Leaf Photosynthetic Rate of Tropical Ferns Is Evolutionarily Linked to Water Transport Capacity

Ferns usually have relatively lower photosynthetic potential than angiosperms. However, it is unclear whether low photosynthetic potential of ferns is linked to leaf water supply. We hypothesized that there is an evolutionary association of leaf water transport capacity with photosynthesis and stomatal density in ferns. In the present study, a series of functional traits relating to leaf anatomy, hydraulics and physiology were assessed in 19 terrestrial and 11 epiphytic ferns in a common garden, and analyzed by a comparative phylogenetics method. Compared with epiphytic ferns, terrestrial ferns had higher vein density (D_vein), stomatal density (SD), stomatal conductance (g_s), and photosynthetic capacity (A_max), but lower values for lower epidermal thickness (LET) and leaf thickness (LT). Across species, all traits varied significantly, but only stomatal length (SL) showed strong phylogenetic conservatism. A_max was positively correlated with D_vein and g_s with and without phylogenetic corrections. SD correlated positively with A_max, D_vein and g_s, with the correlation between SD and D_vein being significant after phylogenetic correction. Leaf water content showed significant correlations with LET, LT, and mesophyll thickness. Our results provide evidence that A_max of the studied ferns is linked to leaf water transport capacity, and there was an evolutionary association between water supply and demand in ferns. These findings add new insights into the evolutionary correlations among traits involving carbon and water economy in ferns.     

Transpiration and stomatal control: a cross-species study of leaf traits in 39 evergreen and deciduous broadleaved subtropical tree species

Key message

Using an extensive dataset for 39 subtropical broadleaved tree species, we found traits of the leaf economics spectrum to be linked to mean stomatal conductance but not to stomatal regulation.

Abstract

The aim of our study was to establish links between stomatal control and functional leaf traits. We hypothesized that mean and maximum stomatal conductance (g _s) varies with the traits described by the leaf economics spectrum, such as specific leaf area and leaf dry matter content, and that high g _s values correspond to species with tender leaves and high photosynthetic capacity. In addition, we hypothesized that species with leaves of low stomata density have more limited stomatal closure than those with high stomata density. In order to account for confounding site condition effects, we made use of a common garden situation in which 39 deciduous and evergreen species of the same age were grown in a biodiversity ecosystem functioning experiment in Jiangxi (China). Daily courses of g _s were measured with porometry, and the species-specific g _s~vpd relationships were modeled. Our results show that mean stomatal conductance can be predicted from leaf traits that represent the leaf economics spectrum, with a positive relationship being related to leaf nitrogen content and a negative relationship with the leaf carbon: nitrogen ratio. In contrast, parameters of stomatal control were related to traits unassociated with the leaf economics spectrum. The maximum of the conductance~vpd curve was positively related to leaf carbon content and vein length. The vpd at the point of inflection of the conductance~vpd curve was lower for species with higher stomata density and higher for species with a high leaf carbon content. Overall, stomata size and density as well as vein length were more effective at explaining stomatal regulation than traits used in the leaf economics spectrum.     

Contrasting correlation networks between leaf structure,nitrogen and chlorophyll in herbaceous and woody canopies

We studied acclimation patterns in leaf dry mass per area (M_A), nitrogen (N_A) and chlorophyll (ζ_A) content per area, and chlorophyll to nitrogen ratio (ζ/N) along vertical light gradients in natural temperate mixed herbaceous canopy and deciduous tree canopy. In the deciduous tree canopy, all leaves are formed at approximately the same time, and the light gradient during the rest of the growing season reflects the differences in light availability during leaf development, whereas in the herbaceous canopy, leaf production continues during most of the growing season and major changes in light conditions occur after leaf maturation. M_A and N_A increased strongly with increasing current light availability (I_D) in the tree canopy. In the herbaceous canopy, M_A and N_A were generally unrelated to I_D. Depending on species, the correlation between chlorophyll content per leaf area (ζ_A) and I_D was positive, negative, or non-significant. Path analyses revealed two opposite effects of I_D on the amount of leaf chlorophyll. In the tree canopy, increasing I_D enhanced ζ_A through changes in M_A and N_A, whereas the direct effect of light was negative in both canopies. The overall correlation network between foliage structural and chemical traits and the relationships with I_D were significantly stronger in the tree canopy, suggesting limited re-acclimation potential in the mixed herbaceous canopy. Within-species acclimation patterns reflected the patterns within the main functional types. These data demonstrate that the relationships of current light availability vs. leaf dry mass per area, leaf nitrogen and chlorophyll contents, and chlorophyll to nitrogen ratio differ among multi-species herbaceous canopies and deciduous tree canopies due to contrasting canopy development.     

Leaf Gas Exchange of Pachyrhizus ahipa and P. erosus Under Water and Temperature Stress

Gas exchange, water relations, and leaf traits were studied in the tuberous-root producing legumes ahipa (Pachyrhizus ahipa) and yambean (P. erosus) under different environmental conditions. Differences in leaf traits (hairiness, leaf area, areal leaf mass, stomatal density) and paraheliotropism were found between ahipa and yambean. Under sufficient water supply, the increase in air temperature and decrease in air humidity increased stomatal conductance (g _s) and net photosynthetic rate (P _N) in yambean but reduced them in ahipa. In a drying soil (14 d after irrigation), inter-specific variation in gas exchange was only observed in the early morning, and yambean showed a greater sensitivity to water restriction than ahipa. High g _s at low humidity increased P _N of P. erosus but resulted in lower water-use efficiency (WUE). However, long-term WUE, estimated by leaf carbon isotope discrimination, showed little variation between species. Daily-irrigated ahipa and yambean grown in the greenhouse did not show significant differences in gas exchange. However, leaf temperature was significantly greater in yambean than in ahipa while a steepper relationship between E and P _N and g _s was observed in ahipa.     

The Photosynthetic Characteristics of Saplings of Eight Canopy Tree Species in a Disturbed Neotropical Rain Forest

Foliar gas exchange characteristics, understorey microclimate, and crown irradiation were assessed for saplings of eight canopy tree species in two plots of neotropical rain forest with different degrees of canopy opening. Species studied belonged to different putative guilds: shade intolerants (both short-lived--pioneers--and long-lived), intermediates, and shade-tolerants. A considerable overlap was recorded between species in values of the photosynthetic rate per unit leaf area (P_N). The highest median P_N (1.26 μmol m^-2 s^-1) was recorded in the pioneer Croton killipianus, while slightly lower median values were recorded in Simarouba amara and Pentaclethra macroloba, and markedly lower values in two species of Vochysiaceae (Qualea paraense and Vochysia ferruginea), both putative intolerants. Highest median stomatal conductance (g_s) was also shown by C. killipianus, while S. amara, P. macroloba, and L. procera exhibited intermediate values, and the lowest g_s was shown by V. ferruginea and Q. paraense. Overall irradiance and crown irradiation, P_N, and g_s of saplings were higher in the plot which had previously received a silvicultural treatment. Most values of photosynthetic photon flux density (PPFD) were <100 μmol m^-2 s^-1 in both plots, with shortlived peaks of up to 2000 μmol m^-2 s^-1 in the treated plot. When the relationship between P_N and irradiance (I) was examined by fitting P_N/I curves, the degree of fit varied markedly between species, values of the regression coefficient r² were between 0.09 and 0.51. No significant differences between species were recorded in P_max and species also demonstrated little variation in the predicted values of dark respiration (R_D), values varying between -0.51 and -1.46 μmol m^-2 s^-1 in Q. paraense and Minquartia guianensis, respectively. Fitted values of apparent quantum efficiency were also fairly uniform, generally falling within the range 0.02-0.03 mol(CO₂) mol^-1(photon). This revised version was published online in June 2006 with corrections to the Cover Date.     

Stomatal Behavior and Water Status of Maize, Sorghum, and Tobacco under Field Conditions: I. At High Soil Water Potential

Diurnal changes in the vertical profiles of irradiance incident upon the adaxial leaf surface (I), stomatal resistance (r_s), leaf water potential (ψ), osmotic potential (π), and turgor potential (P) were followed concurrently in crops of maize (Zea mays L. var. Pa 602A), sorghum (Sorghum bicolor [L.] Moench var. RS610), and tobacco (Nicotiana tabacum L. var. Havanna Seed 211) on several days in 1968 to 1970 when soil water potentials were high. In all three crops the r_s, measured with a ventilated diffusion porometer, the ψ, measured with the pressure chamber, the π, measured with a vapor pressure osmometer, and the calculated P, decreased from sunrise to reach minimum values near midday and then increased again in the afternoon. The diurnal range of all the variables was greater for leaves in the upper canopy than for those in the lower canopy. P was observed to decrease with decreasing ψ, but never became zero. Sorghum had a higher P at a ψ of, say −10 bars, than did maize, and maize had a higher P than tobacco at the same ψ. Moreover, at the same ψ the upper leaves in all canopies had a higher P than the lower leaves. When compared at high irradiances, r_s did not increase as ψ declined to −13, −15, and −10 bars or as P declined to 0.3, 3.5, and 1.2 bars in maize, sorghum, and tobacco, respectively. The relation between r_s and I in the upper, nonsenescent leaves of all three crops fits a hyperbolic curve, but the response varied with species and leaf senescence. The adaxial and abaxial epidermises had the same response of r_s to I in maize and sorghum, whereas in tobacco the adaxial epidermis had a higher r_s than the abaxial epidermis at all values of I. At equal values of I, tobacco had the lowest leaf resistance (r_l) and maize had the highest r_l. Senescent maize leaves had nonfunctional stomata, whereas the lowermost sorghum leaves had higher stomatal resistances on average than the other leaves.     

Functional relationships between hydraulic traits and the timing of diurnal depression of photosynthesis

The hydraulic coordination along the water transport pathway helps trees provide adequate water supply to the canopy, ensuring that water deficits are minimized and that stomata remain open for CO₂ uptake. We evaluated the stem and leaf hydraulic coordination and the linkages between hydraulic traits and the timing of diurnal depression of photosynthesis across seven evergreen tree species in the southern Andes. There was a positive correlation between stem hydraulic conductivity (k_s) and leaf hydraulic conductance (K_Leaf) across species. All species had similar maximum photosynthetic rates (A_max). The species with higher k_s and K_Leaf attained A_max in the morning, whereas the species with lower k_s and K_Leaf exhibited their A_max in the early afternoon concurrently with turgor loss. These latter species had very negative leaf water potentials, but far from the pressure at which the 88% of leaf hydraulic conductance is lost. Our results suggest that diurnal gas exchange dynamics may be determined by leaf hydraulic vulnerability such that a species more vulnerable to drought restrict water loss and carbon assimilation earlier than species less vulnerable. However, under stronger drought, species with earlier CO₂ uptake depression may increase the risk of hydraulic failure, as their safety margins are relatively narrow.     

Genotypic and within canopy variation in leaf carbon isotope discrimination and its relation to short-term leaf gas exchange characteristics in cassava grown under rain-fed conditions in the tropics

In a field rain-fed trial with 15 cassava cultivars, leaf gas exchanges and carbon isotope discrimination (Δ) of the same leaves were determined to evaluate genotypic and within-canopy variations in these parameters. From 3 to 7 months after planting leaf gas exchange was measured on attached leaves from upper, middle, and lower canopy layers. All gas exchange parameters varied significantly among cultivars as well as canopy layers. Net photosynthetic rate (P _N) decreased from top canopy to bottom indicating both shade and leaf age effects. The same trend, but in reverse, was found with respect to Δ, with the highest values in low canopy level and the lowest in upper canopy. There were very significant correlations, with moderate and low values, among almost all these parameters, with P _N negatively associated with intercellular CO₂ concentration (C _i), ratio of C _i to ambient CO₂ concentration C _i/C _a, and Δ. Across all measured leaves, Δ correlated negatively with leaf water use efficiency (WUE = photosynthesis/stomatal conductance, g _s) and with g _s, but positively with C _i and C _i/C _a. The later parameters negatively correlated with leaf WUE. Across cultivars, both P _N and correlated positively with storage root yield. These results are in agreement with trends predicted by the carbon isotope discrimination model.     

Association genetics,geography and ecophysiology link stomatal patterning in Populus trichocarpa with carbon gain and disease resistance trade‐offs

Stomata are essential for diffusive entry of gases to support photosynthesis, but may also expose internal leaf tissues to pathogens. To uncover trade‐offs in range‐wide adaptation relating to stomata, we investigated the underlying genetics of stomatal traits and linked variability in these traits with geoclimate, ecophysiology, condensed foliar tannins and pathogen susceptibility in black cottonwood (Populus trichocarpa). Upper (adaxial) and lower (abaxial) leaf stomatal traits were measured from 454 accessions collected throughout much of the species range. We calculated broad‐sense heritability (H²) of stomatal traits and, using SNP data from a 34K Populus SNP array, performed a genome‐wide association studies (GWAS) to uncover genes underlying stomatal trait variation. H² values for stomatal traits were moderate (average H² = 0.33). GWAS identified genes associated primarily with adaxial stomata, including polarity genes (PHABULOSA), stomatal development genes (BRASSINOSTEROID‐INSENSITIVE 2) and disease/wound‐response genes (GLUTAMATE‐CYSTEINE LIGASE). Stomatal traits correlated with latitude, gas exchange, condensed tannins and leaf rust (Melampsora) infection. Latitudinal trends of greater adaxial stomata numbers and guard cell pore size corresponded with higher stomatal conductance (g_s) and photosynthesis (A_max), faster shoot elongation, lower foliar tannins and greater Melampsora susceptibility. This suggests an evolutionary trade‐off related to differing selection pressures across the species range. In northern environments, more adaxial stomata and larger pore sizes reflect selection for rapid carbon gain and growth. By contrast, southern genotypes have fewer adaxial stomata, smaller pore sizes and higher levels of condensed tannins, possibly linked to greater pressure from natural leaf pathogens, which are less significant in northern ecosystems.     

The effects of intervessel pit characteristics on xylem hydraulic efficiency and photosynthesis in hemiepiphytic and non‐hemiepiphytic Ficus species

Xylem vulnerability to cavitation and hydraulic efficiency are directly linked to fine‐scale bordered pit features in water‐conducting cells of vascular plants. However, it is unclear how pit characteristics influence water transport and carbon economy in tropical species. The primary aim of this study was to evaluate functional implications of changes in pit characteristics for water relations and photosynthetic traits in tropical Ficus species with different growth forms (i.e. hemiepiphytic and non‐hemiepiphytic) grown under common conditions. Intervessel pit characteristics were measured using scanning electron microscopy in five hemiepiphytic and five non‐hemiepiphytic Ficus species to determine whether these traits were related to hydraulics, leaf photosynthesis, stomatal conductance and wood density. Ficus species varied greatly in intervessel pit structure, hydraulic conductivity and leaf physiology, and clear differences were observed between the two growth forms. The area and diameter of pit aperture were negatively correlated with sapwood‐specific hydraulic conductivity, mass‐based net assimilation rate, stomatal conductance (g_s), intercellular CO₂ concentration (C_i) and the petiole vessel lumen diameters (D_v), but positively correlated with wood density. Pit morphology was only negatively correlated with sapwood‐ and leaf‐specific hydraulic conductivity and D_v. Pit density was positively correlated with g_s, C_i and D_v, but negatively with intrinsic leaf water‐use efficiency. Pit and pit aperture shape were not significantly correlated with any of the physiological traits. These findings indicate a significant role of pit characteristics in xylem water transport, carbon assimilation and ecophysiological adaptation of Ficus species in tropical rain forests.     

Prolonged experimental drought reduces plant hydraulic conductance and transpiration and increases mortality in a piñon–juniper woodland

Plant hydraulic conductance (k_s) is a critical control on whole‐plant water use and carbon uptake and, during drought, influences whether plants survive or die. To assess long‐term physiological and hydraulic responses of mature trees to water availability, we manipulated ecosystem‐scale water availability from 2007 to 2013 in a piñon pine (Pinus edulis) and juniper (Juniperus monosperma) woodland. We examined the relationship between k_s and subsequent mortality using more than 5 years of physiological observations, and the subsequent impact of reduced hydraulic function and mortality on total woody canopy transpiration (E_C) and conductance (G_C). For both species, we observed significant reductions in plant transpiration (E) and k_s under experimentally imposed drought. Conversely, supplemental water additions increased E and k_s in both species. Interestingly, both species exhibited similar declines in k_s under the imposed drought conditions, despite their differing stomatal responses and mortality patterns during drought. Reduced whole‐plant k_s also reduced carbon assimilation in both species, as leaf‐level stomatal conductance (g_s) and net photosynthesis (A_n) declined strongly with decreasing k_s. Finally, we observed that chronically low whole‐plant k_s was associated with greater canopy dieback and mortality for both piñon and juniper and that subsequent reductions in woody canopy biomass due to mortality had a significant impact on both daily and annual canopy E_C and G_C. Our data indicate that significant reductions in k_s precede drought‐related tree mortality events in this system, and the consequence is a significant reduction in canopy gas exchange and carbon fixation. Our results suggest that reductions in productivity and woody plant cover in piñon–juniper woodlands can be expected due to reduced plant hydraulic conductance and increased mortality of both piñon pine and juniper under anticipated future conditions of more frequent and persistent regional drought in the southwestern United States.     

Variations in photosynthetic rate and associated parameters with age of oil palm leaves under irrigation

Net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s) in an adult oil palm (Elaeis guineensis) canopy were highest in the 9^th leaf and progressively declined with leaf age. Larger leaf area (LA) and leaf dry mass (LDM) were recorded in middle leaves. P _N showed a significant positive correlation with g _s and a negative relationship with leaf mass per area (ALM). The oil palm leaf remains photosynthetically active for a longer time in the canopy which contributes significantly to larger dry matter production in general and greater fresh fruit bunch yields in particular.     

Late-stage canopy tree species with extremely low δ13C and high stomatal sensitivity to seasonal soil drought in the tropical rainforest of French Guiana

We assessed the daily time‐courses of CO₂ assimilation rate (A), leaf transpiration rate (E), stomatal conductance for water vapour (g_s), leaf water potential ( Ψ _w) and tree transpiration in a wet and a dry season for three late‐stage canopy rainforest tree species in French Guiana differing in leaf carbon isotope composition ( δ ¹³C). The lower sunlit leaf δ ¹³C values found in Virola surinamensis ( ? 29·9‰) and in Diplotropis purpurea ( ? 30·9‰), two light‐demanding species, as compared to Eperua falcata ( ? 28·6‰), a shade‐semi‐tolerant species, were clearly associated with higher maximum g_s values of sunlit leaves in the two former species. These two species were also characterized by a high sensitivity of g_s, sap flow density (Ju) and canopy conductance (g_c) to seasonal soil drought, allowing maintenance of high midday Ψ _w values in the dry season. The data for Diplotropis provided an original picture of increasing midday Ψ _w with increasing soil drought. In Virola, stomata were extremely sensitive to seasonal soil drought, leading to a dramatic decrease in leaf and tree transpiration in the dry season, whereas midday Ψ _w remained close to ? 0·3 MPa. The mechanisms underlying such an extremely high sensitivity of stomata to soil drought remain unknown. In Eperua, g_s of sunlit leaves was non‐responsive to seasonal drought, whereas Ju and g_c were lower in the dry season. This suggests a higher stomatal sensitivity to seasonal drought in shaded leaves than in sunlit ones in this species.     

Stomatal sensitivity to vapour pressure difference over a subambient to elevated CO2 gradient in a C3/C4 grassland

In the present study the response of stomatal conductance (g_s) to increasing leaf‐to‐air vapour pressure difference (D) in early season C₃ (Bromus japonicus) and late season C₄ (Bothriochloa ischaemum) grasses grown in the field across a range of CO₂ (200–550 µmol mol^?1) was examined. Stomatal sensitivity to D was calculated as the slope of the response of g_s to the natural log of externally manipulated D (dg_s/dlnD). Increasing D and CO₂ significantly reduced g_s in both species. Increasing CO₂ caused a significant decrease in stomatal sensitivity to D in Br. japonicus, but not in Bo. ischaemum. The decrease in stomatal sensitivity to D at high CO₂ for Br. japonicus fit theoretical expectations of a hydraulic model of stomatal regulation, in which g_s varies to maintain constant transpiration and leaf water potential. The weaker stomatal sensitivity to D in Bo. ischaemum suggested that stomatal regulation of leaf water potential was poor in this species, or that non‐hydraulic signals influenced guard cell behaviour. Photosynthesis (A) declined with increasing D in both species, but analyses of the ratio of intercellular to atmospheric CO₂ (C_i/C_a) suggested that stomatal limitation of A occurred only in Br. japonicus. Rising CO₂ had the greatest effect on g_s and A in Br. japonicus at low D. In contrast, the strength of stomatal and photosynthetic responses to CO₂ were not affected by D in Bo. ischaemum. Carbon and water dynamics in this grassland are dominated by a seasonal transition from C₃ to C₄ photosynthesis. Interspecific variation in the response of g_s to D therefore has implications for predicting seasonal ecosystem responses to CO₂.     

Survey and synthesis of intra- and interspecific variation in stomatal sensitivity to vapour pressure deficit

Responses of stomatal conductance (g_s) to increasing vapour pressure deficit (D) generally follow an exponential decrease described equally well by several empirical functions. However, the magnitude of the decrease – the stomatal sensitivity – varies considerably both within and between species. Here we analysed data from a variety of sources employing both porometric and sap flux estimates of g_s to evaluate the hypothesis that stomatal sensitivity is proportional to the magnitude of g_s at low D ( ≤ 1 kPa). To test this relationship we used the function g_s = g_sref–m· lnD where m is the stomatal sensitivity and g_sref = g_s at D = 1 kPa. Regardless of species or methodology, m was highly correlated with g_sref (average r² = 0·75) with a slope of approximately 0·6. We demonstrate that this empirical slope is consistent with the theoretical slope derived from a simple hydraulic model that assumes stomatal regulation of leaf water potential. The theoretical slope is robust to deviations from underlying assumptions and variation in model parameters. The relationships within and among species are close to theoretical predictions, regardless of whether the analysis is based on porometric measurements of g_s in relation to leaf-surface D (D_s), or on sap flux-based stomatal conductance of whole trees (G_Si), or stand-level stomatal conductance (G_S) in relation to D. Thus, individuals, species, and stands with high stomatal conductance at low D show a greater sensitivity to D, as required by the role of stomata in regulating leaf water potential.     

Leaf morphology of 40 evergreen and deciduous broadleaved subtropical tree species and relationships to functional ecophysiological traits

We explored potential of morphological and anatomical leaf traits for predicting ecophysiological key functions in subtropical trees. We asked whether the ecophysiological parameters stomatal conductance and xylem cavitation vulnerability could be predicted from microscopy leaf traits. We investigated 21 deciduous and 19 evergreen subtropical tree species, using individuals of the same age and from the same environment in the Biodiversity‐Ecosystem Functioning experiment at Jiangxi (BEF‐China). Information‐theoretic linear model selection was used to identify the best combination of morphological and anatomical predictors for ecophysiological functions. Leaf anatomy and morphology strongly depended on leaf habit. Evergreen species tended to have thicker leaves, thicker spongy and palisade mesophyll, more palisade mesophyll layers and a thicker subepidermis. Over 50% of all evergreen species had leaves with multi‐layered palisade parenchyma, while only one deciduous species (Koelreuteria bipinnata) had this. Interactions with leaf habit were also included in best multi‐predictor models for stomatal conductance (g_s) and xylem cavitation vulnerability. In addition, maximum g_s was positively related to log ratio of palisade to spongy mesophyll thickness. Vapour pressure deficit (vpd) for maximum g_s increased with the log ratio of palisade to spongy mesophyll thickness in species having leaves with papillae. In contrast, maximum specific hydraulic conductivity and xylem pressure at which 50% loss of maximum specific xylem hydraulic conductivity occurred (Ψ₅₀) were best predicted by leaf habit and density of spongy parenchyma. Evergreen species had lower Ψ₅₀ values and lower maximum xylem hydraulic conductivities. As hydraulic leaf and wood characteristics were reflected in structural leaf traits, there is high potential for identifying further linkages between morphological and anatomical leaf traits and ecophysiological responses.     

Regulation of transpiration in coffee hedgerows: covariation of environmental variables and apparent responses of stomata to wind and humidity

Stomatal regulation of transpiration was studied in hedgerow coffee (Coffea arabica L.) at different stages of canopy development encompassing a range of leaf area indices (L) from 0·7 to 6·7. Stomatal (g_s) and crown (g_c) conductance attained maximum values early during the day and then declined as both leaf-to-bulk air water vapour mole fraction difference (V_a) and photosynthetically active photon flux density (I) continued to increase. Covariation of environmental variables during the day, particularly V, I, and wind speed (u), obscured stomatal responses to individual variables. This also caused diurnal hysteresis in the relationship between g_c and individual variables. Normalization of g_s and g_c by I removed the hysteresis and revealed a strong stomatal response to humidity. At the crown scale, transpiration (E) increased linearly with net radiation (R_n) and seemed to increase with increasing wind speed. Increasing wind speed imposed higher leaf interior to leaf surface water vapour mole fraction differences (V_s) at given levels of V_a. However, strong relationships between declining g_c and E and increasing wind speed were obtained when g_c and E were normalized by I and R_n, respectively, without invoking additional potential interactions involving temperature or CO₂ concentration at the leaf surface. Apparent stomatal responses to wind were thus at least partially a reflection of the stomatal response to humidity.     

Coordinated changes in photosynthesis,water relations and leaf nutritional traits of canopy trees along a precipitation gradient in lowland tropical forest

We investigated leaf physiological traits of dominant canopy trees in four lowland Panamanian forests with contrasting mean annual precipitation (1,800, 2,300, 3,100 and 3,500 mm). There was near complete turn-over of dominant canopy tree species among sites, resulting in greater dominance of evergreen species with long-lived leaves as precipitation increased. Mean structural and physiological traits changed along this gradient as predicted by cost–benefit theories of leaf life span. Nitrogen content per unit mass (N_mass) and light- and CO₂-saturated photosynthetic rates per unit mass (P_mass) of upper canopy leaves decreased with annual precipitation, and these changes were partially explained by increasing leaf thickness and decreasing specific leaf area (SLA). Comparison of 1,800 mm and 3,100 mm sites, where canopy access was available through the use of construction cranes, revealed an association among extended leaf longevity, greater structural defense, higher midday leaf water potential, and lower P_mass, N_mass, and SLA at wetter sites. Shorter leaf life spans and more enriched foliar ¹⁵N values in drier sites suggest greater resorption and re-metabolism of leaf N in drier forest. Greater dominance of short-lived leaves with relatively high P_mass in drier sites reflects a strategy to maximize photosynthesis when water is available and to minimize water loss and respiration costs during rainless periods. Overall, our study links coordinated change in leaf functional traits that affect productivity and nutrient cycling to seasonality in lowland tropical forests.     

Stomatal control of water use in olive tree leaves

Little is known about the strategies used by olive trees to overcome the long dry periods typical of the areas where they are cropped. This makes it difficult to optimize the water supply in orchards. To study the control of water consumption by olive trees, measurements of leaf water potential () and stomatal conductance to H₂O (g) were made on 26-year-old t Manzanillo olive trees under three irrigation treatments. The first treatment provided enough water to cover the crop water demand, the next treatment supplied one third of that rate, and the final treatment was no irrigation at all, typical of dry-farming conditions. Under conditions of high vapour pressure deficit of the air (D_a), the olive trees prevented excessive water loss by closing their stomata. Leaves of the current year showed better stomatal control than did the 1-year-old leaves. The upper-bound functional relationships between t g and t D_a and photon flux density (I_P) were obtained by boundary-line analysis, based on a technique of non-linear least squares. Maximum values of t g were observed at relatively low levels of t I_P, from about 500 mol m^-2 s^-1, and a proportional decrease in t g with increasing t D_a was also found, at least for values of up to approximately 3.5 kPa. Higher values of t g were observed in the morning than in the afternoon, for similar levels of t I_P and t D_a. Unirrigated olive trees recovered quickly after the dry season, showing values of and t g similar to those of irrigated trees after just two days.     

Seasonal changes in the photosynthetic response ofMercurialis perennis plants from different light regime conditions

Curves relating net photosynthetic rate to irradiance [P(I) curve relation] were estimated and analysed inMercurialis perennis L. plants stemming from three forest (spruce, beech and ash) stands with different tree leaf canopy development and different light regime. The saturating irradiance (I_s) reached the highest values in plants of all three stands in spring (spruce forest: 438 W m⁻², beech forest: 440 W m⁻² and ash forest: 367 W m⁻²), it declined sharply in the middle of the growing season (283, 285 and 297 W m^-2, respectively) and this I_s level persisted until autumn. A pronounced dynamics in plants from spruce and beech forests made itself manifest also in the adaptation (I_a) and compensating (I_c) irradiances, respectively. After a sudden decline in summer, values in autumn were close to those of the vernal season. The most pronounced parameter, which optimally expressed the adaptation ofMercurialis perennis to various light conditions, was the photosynthetic efficiency (α) calculated as the slope of the linear part of the curve relating net photosynthetic rate to irradiance. At the time of the highest P_{N sat.} value in course of the growing season (August) (spruce forest: 100, beech forest: 98.7 and ash forest: 85.8 μg CO₂ m⁻² s⁻¹), R_D was in its minimum; in autumn P_{N sat.} reached the lowest values which corresponded to the most intensive R_D. It was found thatMercurialis perennis plants stemming from forest stands with different light conditions did not make use equally of the altering light conditions in the course of the growing season. By the underlying analysis of P(I) curves this rhizomatous perennial herb (geophyte) may be characterized as a shade tolerant species.