assessed. The N content at leaf level increases in F. ornus only, which invests resources on photosynthetic machinery, whereas Q. ilex tends to store N in non-photosynthetic biomass, increasing relative growth rate and biomass, resulting in different allometric ratio. This effect may play a role in water stress response. Stomatal conductance of Q. ilex treated with N and subjected to water stress is lower relative to drought treatment. On the contrary, F. ornus takes advantage of N addition that has ameliorative effects on its functionality when drought was imposed. The obtained results, highlighting response mechanisms to multiple stress factors, should help to better understand and assess the performance of forest ecosystems under the foreseen environmental changes.     

Importance of semi-natural habitats for the conservation of butterfly communities in landscapes dominated by pine plantations

While the area of plantation forests continues to increase worldwide, their contribution to the conservation of biodiversity is still controversial. There is a particular concern on the central role played by natural habitat remnants embedded within the plantation matrix in conserving species-rich insect communities. We surveyed butterflies in maritime pine plantation landscapes in south-western France in 83 plots belonging to seven habitat types (five successional stages of pine stands, native deciduous woodlands and herbaceous firebreaks). The effect of plot, habitat and landscape attributes on butterfly species richness, community composition and individual species were analysed with a General Linear Model (GLM), partial Canonical Correspondence Analysis (CCA) and the IndVal method. The most important factors determining butterfly diversity and community composition were the presence of semi-natural habitats (deciduous woodlands and firebreaks) at the landscape scale and the composition of understorey vegetation at the plot scale. Pure effects of plot variables explained the largest part of community variation (12.8%), but landscape factors explained an additional, independent part (6.7%). Firebreaks were characterized by a higher species richness and both firebreaks and deciduous woodlands harboured species not or rarely found in pine stands. Despite the forest-dominated landscape, typical forest butterflies were rare and mainly found in the deciduous woodlands. Threatened species, such as Coenonympha oedippus and Euphydryas aurinia, were found in pine stands and in firebreaks, but were more abundant in the latter. In the studied plantation forest, the conservation of butterflies depends mainly on the preservation of semi-natural habitats, an adequate understorey management and the maintenance of soil moisture levels.     

Temporal dynamics and spatial variability in the enhancement of canopy leaf area under elevated atmospheric CO2

Increased canopy leaf area (L) may lead to higher forest productivity and alter processes such as species dynamics and ecosystem mass and energy fluxes. Few CO₂ enrichment studies have been conducted in closed canopy forests and none have shown a sustained enhancement of L. We reconstructed 8 years (1996–2003) of L at Duke's Free Air CO₂ Enrichment experiment to determine the effects of elevated atmospheric CO₂ concentration ([CO₂]) on L before and after canopy closure in a pine forest with a hardwood component, focusing on interactions with temporal variation in water availability and spatial variation in nitrogen (N) supply. The dynamics of L were reconstructed using data on leaf litterfall mass and specific leaf area for hardwoods, and needle litterfall mass and specific leaf area combined with needle elongation rates, and fascicle and shoot counts for pines. The dynamics of pine L production and senescence were unaffected by elevated [CO₂], although L senescence for hardwoods was slowed. Elevated [CO₂] enhanced pine L and the total canopy L (combined pine and hardwood species; P<0.050); on average, enhancement following canopy closure was ～16% and 14% respectively. However, variation in pine L and its response to elevated [CO₂] was not random. Each year pine L under ambient and elevated [CO₂] was spatially correlated to the variability in site nitrogen availability (e.g. r²=0.94 and 0.87 in 2001, when L was highest before declining due to droughts and storms), with the [CO₂]‐induced enhancement increasing with N (P=0.061). Incorporating data on N beyond the range of native fertility, achieved through N fertilization, indicated that pine L had reached the site maximum under elevated [CO₂] where native N was highest. Thus closed canopy pine forests may be able to increase leaf area under elevated [CO₂] in moderate fertility sites, but are unable to respond to [CO₂] in both infertile sites (insufficient resources) and sites having high levels of fertility (maximum utilization of resources). The total canopy L, representing the combined L of pine and hardwood species, was constant across the N gradient under both ambient and elevated [CO₂], generating a constant enhancement of canopy L. Thus, in mixed species stands, L of canopy hardwoods which developed on lower fertility sites (～3 g N inputs m^?2 yr^?1) may be sufficiently enhanced under elevated [CO₂] to compensate for the lack of response in pine L, and generate an appreciable response of total canopy L (～14%).     

Leaf traits of natural populations of <Emphasis Type="Italic">Adiantum reniforme</Emphasis> var. <Emphasis Type="Italic">sinensis</Emphasis>, endemic to the Three Gorges region in China

Leaf mass per unit area (LMA), carbon and nitrogen contents, leaf construction cost, and photosynthetic capacity (P _max) of Adiantum reniforme var. sinensis, an endangered fern endemic to the Three Gorges region in southwest China, were compared in five populations differing in habitat such as soil moisture and irradiance. The low soil moisture and high irradiance habitat population exhibited significantly higher LMA, area-based leaf construction (CC_A), and carbon content (C_A), but lower leaf nitrogen content per unit dry mass (N_M) than the other habitat populations. The high soil moisture and low irradiance habitat populations had the lowest CC_A, but their cost/benefic ratios of CCA/P _max were similar to the medium soil moisture and irradiance habitat population due to their lower leaf P _max. Hence A. reniforme var. sinensis prefers partially shaded, moist but well-drained, slope habitats. Due to human activities, however, its main habitats now are cliffs or steeply sloped bare rocks with poor and thin soil. The relatively high energy requirements and low photosynthetic capacity in these habitats could limit the capability of the species in extending population or interspecific competition and hence increase its endangerment.     

The stem xylem of Patagonian shrubs operates far from the point of catastrophic dysfunction and is additionally protected from drought‐induced embolism by leaves and roots

Hydraulic architecture was studied in shrub species differing in rooting depth in a cold desert in Southern Argentina. All species exhibited strong hydraulic segmentation between leaves, stems and roots with leaves being the most vulnerable part of the hydraulic pathway. Two types of safety margins describing the degree of conservation of the hydraulic integrity were used: the difference between minimum stem or leaf water potential (Ψ) and the Ψ at which stem or leaf hydraulic function was reduced by 50% (Ψ – Ψ₅₀), and the difference between leaf and stem Ψ₅₀. Leaf Ψ₅₀ – stem Ψ₅₀ increased with decreasing rooting depth. Large diurnal decreases in root‐specific hydraulic conductivity suggested high root vulnerability to embolism across all species. Although stem Ψ₅₀ became more negative with decreasing species‐specific Ψ_soil and minimum stem Ψ, leaf Ψ₅₀ was independent of Ψ and minimum leaf Ψ. Species with embolism‐resistant stems also had higher maximum stem hydraulic conductivity. Safety margins for stems were >2.1 MPa, whereas those for leaves were negative or only slightly positive. Leaves acted as safety valves to protect the integrity of the upstream hydraulic pathway, whereas embolism in lateral roots may help to decouple portions of the plant from the impact of drier soil layers.     

Does turgor limit growth in tall trees?

The gravitational component of water potential contributes a standing 0.01 MPa m^?1 to the xylem tension gradient in plants. In tall trees, this contribution can significantly reduce the water potential near the tree tops. The turgor of cells in buds and leaves is expected to decrease in direct proportion with leaf water potential along a height gradient unless osmotic adjustment occurs. The pressure–volume technique was used to characterize height‐dependent variation in leaf tissue water relations and shoot growth characteristics in young and old Douglas‐fir trees to determine the extent to which growth limitation with increasing height may be linked to the influence of the gravitational water potential gradient on leaf turgor. Values of leaf water potential (Ψ_l), bulk osmotic potential at full and zero turgor, and other key tissue water relations characteristics were estimated on foliage obtained at 13.5 m near the tops of young (approximately 25‐year‐old) trees and at 34.7, 44.2 and 55.6 m in the crowns of old‐growth (approximately 450‐year‐old) trees during portions of three consecutive growing seasons. The sampling periods coincided with bud swelling, expansion and maturation of new foliage. Vertical gradients of Ψ_l and pressure–volume analyses indicated that turgor decreased with increasing height, particularly during the late spring when vegetative buds began to swell. Vertical trends in branch elongation, leaf dimensions and leaf mass per area were consistent with increasing turgor limitation on shoot growth with increasing height. During the late spring (May), no osmotic adjustment to compensate for the gravitational gradient of Ψ_l was observed. By July, osmotic adjustment had occurred, but it was not sufficient to fully compensate for the vertical gradient of Ψ_l. In tall trees, the gravitational component of Ψ_l is superimposed on phenologically driven changes in leaf water relations characteristics, imposing potential constraints on turgor that may be indistinguishable from those associated with soil water deficits.     

Water Relations,CO2 Exchange,Water-use Efficiency and Growth of Welwitschia mirabilis Hook. fil. in three Contrasting Habitats of the Namib Desert1

CO₂ exchange, transpiration and leaf water potential of Welwitschia mirabilis were measured in three contrasting habitats of the Namib desert. From these measurements stomatal conductance, internal CO₂concentration and WUE were calculated. In two of the three habitats photosynthetic CO₂ uptake decreased and transpiration increased with increasing leaf age while in the third habitat CO₂ uptake increased and transpiration decreased with leaf age. Except for the stomata of young leaf sections in this habitat, stomata closed with increasing δw leading to a pronounced midday depression of CO₂ uptake. The high stomatal limitation of photosynthetic CO₂ uptake of glasshouse-grown plants was verified in the natural habitat. Photosynthetic CO₂ uptake saturated between 800 and 1300 μmol photons m^?2 s^?1depending on leaf age and habitat. CO₂ uptake had a broad temperature optimum declining significantly beyond 32 °C. Predawn leaf water potential reflected water availability and atmospheric conditions in the three habitats and ranged from ? 2.5 to ? 6.2 MPa. There was a pronounced diurnal course of leaf water potential in all habitats. During the day a gradient in water potential developed along the leaf axis with the lowest potential at the leaf's tip. With respect to whole plant balances of CO₂ exchange and transpiration, there were marked differences between Welwitschias in the three habitats. Despite a negative CO₂ balance over a period of five months, leaves in the driest habitat grew constantly at the expense of carbon reserves in the plant. Only at the wettest site did carbon gain exceed carbon demand for growth. The WUE of whole plants was insignificant in all habitats. The results were as contrasting as the habitats and plants and did not allow generalisations about adaptational features of Welwitschia mirabilis.     

Differences in the stimulation of cyclic electron flow in two tropical ferns under water stress are related to leaf anatomy

Cyclic electron flow (CEF) plays an important role in photoprotection for angiosperms under environmental stresses. However, ferns are more sensitive to drought and their water transport systems are not as efficient as those of angiosperms, it is unclear whether CEF also contributes to photoprotection in these plants. Using Microsorum punctatum and Paraleptochillus decurrens, we studied the electron fluxes through both photosystem I (PSI) and photosystem II (PSII) under water stress and their leaf anatomies. Our goal was to determine if CEF functions in the photoprotection of these ferns and, if so, whether CEF stimulation is related to leaf anatomy. Compared with P. decurrens, M. punctatum had thicker leaves and cuticles and higher water storage capacity, but lower stomatal density and slower rate of water loss. During induced drought, the decrease in leaf water potential (Ψ_leaf) was more pronounced in P. decurrens than in M. punctatum. For both species, the decline in Ψ_leaf was associated with a lower effective PSII quantum yield, photochemical quantum yield of PSI and electron transport rate (ETR), whereas increases were found in the quantum yield of regulated energy dissipation, CEF and CEF/ETR(II) ratio. Values for CEF and the CEF/ETR(II) ratio peaked in M. punctatum at a light intensity of 500–600 µmol m^?2 s^?1 vs only 150–200 µmol m^?2 s^?1 in P. decurrens. Therefore, our results indicate that the stimulation of CEF in tropical ferns contributes to their photoprotection under water stress, and is related to their respective drought tolerance and leaf anatomy.     

Drought effects on the dynamics of leaf production and senescence in field-grown Medicago arborea and Medicago citrina

Forage shrub production in the Mediterranean region is frequently limited by soil water availability. To study plant responses to water deficit under such conditions is important for improving crop management and for selecting better yielding forage shrub species. Pre-dawn leaf water potential (Ψ_pd), plant leaf area (PLA), leaf area per stem (LA_s), leaf appearance rate (LAR₁;), leaf senescence rate (LSR), individual leaf area (LA) and maximal leaf elongation rate (LER) were studied throughout the year for Medicago arborea (MA) and Medicago citrina (MC) under irrigated (control) and low rainfall field conditions, at the experimental field site of the University of the Balearic Islands in Spain. With irrigation, the highest LA and LER were observed in autumn and spring and the lowest in winter and summer. LAR; was similar for both species in autumn and winter. Throughout the spring, LAR₁ was higher for MC compared to MA. PLA was similar for both species during the autumn, winter and spring seasons; however, during the summer PLA of MA was significantly reduced by 53%. This decline was attributed to higher leaf senescence during seed maturity. As a consequence, MC maintained higher leaf area (∼ 5 m² plant⁻¹) than MA (3 m² plant⁻¹). Under natural field conditions, soil water deficit increased from February to late August. The main effect of water stress was a marked reduction in LAR₁, LA and LER reflected in lower LA_s and PLA. Leaf area was severely reduced for both species during the summer, but much more intensively in MA, which developed full leaf senescence. Thus, MC maintained higher PLA than MA (0.5 m² compared to 0.0 m²). Throughout the year, but especially in the driest months, MC was superior to MA in leaf growth parameters and PLA.     

Reversible decreases in the bulk elastic modulus of mature leaves of deciduous Quercus species subjected to two drought treatments

Changes in leaf water relations under water stress were examined. In experiment 1, water stress was imposed by withholding irrigation to potted seedlings of deciduous oak, Quercus crispula and Q. serrata. Changes in the pressure–volume (P–V) curve in mature leaves were followed. The leaf water potential at turgor loss (Ψ_l,tlp) significantly decreased after 13 d of drought treatment. The bulk elastic modulus (?) significantly decreased, which contributed to the maintenance of cell turgor together with the decrease in osmotic potential. In experiment 2, water stress was imposed by notching a branch of a Q. serrata tree. After the notching, the daily minimum leaf water potential (Ψ_l) decreased, and a significant decrease in Ψ_l,tlp was observed 15 d after notching. The osmotic potential at water saturation (Ψ_π,sat) did not decrease significantly until 25 d after notching whereas, ? had already decreased significantly within 15 d after notching and increased promptly after substantial precipitation. It was confirmed that ? of mature leaves decreased reversibly in water stress. This response of ? was more rapid than that of the osmotic potential and, thus, effectively maintained cell turgor when water stress was suddenly imposed on the leaves.     

Stomatal conductance and photosynthesis vary linearly with plant hydraulic conductance in ponderosa pine

Recent work has shown that stomatal conductance (g_s) and assimilation (A) are responsive to changes in the hydraulic conductance of the soil to leaf pathway (K_L), but no study has quantitatively described this relationship under controlled conditions where steady‐state flow is promoted. Under steady‐state conditions, the relationship between g_s, water potential (Ψ) and K_L can be assumed to follow the Ohm's law analogy for fluid flow. When boundary layer conductance is large relative to g_s, the Ohm's law analogy leads to g_s = K_L (Ψ_soil?Ψ_leaf)/D, where D is the vapour pressure deficit. Consequently, if stomata regulate Ψ_leaf and limit A, a reduction in K_L will cause g_s and A to decline. We evaluated the regulation of Ψ_leaf and A in response to changes in K_L in well‐watered ponderosa pine seedlings (Pinus ponderosa). To vary K_L, we systematically reduced stem hydraulic conductivity (k) using an air injection technique to induce cavitation while simultaneously measuring Ψ_leaf and canopy gas exchange in the laboratory under constant light and D. Short‐statured seedlings (< 1 m tall) and hour‐long equilibration times promoted steady‐state flow conditions. We found that Ψ_leaf remained constant near ? 1·5 MPa except at the extreme 99% reduction of k when Ψ_leaf fell to ? 2·1 MPa. Transpiration, g_s, A and K_L all declined with decreasing k (P < 0·001). As a result of the near homeostasis in bulk Ψ_leaf, g_s and A were directly proportional to K_L (R² > 0·90), indicating that changes in K_L may affect plant carbon gain.     

Performance and fate of tree seedlings on and around nests of the leaf‐cutting ant Atta cephalotes: Ecological filters in a fragmented forest

Habitat fragmentation is currently the most pervasive anthropogenic disturbance in tropical forests and some species of leaf‐cutting ants of the genus Atta (dominant herbivores in the neotropics) have become hyper‐abundant in forest edges where their nests directly impact up to 6% of the forest area. Yet, their impacts on the regeneration dynamics of fragmented forests remain poorly investigated. Here we examine the potential of Atta cephalotes nests to function as ecological filters impacting tree recruitment. Growth, survival and biomass partitioning of experimentally planted seedlings (six tree species) were examined at eight spatially independent A. cephalotes colonies in a large Atlantic Forest fragment. Seedling performance and fate (leaf numbers and damage) were monitored up to 27 months across three habitats (nest centre, nest edge and forest understorey). Plants at illuminated nest centres showed twice the gross leaf gain as understorey individuals. Simultaneously, seedlings of all species lost many more leaves at nests than in the forest understorey, causing a negative net leaf gain. Net leaf gain in the shaded understorey ranged from zero (Licania and Thyrsodium species) to substantial growth for Copaifera and Virola, and intermediate levels little above zero for Protium and Pouteria. Also seedling survival differed across habitats and species, being typically low in the centre and at the edge of nests where seedlings were often completely defoliated by the ants. Lastly, seedling survival increased strongly with seed size at nest edges while there was no such correlation in the forest. Our results suggest that Atta nests operate as ecological filters by creating a specific disturbance regime that differs from other disturbances in tropical forests. Apparently, Atta nests favour large‐seeded tree species with resprouting abilities and the potential to profit from a moderate, nest‐mediated increase in light availability.     

Genotype differences in 13C discrimination between atmosphere and leaf matter match differences in transpiration efficiency at leaf and whole‐plant levels in hybrid Populus deltoides ×nigra

¹³C discrimination between atmosphere and bulk leaf matter (Δ¹³C_lb) is frequently used as a proxy for transpiration efficiency (TE). Nevertheless, its relevance is challenged due to: (1) potential deviations from the theoretical discrimination model, and (2) complex time integration and upscaling from leaf to whole plant. Six hybrid genotypes of Populus deltoides×nigra genotypes were grown in climate chambers and tested for whole‐plant TE (i.e. accumulated biomass/water transpired). Net CO₂ assimilation rates (A) and stomatal conductance (g_s) were recorded in parallel to: (1) ¹³C in leaf bulk material (δ¹³C_lb) and in soluble sugars (δ¹³C_ss) and (2) ¹⁸O in leaf water and bulk leaf material. Genotypic means of δ¹³C_lb and δ¹³C_ss were tightly correlated. Discrimination between atmosphere and soluble sugars was correlated with daily intrinsic TE at leaf level (daily mean A/g_s), and with whole‐plant TE. Finally, g_s was positively correlated to ¹⁸O enrichment of bulk matter or water of leaves at individual level, but not at genotype level. We conclude that Δ¹³C_lb captures efficiently the genetic variability of whole‐plant TE in poplar. Nevertheless, scaling from leaf level to whole‐plant TE requires to take into account water losses and respiration independent of photosynthesis, which remain poorly documented.     

滇中喀斯特41种不同生长型植物叶性状研究

该研究以云南石林喀斯特地区41种主要植物(乔木19种、灌木10种、藤本4种、草本8种)为对象,对其叶面积(AR)、叶鲜重(FW)、叶干重(DW)、叶厚度(TH)、叶绿素含量(Chlc)等进行了测定,得到不同植物比叶面积(SLA)、叶干物质含量(DMC)等指标。结果表明:(1)在物种水平上7项植物叶性状值变化范围较大,不同生长型植物差异显著。(2) 41种植物的SLA与DMC之间呈极显著负相关(P0.01),AR与FW、DW呈极显著正相关(P0.01),TH与Chlc呈极显著正相关(P0.01),其他因子间相关性不显著。(3)该研究区植物叶性状变异范围为15.82%～139.14%,多为高变异系数(84.40%～131.01%),叶性状变异中AR与FW贡献率较高,分别为84.40%、90.28%,不同生长型植物在TH上稳定性较好。(4)石林喀斯特地区植物叶片适应特殊生境时表现出较低的TH、SLA、Chlc和较高的DMC等特征。这说明石林喀斯特地区植物通过形成不同的叶性状特征来适应特殊生境,可为喀斯特地区的植被保护与恢复提供理论指导。     

Interactions between water stress, sun-shade acclimation, heat tolerance and photoinhibition in the sclerophyll Heteromeles arbutifolia

Gas exchange and chlorophyll fluorescence techniques were used to evaluate the acclimation capacity of the schlerophyll shrub Heteromeles arbutifolia M. Roem. to the multiple co-occurring summer stresses of the California chaparral. We examined the influence of water, heat and high light stresses on the carbon gain and survival of sun and shade seedlings via a factorial experiment involving a slow drying cycle applied to plants grown outdoors during the summer. The photochemical efficiency of PSII exhibited a diurnal, transient decrease (δF/F_m′) and a chronic decrease or photoinhibition (F_v/F_m) in plants exposed to full sunlight. Water stress enhanced both transient decreases of δF/F_m’and photoinhibition. Effects of decreased δF/F_m’and F_v/F_m on carbon gain were observed only in well-watered plants since in water-stressed plants they were overidden by stomatal closure. Reductions in photochemical efficiency and stomatal conductance were observed in all plants exposed to full sunlight, even in those that were well-watered. This suggested that H. arbutifolia sacrificed carbon gain for water conservation and photoprotection (both structurally via shoot architecture and physiologically via down-regulation) and that this response was triggered by a hot and dry atmosphere together with high PFD, before severe water, heat or high PFD stresses occur. We found fast adaptive adjustments of the thermal stability of PSII (diurnal changes) and a superimposed long-term acclimation (days to weeks) to high leaf temperatures. Water stress enhanced resistance of PSII to high temperatures both in the dark and over a wide range of PFD. Low PFD protected photochemical activity against inactivation by heat while high PFD exacerbated damage of PSII by heat. The greater interception of radiation by horizontally restrained leaves relative to the steep leaves of sun-acclimated plants caused photoinhibition and increased leaf temperature. When transpirational cooling was decreased by water stress, leaf temperature surpassed the limits of chloroplast thermostability. The remarkable acclimation of water-stressed plants to high leaf temperatures proved insufficient for the semi-natural environmental conditions of the experiment. Summer stresses characteristic of Mediterranean-type climates (high leaf temperatures in particular) are a potential limiting factor for seedling survival in H. arbutifolia, especially for shade seedlings lacking the crucial structural photoprotection provided by steep leaf angles.     

Differences in hydraulic architecture account for near-isohydric and anisohydric behaviour of two field-grown Vitis vinifera L. cultivars during drought

A comparative study on stomatal control under water deficit was conducted on grapevines of the cultivars Grenache, of Mediterranean origin, and Syrah of mesic origin, grown near Montpellier, France and Geisenheim, Germany. Syrah maintained similar maximum stomatal conductance (g_max) and maximum leaf photosynthesis (A_max) values than Grenache at lower predawn leaf water potentials, Ψ_leaf, throughout the season. The Ψ_leaf of Syrah decreased strongly during the day and was lower in stressed than in watered plants, showing anisohydric stomatal behaviour. In contrast, Grenache showed isohydric stomatal behaviour in which Ψ_leaf did not drop significantly below the minimum Ψ_leaf of watered plants. When g was plotted versus leaf specific hydraulic conductance, K_l, incorporating leaf transpiration rate and whole‐plant water potential gradients, previous differences between varieties disappeared both on a seasonal and diurnal scale. This suggested that isohydric and anisohydric behaviour could be regulated by hydraulic conductance. Pressure‐flow measurements on excised organs from plants not previously stressed revealed that Grenache had a two‐ to three‐fold larger hydraulic conductance per unit path length (K_h) and a four‐ to six‐fold larger leaf area specific conductivity (LSC) in leaf petioles than Syrah. Differences between internodes were only apparent for LSC and were much smaller. Cavitation detected as ultrasound acoustic emissions on air‐dried shoots showed higher rates for Grenache than Syrah during the early phases of the dry‐down. It is hypothesized that the differences in water‐conducting capacity of stems and especially petioles may be at the origin of the near‐isohydric and anisohydric behaviour of g.     

Different leaf cost–benefit strategies of ferns distributed in contrasting light habitats of sub-tropical forests

Background and Aims Ferns are abundant in sub-tropical forests in southern China, with some species being restricted to shaded understorey of natural forests, while others are widespread in disturbed, open habitats. To explain this distribution pattern, we hypothesize that ferns that occur in disturbed forests (FDF) have a different leaf cost–benefit strategy compared with ferns that occur in natural forests (FNF), with a quicker return on carbon investment in disturbed habitats compared with old-growth forests.Methods We chose 16 fern species from contrasting light habitats (eight FDF and eight FNF) and studied leaf functional traits, including leaf life span (LLS), specific leaf area (SLA), leaf nitrogen and phosphorus concentrations (N and P), maximum net photosynthetic rates (A), leaf construction cost (CC) and payback time (PBT), to conduct a leaf cost–benefit analysis for the two fern groups.Key Results The two groups, FDF and FNF, did not differ significantly in SLA, leaf N and P, and CC, but FDF had significantly higher A, greater photosynthetic nitrogen- and phosphorus-use efficiencies (PNUE and PPUE), and shorter PBT and LLS compared with FNF. Further, across the 16 fern species, LLS was significantly correlated with A, PNUE, PPUE and PBT, but not with SLA and CC.Conclusions Our results demonstrate that leaf cost–benefit analysis contributes to understanding the distribution pattern of ferns in contrasting light habitats of sub-tropical forests: FDF employing a quick-return strategy can pre-empt resources and rapidly grow in the high-resource environment of open habitats; while a slow-return strategy in FNF allows their persistence in the shaded understorey of old-growth forests.     

Field-acclimated Gossypium hirsutum cultivars exhibit genotypic and seasonal differences in photosystem II thermostability

Previous investigations have demonstrated that photosystem II (PSII) thermostability acclimates to prior exposure to heat and drought, but contrasting results have been reported for cotton (Gossypium hirsutum). We hypothesized that PSII thermotolerance in G. hirsutum would acclimate to environmental conditions during the growing season and that there would be differences in PSII thermotolerance between commercially-available U.S. cultivars. To this end, three cotton cultivars were grown under dryland conditions in Tifton Georgia, and two under irrigated conditions in Marianna Arkansas. At Tifton, measurements included PSII thermotolerance (T₁₅, the temperature causing a 15% decline in maximum quantum yield), leaf temperatures, air temperatures, midday (1200 to 1400 h) leaf water potentials (Ψ_MD), leaf-air vapor pressure deficit (VPD), actual quantum yield (Φ_PSII) and electron transport rate through PSII (ETR) on three sample dates. At Marianna, T₁₅ was measured on two sample dates. Optimal air and leaf temperatures were observed on all sample dates in Tifton, but PSII thermotolerance increased with water deficit conditions (Ψ_MD = −3.1 MPa), and ETR was either unaffected or increased under water-stress. Additionally, T₁₅ for PHY 499 was ∼5 °C higher than for the other cultivars examined (DP 0912 and DP 1050). The Marianna site experienced more extreme high temperature conditions (20–30 days T_max ≥ 35 °C), and showed an increase in T₁₅ with higher average T_max. When average T₁₅ values for each location and sample date were plotted versus average daily T_max, strong, positive relationships (r² from .954 to .714) were observed between T_max and T₁₅. For all locations T₁₅ was substantially higher than actual field temperature conditions. We conclude that PSII thermostability in G. hirsutum acclimates to pre-existing environmental conditions; PSII is extremely tolerant to high temperature and water-deficit stress; and differences in PSII thermotolerance exist between commercially-available cultivars.