The functional ecology of shoot architecture in sun and shade plants of Heteromeles arbutifolia M. Roem., a Californian chaparral shrub

The functional roles of the contrasting morphologies of sun and shade shoots of the evergreen shrub Heteromeles arbutifolia were investigated in chaparral and understory habitats by applying a three-dimensional plant architecture simulation model, YPLANT. The simulations were shown to accurately predict the measured frequency distribution of photosynthetic photon flux density (PFD) on both the leaves and a horizontal surface in the open, and gave reasonably good agreement for the more complex light environment in the shade. The sun shoot architecture was orthotropic and characterized by steeply inclined (mean = 71^o) leaves in a spiral phyllotaxy with short internodes. This architecture resulted in relatively low light absorption efficiencies (E _A) for both diffuse and direct PFD, especially during the summer when solar elevation angles were high. Shade shoots were more plagiotropic with longer internodes and a pseudo-distichous phyllotaxis caused by bending of the petioles that positioned the leaves in a nearly horizontal plane (mean = 5^o). This shade-shoot architecture resulted in higher E _A values for both direct and diffuse PFD as compared to those of the sun shoots. Differences in E _A between sun and shade shoots and between summer and winter were related to differences in projection efficiencies as determined by leaf and solar angles, and by differences in self shading resulting from leaf overlap. The leaves exhibited photosynthetic acclimation to the sun and the shade, with the sun leaves having higher photosynthetic capacities per unit area, higher leaf mass per unit area and lower respiration rates per unit area than shade leaves. Despite having 7 times greater available PFD, sun shoots absorbed only 3 times more and had daily carbon gains only double of those of shade shoots. Simulations showed that sun and shade plants performed similarly in the open light environment, but that shade shoots substantially outperformed sun shoots in the shade light environment. The shoot architecture observed in sun plants appears to achieve an efficient compromise between maximizing carbon gain while minimizing the time that the leaf surfaces are exposed to PFDs in excess of those required for light saturation of photosynthesis and therefore potentially photoinhibitory. Received: 8 June 1997 / Accepted: 2 November 1997     

Scaling sun and shade photosynthetic acclimation of Alocasia macrorrhiza to whole-plant performance – II. Simulation of carbon balance and growth at different photon flux densities

A whole-plant carbon balance model incorporating a light acclimation response was developed for Alocasia macrorrhiza based on empirical data and the current understanding of light acclimation in this species. The model was used to predict the relative growth rate (RGR) for plants that acclimated to photon flux density (PFD) by changing their leaf type, and for plants that produced only sun or shade leaves regardless of PFD. The predicted RGR was substantially higher for plants with shade leaves than for those with sun leaves at low PFD. However, the predicted RGR was not higher, and in fact was slightly lower, for plants with sun leaves than for those with shade leaves at high PFD. The decreased leaf area ratios (LARs) of the plants with sun leaves counteracted their higher photosynthetic capacities per unit leaf area (A_max). The model was manipulated by changing parameters to examine the sensitivity of RGR to variation in single factors. Overall, RGR was most sensitive to LAR and showed relatively little sensitivity to variation in A_max or maintenance respiration. Similarly, RGR was relatively insensitive to increases in leaf life-span beyond those observed. Respiration affected RGR only at low PFD, whereas A_max was moderately important only at high PFD.     

Photosynthetic responses to light in seedlings of selected Amazonian and Australian rainforest tree species

Summary Seedlings of the Caesalpinoids Hymenaea courbaril, H. parvifolia and Copaifera venezuelana, emergent trees of Amazonian rainforest canopies, and of the Araucarian conifers Agathis microstachya and A. robusta, important elements in tropical Australian rainforests, were grown at 6% (shade) and 100% full sunlight (sun) in glasshouses. All species produced more leaves in full sunlight than in shade and leaves of sun plants contained more nitrogen and less chlorophyll per unit leaf area, and had a higher specific leaf weight than leaves of shade plants. The photosynthetic response curves as a function of photon flux density for leaves of shade-grown seedlings showed lower compensation points, higher quantum yields and lower respiration rates per unit leaf area than those of sun-grown seedlings. However, except for A. robusta, photosynthetic acclimation between sun and shade was not observed; the light saturated rates of assimilation were not significantly different. Intercellular CO₂ partial pressure was similar in leaves of sun and shade-grown plants, and assimilation was limited more by intrinsic mesophyll factors than by stomata. Comparison of assimilation as a function of intercellular CO₂ partial pressure in sun- and shade-grown Agathis spp. showed a higher initial slope in leaves of sun plants, which was correlated with higher leaf nitrogen content. Assimilation was reduced at high transpiration rates and substantial photoinhibition was observed when seedlings were transferred from shade to sun. However, after transfer, newly formed leaves in A. robusta showed the same light responses as leaves of sun-grown seedlings. These observations on the limited potential for acclimation to high light in leaves of seedlings of rainforest trees are discussed in relation to regeneration following formation of gaps in the canopy.     

Leaf morphology of a facultative rheophyte,Farfugium japonicum var.luchuense (Compositae)

The gross morphology and anatomy of leaves in a facultative rheophyte,Farfugium japonicum var.luchuense, were examined in order to clarify how the two characters are correlated with one another and how the facultative rheophyte differs from obligate rheophytes in leaf morphology and anatomy. Most rheophytes of the variety have narrowly cuneate leaf base, while most inland plants usually have truncate to cordate one, although the habitat-morphology correlation is not so clear. The leaf shape or the divergence angle of leaf base is strongly correlated with the number of primary veins and intervein-distance dependent on the number of cells intervening between veins. This makes a marked contrast to many reported cases of obligate rheophytes in which the leaf shape is strongly correlated with cell size. There is a rough tendency that narrower leaves of rheophytes have a thicker cuticular layer. However, the cell size and the volume (area) of intercellular space differ only slightly with the leaf shape.     

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.     

Photosynthetically versatile thin shade leaves: A paradox of irradiance-response curves

Thick sun leaves have a larger construction cost per unit leaf area than thin shade leaves. To re-evaluate the adaptive roles of sun and shade leaves, we compared the photosynthetic benefits relative to the construction cost of the leaves. We drew photosynthetically active radiation (PAR)-response curves using the leaf-mass-based photosynthetic rate to reflect the cost. The dark respiration rates of the sun and shade leaves of mulberry (Morus bombycis Koidzumi) seedlings did not differ significantly. At irradiances below 250 μmol m⁻² s⁻¹, the shade leaves tended to have a significantly larger net photosynthetic rate (P _N) than the sun leaves. At irradiances above 250 μmol m⁻² s⁻¹, the P _N did not differ significantly. The curves indicate that plants with thin shade leaves have a larger daily CO₂ assimilation rate per construction cost than those with thick sun leaves, even in an open habitat. These results are consistently explained by a simple model of PAR extinction in a leaf. We must target factors other than the effective assimilation when we consider the adaptive roles of thick sun leaves.     

Leaf anatomy and light acclimation in woody seedlings after gap formation in a cool-temperate deciduous forest

The photosynthetic light acclimation of fully expanded leaves of tree seedlings in response to gap formation was studied with respect to anatomical and photosynthetic characteristics in a natural cool-temperate deciduous forest. Eight woody species of different functional groups were used; two species each from mid-successional canopy species (Kalopanax pictus and Magnolia obovata), from late-successional canopy species (Quercus crispula and Acer mono), from sub-canopy species (Acer japonicum and Fraxinus lanuginosa) and from vine species (Schizophragma hydrangeoides and Hydrangea petiolaris). The light-saturated rate of photosynthesis (P _max) increased significantly after gap formation in six species other than vine species. Shade leaves of K. pictus, M. obovata and Q. crispula had vacant spaces along cell walls in mesophyll cells, where chloroplasts were absent. The vacant space was filled after the gap formation by increased chloroplast volume, which in turn increased P _max. In two Acer species, an increase in the area of mesophyll cells facing the intercellular space enabled the leaves to increase P _max after maturation. The two vine species did not significantly change their anatomical traits. Although the response and the mechanism of acclimation to light improvement varied from species to species, the increase in the area of chloroplast surface facing the intercellular space per unit leaf area accounted for most of the increase in P _max, demonstrating the importance of leaf anatomy in increasing P _max.     

Determination of mesophyll diffusion resistance in Chamaerion angustifolium by the method of three-dimensional reconstruction of the leaf cell packing

A detailed quantitative analysis of the three-dimensional organization of the mesophyll was performed, and mesophyll diffusion resistance to CO₂ in the leaves of Chamaerion angustifolium formed under different irradiance was calculated using an original method of stereometric cellular packing. For each type of leaves (sun and shade), we determined structural components of gas exchange: the volume of mesophyll per unit leaf area (V _mes), the volume of the intercellular space in the mesophyll (V _is), the area of the total mesophyll surface (S), the area of the free mesophyll surface facing the intercellular spaces (S _mes), and the ratios of the total and the free mesophyll surfaces to its volume (S/V and S _mes/V). As compared with sun leaves, in the shade leaves of Ch. angustifolium, S and V _mes decreased twofold, tissue density was reduced twofold, and the share of the intercellular space in the mesophyll rose from 49 to 72%. In shade, the diffusion resistance of the mesophyll increased by 1.8 times because of changes in the leaf structure. At the same time, the ratio S _mes/V was found to increase by 1.4 times, which facilitated the diffusion of CO₂. In the shade leaves of Ch. angustifolium, the diffusion resistance of the intercellular air spaces was reduced twofold as a result of an increase in their share in the leaf mesophyll and simplification of their geometry. Thus, the method of three-dimensional reconstruction of sun and shade leaves of Ch. angustifolium showed a comprehensive rearrangement of the mesophyll spatial organization in shade and revealed the structural mechanisms of changes in the resistance to CO₂ diffusion within the leaf.     

Some ecophysiological features in sun and shade leaves of tall beech trees

Some ecophysiological features in sun and shade leaves of tall European beech trees (Fagus sylvatica L.) growing in a natural forest stand were investigated. Quantitative leaf characteristics were followed in the field and under controlled conditions. In the sun leaves significantly higher rates of photosynthesis, photorespiration and dark respiration, and also photosynthetic CO₂ fixation capacity, photosynthetic productivity, and saturating, adaptation and compensating irradiances were found. Specific leaf mass, mean leaf area, stomata density and size as well as the chlorophyll content per unit dry mass were also significantly different in both types of the leaves. Higher photosynthetic efficiency in the shade leaves allows them a better utilization of the lower irradiance for carbon dioxide uptake. The importance of these findings for annual carbon gain of the shade tolerant European beech species is also discussed.     

Costs and benefits of photosynthetic light acclimation by tree seedlings in response to gap formation

Some shade leaves increase their photosynthetic capacity (P _max) when exposed to a higher irradiance. The increase in P _max is associated with an increase in chloroplast size or number. To accommodate those chloroplasts, plants need to make thick leaves in advance. We studied the cost and benefit of photosynthetic acclimation in mature leaves of a tree species, Kalopanax pictus Nakai, in a cool-temperate deciduous forest. Costs were evaluated as the additional investment in biomass required to make thick leaves, while the benefit was evaluated as an increase in photosynthetic carbon gain. We created gaps by felling canopy trees and examined the photosynthetic responses of mature leaves of the understorey seedlings. In the shade, leaves of K. pictus had vacant spaces that were not filled by chloroplasts in the mesophyll cells facing the intercellular space. When those leaves were exposed to higher irradiance after gap formation, the area of the mesophyll surface covered by chloroplasts increased by 17% and P _max by 27%. This increase in P _max led to an 11% increase in daily carbon gain, which was greater than the amount of biomass additionally invested to construct thicker leaves. We conclude that the capacity of a plant to acclimate to light (photosynthetic acclimation) would contribute to rapid growth in response to gap formation.     

Leaf expansion and carbon assimilation in cotton leaves grown at two photosynthetic photon flux densities

Increasing photosynthetic photon flux density (PPFD) received during development from 5.5 to 31.2 mol m^-2 d^-1 resulted in greater leaf and mesophyll cell surface areas in cotton (Gossypium hirsutum L.). The relationships between the amounts of these surface areas and potential CO₂ assimilation by these leaves were evaluated. Leaf area (epidermal surface area of one side of a leaf), mesophyll cell surface area, and net rate of CO₂ uptake (P_n) were measured from the time leaves first unfolded until P., was substantially reduced. At the higher PPFD, leaf and mesophyll surface areas increased more rapidly during expansion, and P_n per unit leaf area was greater than at the lower PPFD. Although leaves at the higher PPFD reached the maximum P., per unit mesophyll cell surface area 4 to 5 days earlier than leaves at the lower PPFD, the maxima for these P., were similar. Leaves grown at the higher PPFD had the potential to assimilate 2.2, 3.5, or 5.8 times the amount of CO₂ as leaves from the lower PPFD when P., was expressed per unit mesophyll surface, per unit leaf surface, or per whole leaf, respectively. Greater and earlier development of both P., and mesophyll cell surface area at higher PPFD apparently had a compounding effect on the potential for carbon assimilation by a leaf.     

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).     

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.     

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.     

The induction of sun and shade leaves of the European beech (Fagus sylvatica L.): anatomical studies

Summary Primordia from buds of sun and shade twigs of European beech (Fagus sylvatica L.) were collected six times a year for anatomical investigations. Differentiation into sun-leaf and shade-leaf primordia was first observed in early August. Sun-leaf primordia had five, and shade-leaf primordia four layers of mesophyll meristem cells. With potted graft unions of beeches possible structural changes of leaf primordia were investigated. Trees adapted to shade develop sun-leaf primordia when put into full daylight, provided the transfer happened before July. Trees adapted to full daylight developed leaf primordia which remained structurally sun-leaf primordia when the plant was kept under shade conditions. Shadeleaf branches of young beech trees cut in February in order to expose the shade buds to full daylight developed either shade leaves or intermediate shade/sun leaves. These experiments show that the subtending leaf may provide the developing axillary bud with photoassimilates, but its character, whether sun or shade leaf, has no influence on the character of the developing leaf primordia.     

The 'hydrology' of leaves: co-ordination of structure and function in temperate woody species

The hydraulic conductance of the leaf lamina (K_lamina) substantially constrains whole‐plant water transport, but little is known of its association with leaf structure and function. K_lamina was measured for sun and shade leaves of six woody temperate species growing in moist soil, and tested for correlation with the prevailing leaf irradiance, and with 22 other leaf traits. K_lamina varied from 7.40 × 10^?5 kg m^?2 s^?1 MPa^?1 for Acer saccharum shade leaves to 2.89 × 10^?4 kg m^?2 s^?1 MPa^?1 for Vitis labrusca sun leaves. Tree sun leaves had 15–67% higher K_lamina than shade leaves. K_lamina was co‐ordinated with traits associated with high water flux, including leaf irradiance, petiole hydraulic conductance, guard cell length, and stomatal pore area per lamina area. K_lamina was also co‐ordinated with lamina thickness, water storage capacitance, 1/mesophyll water transfer resistance, and, in five of the six species, with lamina perimeter/area. However, for the six species, K_lamina was independent of inter‐related leaf traits including leaf dry mass per area, density, modulus of elasticity, osmotic potential, and cuticular conductance. K_lamina was thus co‐ordinated with structural and functional traits relating to liquid‐phase water transport and to maximum rates of gas exchange, but independent of other traits relating to drought tolerance and to aspects of carbon economy.     

Photosynthetic activity,chloroplast ultrastructure,and leaf characteristics of high-light and low-light plants and of sun and shade leaves

The photosynthetic CO₂-fixation rates, chlorophyll content, chloroplast ultrastructure and other leaf characteristics (e.g. variable fluorescence, stomata density, soluble carbohydrate content) were studied in a comparative way in sun and shade leaves of beech (Fagus sylvatica) and in high-light and low-light seedlings.

1. Sun leaves of the beech possess a smaller leaf area, higher dry weight, lower water content, higher stomata density, higher chlorophyll a/b ratios and are thicker than the shade leaves. Sun leaves on the average contain more chlorophyll in a leaf area unit; the shade leaf exhibits more chlorophyll on a dry weight basis. Sun leaves show higher rates for dark respiration and a higher light saturation of photosynthetic CO₂-fixation. Above 2000 lux they are more efficient in photosynthetic quantum conversion than the shade leaves.
2. The development of HL-radish plants proceeds much faster than that of LL-plants. The cotyledons of HL-plants show a higher dry weight, lower water content, a higher ratio of chlorophyll a/b and a higher gross photosynthesis rate than the cotyledons of the LL-plants, which possess a higher chlorophyll content per dry weight basis. The large area of the HL-cotyledon on the one hand, as well as the higher stomata density and the higher respiration rate in the LL-cotyledon on the other hand, are not in agreement with the characteristics of sun and shade leaves respectively.
3. The development, growth and wilting of wheat leaves and the appearance of the following leaves (leaf succession) is much faster at high quanta fluence rates than in weak light. The chlorophyll content is higher in the HL-leaf per unit leaf area and in the LL-leaf per g dry weight. There are no differences in the stomata density and leaf area between the HL- and LL-leaf. There are fewer differences between HL- and LL-leaves than in beech or radish leaves.
4. The chloroplast ultrastructure of shade-type chloroplasts (shade leaves, LL-leaves) is not only characterized by a much higher number of thylakoids per granum and a higher stacking degree of thylakoids, but also by broader grana than in sun-type chloroplasts (sun leaves, HL-leaves). The chloroplasts of sun leaves and of HL-leaves exhibit large starch grains.
5. Shade leaves and LL-leaves exhibit a higher maximum chlorophyll fluorescence and it takes more time for the fluorescence to decline to the steady state than in sun and HL-leaves. The variable fluorescence VF (ratio of fluorescence decrease to steady state fluorescence) is always higher in the sun and HL-leaf of the same physiological stage (maximum chlorophyll content of the leaf) than in the shade and LL-leaf. The fluorescence emission spectra of sun and HL-leaves show a higher proportion of chlorophyli fluorescence in the second emission maximum F₂ than shade and LL-leaves.
6. The level of soluble carbohydrates (reducing sugars) is significantly higher in sun and HL-leaves than in shade and LL-leaves and even reflects changes in the amounts of the daily incident light.
7. Some but not all characteristics of mature sun and shade leaves are found in HL- and LL-leaves of seedlings. Leaf thickness, dry weight, chlorophyll content, soluble carbohydrate level, photosynthetic CO₂-fixation, height and width of grana stacks and starch content, are good parameters to describe the differences between LL- and HL-leaves; with some reservations concerning age and physiological stage of leaf, a/b ratios, chlorophyll content per leaf area unit and the variable fluorescence are also suitable.

     

No photosynthetic down-regulation in sweetgum trees (Liquidambar styraciflua L.) after three years of CO2 enrichment at the Duke Forest FACE experiment

Photosynthetic capacity and leaf properties of sun and shade leaves of overstorey sweetgum trees (Liquidambar styraciflua L.) were compared over the first 3 years of growth in ambient or ambient + 200 μL L^?1 CO₂ at the Duke Forest Free Air CO₂ Enrichment (FACE) experiment. We were interested in whether photosynthetic down‐regulation to CO₂ occurred in sweetgum trees growing in a forest ecosystem, whether shade leaves down‐regulated to a greater extent than sun leaves, and if there was a seasonal component to photosynthetic down‐regulation. During June and September of each year, we measured net photosynthesis (A) versus the calculated intercellular CO₂ concentration (C_i) in situ and analysed these response curves using a biochemical model that described the limitations imposed by the amount and activity of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Vc_max) and by the rate of ribulose‐1,5‐bisphosphate (RuBP) regeneration mediated by electron transport (J_max). There was no evidence of photosynthetic down‐regulation to CO₂ in either sun or shade leaves of sweetgum trees over the 3 years of measurements. Elevated CO₂ did not significantly affect Vc_max or J_max. The ratio of Vc_max to J_max was relatively constant, averaging 2·12, and was not affected by CO₂ treatment, position in the canopy, or measurement period. Furthermore, CO₂ enrichment did not affect leaf nitrogen per unit leaf area (N_a), chlorophyll or total non‐structural carbohydrates of sun or shade leaves. We did, however, find a strong relationship between N_a and the modelled components of photosynthetic capacity, Vc_max and J_max. Our data over the first 3 years of this experiment corroborate observations that trees rooted in the ground may not exhibit symptoms of photosynthetic down‐regulation as quickly as tree seedlings growing in pots. There was a strong sustained enhancement of photosynthesis by CO₂ enrichment whereby light‐saturated net photosynthesis of sun leaves was stimulated by 63% and light‐saturated net photosynthesis of shade leaves was stimulated by 48% when averaged over the 3 years. This study suggests that this CO₂ enhancement of photosynthesis will be sustained in the Duke Forest FACE experiment as long as soil N availability keeps pace with photosynthetic and growth processes.     

Does the photosynthetic light‐acclimation need change in leaf anatomy?

There is a strong correlation between leaf thickness and the light‐saturated rate of photosynthesis per unit leaf area (P_max). However, when leaves are exposed to higher light intensities after maturation, P_max often increases without increasing leaf thickness. To elucidate the mechanism with which mature leaves increase P_max, the change in anatomical and physiological characteristics of mature leaves of Chenopodium album, which was transferred from low to high light condition, were examined. When compared with leaves subjected to low light continuously (LL leaves), the leaves transferred from low to high light (LH leaves) significantly increased P_max. The transfer also increased the area of chloroplasts facing the intercellular space (S_c) and maintained a strong correlation between P_max and S_c. The mesophyll cells of LL leaves had open spaces along cell walls where chloroplasts were absent, which enabled the leaves to increase P_max when they were exposed to high light (LH). However, the LH leaves were not thick enough to allow further increase in P_max to the level in HH leaves. Thus leaf thickness determines an upper limit of P_max of leaves subjected to a change from low to high light conditions. Shade leaves would only increase P_max when they have open space to accommodate chloroplasts which elongate after light conditions improve.     

The effects of light acclimation during and after foliage expansion on photosynthesis ofAbies amabilis foliage within the canopy

Variation in the photosynthetic function ofAbies amabilis foliage within a canopy was examined and related to three different processes that affect foliage function: foliage aging, sun-shade acclimation that occurred while foliage was expanding, and reacclimation after expansion was complete. Foliage produced in the sun had higher photosynthesis at light saturation (A _max, mol·m^-2·s^-1), dark respiration (mol·m^-2·s^-1), nitrogen content (g·m^-2), chlorophyll content (g·m^-2), and chlorophylla:b ratio, and a lower chlorophyll to nitrogen ratio (chl:N), than foliage produced in the shade. As sun foliage becomes shaded, it becomes physiologically similar to shade foliage, even though it still retains a sun morphology. Shaded sun foliage exhibited lowerA _max, dark respiration, nitrogen content, and chlorophylla:b ratio, and a higher chl:N ratio than sun foliage of the same age remaining in the open. However, shaded sun foliage had a higher chlorophyll content than sun foliage remaining in the open, even though true shade foliage had a lower chlorophyll content than sun foliage. This anomaly arises because as sun foliage becomes shaded, it retains a higher nitrogen content than shade foliage in a similar light environment, but the two forms have similar chl:N ratios. Within the canopy, most physiological indicators were more strongly correlated with the current light environment than with foliage age or leaf thickness, with the exception of chlorophyll content.A _max decreased significantly with both decreasing current light environment of the foliage and increasing foliage age. The same trend with current light and age was found for the chlorophylla:b ratio. Foliage nitrogen content also decreased with a decrease in current light environment, but no distinct pattern was found with foliage age. Leaf thickness was also important for predicting leaf nitrogen content: thicker leaves had more nitrogen than thinner leaves regardless of light environment or age. The chl:N ratio had a strong negative correlation with the current light environment, and, as with nitrogen content, no distinct pattern was found with foliage age. Chlorophyll content of the foliage was not well correlated with any of the three predictor variables: current light environment, foliage age or leaf thickness. On the other hand, chlorophyll content was positively correlated with the amount of nitrogen in a leaf, and once nitrogen was considered, the current light environment was also highly significant in explaining the variation in chlorophyll content. It has been suggested that the redistribution of nitrogen both within and between leaves is a mechanism for photosynthetic acclimation to the current light environment. Within theseA. amabilis canopies, both leaf nitrogen and the chl:N ratio were strongly correlated with the current light environment, but only weakly with leaf age, supporting the idea that changing light is the driving force for the redistribution of nitrogen both within and between leaves. Thus, our results support previous theories on nitrogen distribution and partitioning. However,A _max was significantly affected by both foliage age and the current light environment, indicating that changes in light alone are not enough to explain changes inA _max with time.