Impact of light quality on leaf and shoot hydraulic properties: a case study in silver birch (Betula pendula)

Responses of leaf and shoot hydraulic conductance to light quality were examined on shoots of silver birch (Betula pendula), cut from lower (‘shade position’) and upper thirds of the crowns (‘sun position’) of trees growing in a natural temperate forest stand. Hydraulic conductances of leaf blades (K_lb), petioles (K_P) and branches (i.e. leafless stem; K_B) were determined using a high pressure flow meter in steady state mode. The shoots were exposed to photosynthetic photon flux density of 200–250 µmol m^?2 s^?1 using white, blue or red light. K_lb depended significantly on both light quality and canopy position (P < 0.001), K_B on canopy position (P < 0.001) and exposure time (P = 0.014), and none of the three factors had effect on K_P. The highest values of K_lb were recorded under the blue light (3.63 and 3.13 × 10^?4 kg m^?2 MPa^?1 s^?1 for the sun and shade leaves, respectively), intermediate values under white light (3.37 and 2.46 × 10^?4 kg m^?2 MPa^?1 s^?1, respectively) and lowest values under red light (2.83 and 2.02 × 10^?4 kg m^?2 MPa^?1 s^?1, respectively). Light quality has an important impact on leaf hydraulic properties, independently of light intensity or of total light energy, and the specific light receptors involved in this response require identification. Given that natural canopy shade depletes blue and red light, K_lb may be decreased both by reduced fluence and shifts in light spectra, indicating the need for studies of the natural heterogeneity of K_lb within and under canopies, and its impacts on gas exchange.     

Hydraulic architecture and water flow in growing grass tillers (Festuca arundinacea Schreb.)

The water relations and hydraulic architecture of growing grass tillers (Festuca arundinacea Schreb.) are reported. Evaporative flux density, E (mmol s^?1 m^?2), of individual leaf blades was measured gravimetrically by covering or excision of entire leaf blades. Values of E were similar for mature and elongating leaf blades, averaging 2·4 mmol s^?1 m^?2. Measured axial hydraulic conductivity, K_h (mmol s^?1 mm MPa^?1), of excised leaf segments was three times lower than theoretical hydraulic conductivity (K_t) calculated using the Poiseuille equation and measurements of vessel number and diameter. K_t was corrected (K_t*) to account for the discrepancy between K_h and K_t and for immature xylem in the basal expanding region of elongating leaves. From base to tip of mature leaves the pattern of K_t* was bell‐shaped with a maximum near the sheath–blade joint (≈ 19 mmol s^?1 mm MPa^?1). In elongating leaves, immature xylem in the basal growing region led to a much lower K_t*. As the first metaxylem matured, K_t* increased by 10‐fold. The hydraulic conductances of the whole root system, (mmol s^?1 MPa^?1) and leaf blades, (mmol s^?1 MPa^?1) were measured by a vacuum induced water flow technique. and were linearly related to the leaf area downstream. Approximately 65% of the resistance to water flow within the plant resided in the leaf blade. An electric‐analogue computer model was used to calculate the leaf blade area‐specific radial hydraulic conductivity, (mmol s^?1 m^?2 MPa^?1), using , K_t* and water flux values. values decreased with leaf age, from 21·2 mmol s^?1 m^?2 MPa^?1 in rapidly elongating leaf to 7·2 mmol s^?1 m^?2 MPa^?1 in mature leaf. Comparison of and values showed that ≈ 90% of the resistance to water flow within the blades resided in the liquid extra‐vascular path. The same algorithm was then used to compute the xylem and extravascular water potential drop along the liquid water path in the plant under steady state conditions. Predicted and measured water potentials matched well. The hydraulic design of the mature leaf resulted in low and quite constant xylem water potential gradient (≈ 0·3 MPa m^?1) throughout the plant. Much of the water potential drop within mature leaves occurred within a tenth of millimetre in the blade, between the xylem vessels and the site of water evaporation within the mesophyll. In elongating leaves, the low K_t* in the basal growth zone dramatically increased the local xylem water potential gradient (≈ 2·0 MPa m^?1) there. In the leaf elongation zone the growth‐induced water potential difference was ≈ 0·2 MPa.     

Coupled response of stomatal and mesophyll conductance to light enhances photosynthesis of shade leaves under sunflecks

Light gradients within tree canopies play a major role in the distribution of plant resources that define the photosynthetic capacity of sun and shade leaves. However, the biochemical and diffusional constraints on gas exchange in sun and shade leaves in response to light remain poorly quantified, but critical for predicting canopy carbon and water exchange. To investigate the CO₂ diffusion pathway of sun and shade leaves, leaf gas exchange was coupled with concurrent measurements of carbon isotope discrimination to measure net leaf photosynthesis (A_n), stomatal conductance (g_s) and mesophyll conductance (g_m) in Eucalyptus tereticornis trees grown in climate controlled whole‐tree chambers. Compared to sun leaves, shade leaves had lower A_n, g_m, leaf nitrogen and photosynthetic capacity (A_max) but g_s was similar. When light intensity was temporarily increased for shade leaves to match that of sun leaves, both g_s and g_m increased, and A_n increased to values greater than sun leaves. We show that dynamic physiological responses of shade leaves to altered light environments have implications for up‐scaling leaf level measurements and predicting whole canopy carbon gain. Despite exhibiting reduced photosynthetic capacity, the rapid up‐regulation of g_m with increased light enables shade leaves to respond quickly to sunflecks.     

Contributions of diffusional limitation, photoinhibition and photorespiration to midday depression of photosynthesis in Arisaema heterophyllum in natural high light

Diurnal changes in photosynthetic gas exchange and chlorophyll fluorescence were measured under full sunlight to reveal diffusional and non‐diffusional limitations to diurnal assimilation in leaves of Arisaema heterophyllum Blume plants grown either in a riparian forest understorey (shade leaves) or in an adjacent deforested open site (sun leaves). Midday depressions of assimilation rate (A) and leaf conductance of water vapour were remarkably deeper in shade leaves than in sun leaves. To evaluate the diffusional (i.e. stomatal and leaf internal) limitation to assimilation, we used an index [1–A/A₃₅₀], in which A₃₅₀ is A at a chloroplast CO₂ concentration of 350 μ mol mol ^{? 1}. A₃₅₀ was estimated from the electron transport rate (J_T), determined fluorometrically, and the specificity factor of Rubisco (S), determined by gas exchange techniques. In sun leaves under saturating light, the index obtained after the ‘peak’ of diurnal assimilation was 70% greater than that obtained before the ‘peak’, but in shade leaves, it was only 20% greater. The photochemical efficiency of photosystem II ( Δ F/F_m ′ ) and thus J_T was considerably lower in shade leaves than in sun leaves, especially after the ‘peak’. In shade leaves but not in sun leaves, A at a photosynthetically active photon flux density (PPFD) > 500 μ mol m ^{? 2} s ^{? 1} depended positively on J_T throughout the day. Electron flows used by the carboxylation and oxygenation (J_O) of RuBP were estimated from A and J_T. In sun leaves, the J_O/J_T ratio was significantly higher after the ‘peak’, but little difference was found in shade leaves. Photorespiratory CO₂ efflux in the absence of atmospheric CO₂ was about three times higher in sun leaves than in shade leaves. We attribute the midday depression of assimilation in sun leaves to the increased rate of photorespiration caused by stomatal closure, and that in shade leaves to severe photoinhibition. Thus, for sun leaves, increased capacities for photorespiration and non‐photochemical quenching are essential to avoid photoinhibitory damage and to tolerate high leaf temperatures and water stress under excess light. The increased Rubisco content in sun leaves, which has been recognized as raising photosynthetic assimilation capacity, also contributes to increase in the capacity for photorespiration.     

The Péclet effect on leaf water enrichment correlates with leaf hydraulic conductance and mesophyll conductance for CO(2)

Leaf water gets isotopically enriched through transpiration, and diffusion of enriched water through the leaf depends on transpiration flow and the effective path length (L). The aim of this work was to relate L with physiological variables likely to respond to similar processes. We studied the response to drought and vein severing of leaf lamina hydraulic conductance (K_lamina), mesophyll conductance for CO₂ (g_m) and leaf water isotope enrichment in Vitis vinifera L cv. Grenache. We hypothesized that restrictions in water pathways would reduce K_lamina and increase L. As a secondary hypothesis, we proposed that, given the common pathways for water and CO₂ involved, a similar response should be found in g_m. Our results showed that L was strongly related to mesophyll variables, such as K_lamina or g_m across experimental drought and vein‐cutting treatments, showing stronger relationships than with variables included as input parameters for the models, such as transpiration. Our findings were further supported by a literature survey showing a close link between L and leaf hydraulic conductance (K_leaf = 31.5 × L^?0.43, r² = 0.60, n = 24). The strong correlation found between L, K_lamina and g_m supports the idea that water and CO₂ share an important part of their diffusion pathways through the mesophyll.     

Bundle sheath extensions affect leaf structural and physiological plasticity in response to irradiance

Coordination between structural and physiological traits is key to plants' responses to environmental fluctuations. In heterobaric leaves, bundle sheath extensions (BSEs) increase photosynthetic performance (light‐saturated rates of photosynthesis, A_max) and water transport capacity (leaf hydraulic conductance, K_leaf). However, it is not clear how BSEs affect these and other leaf developmental and physiological parameters in response to environmental conditions. The obscuravenosa (obv) mutation, found in many commercial tomato varieties, leads to absence of BSEs. We examined structural and physiological traits of tomato heterobaric and homobaric (obv) near‐isogenic lines grown at two different irradiance levels. K_leaf, minor vein density, and stomatal pore area index decreased with shading in heterobaric but not in homobaric leaves, which show similarly lower values in both conditions. Homobaric plants, on the other hand, showed increased A_max, leaf intercellular air spaces, and mesophyll surface area exposed to intercellular airspace (S_mes) in comparison with heterobaric plants when both were grown in the shade. BSEs further affected carbon isotope discrimination, a proxy for long‐term water‐use efficiency. BSEs confer plasticity in traits related to leaf structure and function in response to irradiance levels and might act as a hub integrating leaf structure, photosynthetic function, and water supply and demand.     

Size-dependent variability of leaf and shoot hydraulic conductance in silver birch

Variation in leaf and shoot hydraulic conductance was examined on detached shoots of silver birch (Betula pendula Roth), cut from the lower third (shade leaves) and upper third of the crown (sun leaves) of large trees growing in a natural temperate forest stand. Hydraulic conductances of whole shoots (K _S), leaf blades (K _lb), petioles (K _P) and branches (i.e. leafless stem; K _B) were determined by water perfusion using a high-pressure flow meter in quasi-steady state mode. The shoots were exposed to irradiance of photosynthetic photon flux density of 200–250 μmol m⁻² s⁻¹, using different light sources. K _lb depended significantly (P < 0.001) on light quality, canopy position and leaf blade area (A _L). K _lb increased from crown base to tree top, in parallel with vertical patterns of A _L. However, the analysis of data on shade and sun leaves separately revealed an opposite trend: the bigger the A _L the higher K _lb. Leaf anatomical study of birch saplings revealed that this trend is attributable to enhanced vascular development with increasing leaf area. Hydraulic traits (K _S, K _B, K _lb) of sun shoots were well co-ordinated and more strongly correlated with characteristics of shoot size than those of shade shoots, reflecting their greater evaporative load and need for stricter adjustment of hydraulic capacity with shoot size. K _S increased with increasing xylem cross-sectional area to leaf area ratio (Huber value; P < 0.01), suggesting a preferential investment in water-conducting tissue (sapwood) relative to transpiring tissue (leaves), and most likely contributing to the functional stability of the hydraulic system, essential for fast-growing pioneer species.     

Scaling carbon dioxide and water vapour exchange from leaf to canopy in a deciduous forest. I. Leaf model parametrization

In order to parametrize a leaf submodel of a canopy level gas-exchange model, a series of photosynthesis and stomatal conductance measurements were made on leaves of white oak (Quercus alba L.) and red maple (Acer rubrum L.) in a mature deciduous forest near Oak Ridge, TN. Gas-exchange characteristics of sun leaves growing at the top of a 30 m canopy and of shade leaves growing at a depth of 3–4 m from the top of the canopy were determined. Measured rates of net photosynthesis at a leaf temperature of 30°C and saturating photosynthetic photon flux density, expressed on a leaf area basis, were significantly lower (P = 0.01; n = 8) in shade leaves (7.9μmol m^?2 s^?1) than in sun leaves (11–5μmol m^?2 s^?1). Specific leaf area increased significantly with depth in the canopy, and when photosynthesis rates were expressed on a dry mass basis, they were not significantly different for shade and sun leaves. The percentage leaf nitrogen did not vary significantly with height in the canopy; thus, rates expressed on a per unit nitrogen basis were also not significantly different in shade and sun leaves. A widely used model integrating photosynthesis and stomatal conductance was parametrized independently for sun and shade leaves, enabling us to model successfully diurnal variations in photosynthesis and evapotranspiration of both classes of leaves. Key photosynthesis model parameters were found to scale with leaf nitrogen levels. The leaf model parametrizations were then incorporated into a canopy-scale gas-exchange model that is discussed and tested in a companion paper (Baldocchi & Harley 1995, Plant, Cell and Environment 18, 1157–1173).     

Variations in leaf morpho‐anatomy and photosynthetic traits between sun and shade populations of Eurya japonica (Pentaphylacaceae) whose seeds are dispersed by birds across habitats

Eurya japonica occurs in diverse light environments through seed dispersal by birds. As the seed size is extremely small, we hypothesized that newly germinated seedlings with restricted depth of roots and length of the hypocotyl would suffer high mortality due to increased transpiration in sunny habitats and low light in shady habitats. We also expected that surviving seedlings would differ in leaf traits between habitats as a result of selection. We aimed to determine how photosynthetic traits differ between habitats and how leaf structure is related to this difference. We examined photosynthesis and leaf morpho‐anatomy for plants cloned from cuttings collected from the forest understory (shade population) and neighboring roadsides and cut‐over areas (sun population) and then grown under two irradiances (18.5% and 100% sunlight) in an experimental garden. Under growth in 100% sunlight, cloned plants from the sun population exhibited significantly greater area‐based photosynthetic capacity compared to cloned plants from the shade population at a comparable stomatal conductance, which was attributable to a higher area‐based leaf nitrogen concentration. On the other hand, mean values of photosynthetic capacity did not significantly differ between the two populations. Cloned plants from the sun population had significantly thicker leaf laminas and spongy tissue and lower stomatal density compared to cloned plants from the shade population. Thickened leaf lamina might have increased leaf tolerance to physical stresses in open habitats. The variation in leaf morpho‐anatomy between the two populations can be explained in terms of the economy of leaf photosynthetic tissue.     

Productivity, carbon isotope discrimination and leaf traits of trees of Eucalyptus globulus Labill. in relation to water availability

The stand basal area, carbon isotope discrimination (Δ) in tree rings and leaves, leaf area index and leaf traits of trees were measured in 6‐ to 8‐year‐old stands of Eucalyptus globulus Labill. across a gradient of rainfall of 600–1400 mm year^?1 in south‐western Australia to better understand the importance of leaf traits and gas‐exchange as determinants of stand productivity. Δ ranged from 17‰ to 21‰. Δ and basal area were highly, positively correlated with each other and the ratio of mean annual rainfall to potential evaporation (P/PE). Leaf area index, soil water holding capacity and leaf nitrogen content were only weakly correlated with basal area. Δ and P/PE were negatively correlated with leaf nitrogen content. Δ was negatively correlated with leaf density but positively correlated with specific leaf area. This is consistent with the theory that larger leaf nitrogen content and smaller specific leaf area are associated with increased photosynthetic capacity and increased leaf‐scale water‐use‐efficiency, and that Δ is influenced by mesophyll conductance. It is concluded that canopy conductance is a more important determinant of growth in water‐limited conditions than either leaf area index or leaf traits in fertilized stands of E. globulus. Water availability was dictated more by rainfall than soil type.     

Bundle sheath lignification mediates the linkage of leaf hydraulics and venation

The lignification of the leaf vein bundle sheath (BS) has been observed in many species and would reduce conductance from xylem to mesophyll. We hypothesized that lignification of the BS in lower‐order veins would provide benefits for water delivery through the vein hierarchy but that the lignification of higher‐order veins would limit transport capacity from xylem to mesophyll and leaf hydraulic conductance (K_leaf). We further hypothesized that BS lignification would mediate the relationship of K_leaf to vein length per area. We analysed the dependence of K_leaf, and its light response, on the lignification of the BS across vein orders for 11 angiosperm tree species. Eight of 11 species had lignin deposits in the BS of the midrib, and two species additionally only in their secondary veins, and for six species up to their minor veins. Species with lignification of minor veins had a lower hydraulic conductance of xylem and outside‐xylem pathways and lower K_leaf. K_leaf could be strongly predicted by vein length per area and highest lignified vein order (R² = .69). The light‐response of K_leaf was statistically independent of BS lignification. The lignification of the BS is an important determinant of species variation in leaf and thus whole plant water transport.     

Acclimation of photosynthesis and respiration to elevated atmospheric CO2 in two Scrub Oaks

Abstract For two species of oak, we determined whether increasing atmospheric CO₂ concentration (C_a) would decrease leaf mitochondrial respiration (R) directly, or indirectly owing to their growth in elevated C_a, or both. In particular, we tested whether acclimatory decreases in leaf‐Rubisco content in elevated C_a would decrease R associated with its maintenance. This hypothesis was tested in summer 2000 on sun and shade leaves of Quercus myrtifolia Willd. and Quercus geminata Small. We also measured R on five occasions between summer 1999 and 2000 on leaves of Q. myrtifolia. The oaks were grown in the field for 4 years, in either current ambient or elevated (current ambient + 350 µmol mol^?1) C_a, in open‐top chambers (OTCs). For Q. myrtifolia, an increase in C_a from 360 to 710 µmol mol^?1 had no direct effect on R at any time during the year. In April 1999, R in young Q. myrtifolia leaves was significantly higher in elevated C_a—the only evidence for an indirect effect of growth in elevated C_a. Leaf R was significantly correlated with leaf nitrogen (N) concentration for the sun and shade leaves of both the species of oak. Acclimation of photosynthesis in elevated C_a significantly reduced maximum RuBP‐saturated carboxylation capacity (V_{c max}) for both the sun and shade leaves of only Q. geminata. However, we estimated that only 11–12% of total leaf N was invested in Rubisco; consequently, acclimation in this plant resulted in a small effect on N and an insignificant effect on R. In this study measurements of respiration and photosynthesis were made on material removed from the field; this procedure had no effect on gas exchange properties. The findings of this study were applicable to R expressed either per unit leaf area or unit dry weight, and did not support the hypothesis that elevated C_a decreases R directly, or indirectly owing to acclimatory decreases in Rubisco content.     

Diurnal depression of leaf hydraulic conductance in a tropical tree species

Diurnal patterns of hydraulic conductance of the leaf lamina (K_leaf) were monitored in a field‐grown tropical tree species in an attempt to ascertain whether the dynamics of stomatal conductance (g_s) and CO₂ uptake (A_leaf) were associated with short‐term changes in K_leaf. On days of high evaporative demand mid‐day depression of K_leaf to between 40 and 50% of pre‐dawn values was followed by a rapid recovery after 1500 h. Leaf water potential during the recovery stage was less than ?1 MPa implying a refilling mechanism, or that loss of K_leaf was not linked to cavitation. Laboratory measurement of the response of K_leaf to Ψ_leaf confirmed that leaves in the field were operating at water potentials within the depressed region of the leaf ‘vulnerability curve’. Diurnal courses of K_leaf and Ψ_leaf predicted from measured transpiration, xylem water potential and the K_leaf vulnerability function, yielded good agreement with observed trends in both leaf parameters. Close correlation between depression of K_leaf, g_s and A_leaf suggests that xylem dysfunction in the leaf may lead to mid‐day depression of gas exchange in this species.     

Differential coordination of stomatal conductance,mesophyll conductance,and leaf hydraulic conductance in response to changing light across species

Stomatal conductance (g_s) and mesophyll conductance (g_m) represent major constraints to photosynthetic rate (A), and these traits are expected to coordinate with leaf hydraulic conductance (K_leaf) across species, under both steady‐state and dynamic conditions. However, empirical information about their coordination is scarce. In this study, K_leaf, gas exchange, stomatal kinetics, and leaf anatomy in 10 species including ferns, gymnosperms, and angiosperms were investigated to elucidate the correlation of H₂O and CO₂ diffusion inside leaves under varying light conditions. Gas exchange, K_leaf, and anatomical traits varied widely across species. Under light‐saturated conditions, the A, g_s, g_m, and K_leaf were strongly correlated across species. However, the response patterns of A, g_s, g_m, and K_leaf to varying light intensities were highly species dependent. Moreover, stomatal opening upon light exposure of dark‐adapted leaves in the studied ferns and gymnosperms was generally faster than in the angiosperms; however, stomatal closing in light‐adapted leaves after darkening was faster in angiosperms. The present results show that there is a large variability in the coordination of leaf hydraulic and gas exchange parameters across terrestrial plant species, as well as in their responses to changing light.     

High-light effects on CO2 fixation gradients across leaves

Chlorophyll fluorescence and internal patterns of ¹⁴CO₂ fixation were measured in sun and shade leaves of spinach after treatment with various light intensities. When sun leaves were irradiated with 2000μmol m^?2 s^?1 for 2h, F_V/F_M decreased by about 15%, but ¹⁴CO₂ fixation was unaffected, whereas shade leaves exhibited a 21% decrease in F_v/F_M and a 25% decrease in ¹⁴CO₂ fixation. Irradiation of sun and shade leaves with 4000μmol m^?1 for 4 h decreased F_V/F_M by 30% in sun leaves and 40% in shade leaves, while total ¹⁴CO₂ fixation decreased by 41% in sun leaves and 55% in shade leaves. After light treatment, gradients of CO₂ fixation across leaves were determined by measuring ¹⁴CO₂ fixed in paradermal leaf sections after a 10s pulse of ¹⁴CO₂. Gradients of ¹⁴CO₂ fixation in control sun and shade leaves were identified when expressed on a relative basis and normalized for leaf depth. Treatment of leaves with 2000 μmol PAR m^?2 s^?1 for 2h did not after patterns of carbon fixation across sun leaves, but slightly altered the pattern in shade leaves. In contrast, treatment of sun and shade leaves with 4000μmol m^?2 s^?1 for 4h decreased carbon fixation more in the palisade mesophyll cells than in the spongy mesophyll cells of sun and shade leaves, and fixation in medial tissue of shade leaves was dramatically decreased compared to the adaxial and abaxial tissue. The interaction between leaf anatomy and biochemical parameters involved in tolerance to photoinhibition in spinach is discussed.     

Leaf plasticity in response to light of three evergreen species of the Mediterranean maquis

Morphological, anatomical, biochemical and physiological traits of sun and shade leaves of adult Quercus ilex, Phillyrea latifolia and Pistacia lentiscus shrub species co-occurring in the Mediterranean maquis at Castelporziano (Latium) were studied. Fully expanded sun leaves had 47% (mean of the three species) greater leaf mass area (LMA) and 31% lower specific leaf area (SLA) than shade leaves. Palisade parenchyma thickness contributed on an average 42% to the total leaf thickness, spongy layer 43%, upper epidermal cells 5%, and upper cuticle thickness 3%. Stomatal size was greater in sun (25.5 μm) than in shade leaves (23.6 μm). Total chlorophyll content per fresh mass was 71% greater in shade than in sun leaves, and nitrogen content was the highest in sun (13.7 mg g⁻¹) than in shade leaves (11.8 mg g⁻¹). Difference of net photosynthetic rates (P _N) between sun and shade leaves was 97% (mean of the three species). The plasticity index (sensu Valladares et al., New Phytol 148:79–91, 2000a) was the highest for physiological leaf traits (0.86) than for morphological, anatomical and biochemical ones. Q. ilex had the highest plasticity index of morphological, anatomical and physiological leaf traits (0.37, 0.28 and 0.71, respectively) that might explain its wider ecological distribution. The higher leaf plasticity of Q. ilex might be advantageous in response to varying environmental conditions, including global change.     

Stem hydraulic supply is linked to leaf photosynthetic capacity: evidence from New Caledonian and Tasmanian rainforests

A strong relationship between hydraulic supply of water to leaves and maximum photosynthetic capacity was found in a group of seven conifers and 16 angiosperm species, including two vessel‐less taxa, from similar rainforest communities in New Caledonia and Tasmania (Australia). Stem hydraulic supply was expressed as the hydraulic conductivity of branches in terms of leaf area supplied (K_L) and leaf photosynthetic capacity was measured as the mean quantum yield of PSII (Ø_PSII) in leaves exposed to full sun, as determined by chlorophyll fluorescence analysis. A single, highly significant linear regression (r² = 0·74) described the relationship between hydraulic conductivity and quantum yield in all species. This suggests that the maximum photosynthetic rate of leaves is constrained by their vascular supply. In both rainforest locations, the K_L of conifer wood overlapped broadly with that of associated vessel‐bearing and vessel‐less angiosperms indicating a degree of hydraulic convergence in these forests.     

The ability of abaxial and adaxial epidermis of sun and shade leaves to attenuate UV-A and UV-B radiation in relation to the UV absorbing capacity of the whole leaf methanolic extracts

The UV‐absorbing capacity (measured as A₃₁₀ cm^?2 and A₃₆₅ cm^?2 or A_UVR cm^?2) of the shade leaves of four representative evergreen sclerophylls of the Mediterranean region (Quercus coccifera, Q. ilex, Arbutus andrachne and A. unedo) was considerably lower than the corresponding one of sun leaves of the same species. However, fibre optic microprobe measurements showed that adaxial as well as abaxial epidermis of shade leaves of all examined plants, except abaxial epidermis of A. andrachne, were almost as effective as the corresponding ones of the sun leaves in screening out most of the incident UV‐B radiation. There is probably a threshold, under which the concentration of the UV‐B absorbing compounds in the protective tissues is not furthermore reduced, in spite of the low levels of the stress factor (UV‐B radiation) in the environment. On the other hand, the ability of both abaxial and adaxial epidermis to attenuate UV‐A radiation, except of adaxial leaf epidermis of Quercus species, depended on the UV absorbing capacity of the whole‐leaf extracts, with different correlation patterns between the two Quercus species and the two Arbutus species. This could be explained by the fact that shade leaves showed not only quantitative, but also qualitative differences (higher A₃₁₀/A₃₆₅ ratio) in the absorbance of their methanolic extracts compared to these of sun leaves. The results of the present study showed that we should not always correlate the depth of penetration of UV radiation into sun and shade leaves according to the corresponding UV absorbing capacity of the whole leaf methanolic extracts, without taking into account all the anatomical, developmental and biochemical (such as different composition and distribution of the UV‐absorbing compounds among the different protective tissues) peculiarities of the leaves of each species.     

Interaction of ozone pollution and light effects on photosynthesis in a forest canopy experiment

Ozone pollution may reduce net carbon gain in forests, yet data from mature trees are rare and the effects of irradiance on the response of photosynthesis to ozone remain untested. We used an open-air system to expose 10 branches within the upper canopy of an 18-m-tall stand of sugar maple (Acer saccharum Marsh.) to twice-ambient concentrations of ozone (95nmol mol^?1, 0900 to 1700, 1 h mean) relative to 10 paired, untreated controls (45nmol mol^?1) over 3 months. The branch pairs were selected along a gradient from relatively high irradiance (PPFD 14.5 mol m^?2 d^?1) to deep shade (0.7mol m^?2 d^?1). Ozone reduced light-saturated rates of net photosynthesis (A_sat) and increased dark respiration by as much as 56 and 40%, respectively. Compared to sun leaves, shade leaves exhibited greater proportional reductions in A_sat and had lower chlorophyll concentrations, quantum efficiencies, and leaf absorptances when treated with ozone relative to controls. With increasing ozone dose over time, A_sat became uncoupled from stomatal conductance as ratios of internal to external concentrations of carbon dioxide increased, reducing water-use efficiency. Ozone reduced net photosynthesis and impaired stomatal function, with these effects depending on the irradiance environment of the canopy leaves. Increased ozone sensitivity of shade leaves compared to sun leaves has consequences for net carbon gain in canopies.     

Acclimation to humidity modifies the link between leaf size and the density of veins and stomata

The coordination of veins and stomata during leaf acclimation to sun and shade can be facilitated by differential epidermal cell expansion so large leaves with low vein and stomatal densities grow in shade, effectively balancing liquid‐ and vapour‐phase conductances. As the difference in vapour pressure between leaf and atmosphere (VPD) determines transpiration at any given stomatal density, we predict that plants grown under high VPD will modify the balance between veins and stomata to accommodate greater maximum transpiration. Thus, we examined the developmental responses of these traits to contrasting VPD in a woody angiosperm (Toona ciliata M. Roem.) and tested whether the relationship between them was altered. High VPD leaves were one‐third the size of low VPD leaves with only marginally greater vein and stomatal density. Transpirational homeostasis was thus maintained by reducing stomatal conductance. VPD acclimation changed leaf size by modifying cell number. Hence, plasticity in vein and stomatal density appears to be generated by plasticity in cell size rather than cell number. Thus, VPD affects cell number and leaf size without changing the relationship between liquid‐ and vapour‐phase conductances. This results in inefficient acclimation to VPD as stomata remain partially closed under high VPD.