REACTION NORMS OF MORPHOLOGICAL AND LIFE-HISTORY TRAITS TO LIGHT AVAILABILITY IN IMPATIENS CAPENSIS

For plants, light availability is an important environmental factor that varies both within and between populations. Although the existence of sun and shade “ecotypes” is controversial, it is often assumed that trade-offs may exist between performance in sun and in shade. This study therefore investigated variation in reaction norms to light availability within and between two neighboring natural populations of the annual Impatiens capensis, one in full sun and the other in a forest understory. Seedlings were collected randomly from both populations and grown to maturity in a greenhouse under two light conditions: full light and 18% of full light. Selfed full-sib seed families were collected from plants from both populations grown in both parental light environments. To characterize family reaction norms, seedlings from each family were divided into the same two light treatments and individuals were scored for a variety of morphological and life-history traits. The maternal light environment had little impact on progeny reaction norms. However, the two study populations differed both qualitatively and quantitatively in plastic response to light availability (indicated by significant population x environment interactions in mixed-model ANCOVA). Much of this difference was attributable to population differences in light sensitivity of axillary meristem allocation patterns, which produced concurrent differences in reaction norms for a suite of developmentally linked traits. Within each population, different sets of traits displayed significant variation in plasticity (indicated by significant family x environment interactions). Thus, the genetic potential for evolutionary response to selection in heterogeneous light environments may differ dramatically between neighboring plant populations. Between-environment genetic correlations were largely positive in the woods population and positive or nonsignificant in the sun population; there was no evidence for performance trade-offs across environments or sun or shade “specialist” genotypes within either population. There was little evidence that population differences represented adaptive differentiation for sun or shade; rather, the results suggested the hypothesis of differential selection on patterns of meristem allocation caused by population differences in timing of mortality and intensity of competition.     

Effect of local irradiance on CO(2) transfer conductance of mesophyll in walnut

The acclimation responses of walnut leaf photosynthesis to the irradiance microclimate were investigated by characterizing the photosynthetic properties of the leaves sampled on young trees (Juglans nigraxregia) grown in simulated sun and shade environments, and within a mature walnut tree crown (Juglans regia) in the field. In the young trees, the CO(2) compensation point in the absence of mitochondrial respiration (Gamma*), which probes the CO(2) versus O(2) specificity of Rubisco, was not significantly different in sun and shade leaves. The maximal net assimilation rates and stomatal and mesophyll conductances to CO(2) transfer were markedly lower in shade than in sun leaves. Dark respiration rates were also lower in shade leaves. However, the percentage inhibition of respiration by light during photosynthesis was similar in both sun and shade leaves. The extent of the changes in photosynthetic capacity and mesophyll conductance between sun and shade leaves under simulated conditions was similar to that observed between sun and shade leaves collected within the mature tree crown. Moreover, mesophyll conductance was strongly correlated with maximal net assimilation and the relationships were not significantly different between the two experiments, despite marked differences in leaf anatomy. These results suggest that photosynthetic capacity is a valuable parameter for modelling within-canopies variations of mesophyll conductance due to leaf acclimation to light.     

Photosynthetic Adaptation to Light Intensity in Plants Native to Shaded and Exposed Habitats

Photosynthetic adaptation to light intensity has been studied in clones of populations from shaded and exposed habitats of Rumex acetosa and Geum rivale. Clones of the shade species Lamium galeobdolon and the sun species Plantago lanceolata were also included for comparison. The plants were grown under controlled conditions at a high and a low light intensity. The capacity of photosynthetic carbon dioxide uptake at low as well as at saturating light intensities was determined on single attached leaves. As was previously demonstrated in Solidago virgaurea, clones of populations native to shaded and to exposed environments show differences in the photosynthetic response to light intensity during growth. The data provide evidence that populations of the same species native to habitats with contrasting light intensities differ in their photosynthetic properties in an adaptive manner Ln a similar mode as sun and shade species.     

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.     

Photosynthetic acclimation of the liana Stigmaphyllon lindenianum to light changes in a tropical dry forest canopy

Tropical plant canopies show abrupt changes in light conditions across small differences in spatial and temporal scales. Given the canopy light heterogeneity, plants in this stratum should express a high degree of plasticity, both in space (allocation to plant modules as a function of opportunity for resource access) and time (photosynthetic adjustment to temporal changes in the local environment). Using a construction crane for canopy access, we studied light acclimation of the liana Stigmaphyllon lindenianum to sun and shade environments in a tropical dry forest in Panama during the wet season. Measured branches were randomly distributed in one of four light sequences: high- to low-light branches started the experiment under sun and were transferred to shade during the second part of the experiment; low- to high-light branches (LH) were exposed to the opposite sequence of light treatments; and high-light and low-light controls , which were exposed only to sun and shade environments, respectively, throughout the experiment. Shade branches were set inside enclosures wrapped in 63% greenhouse shade cloth. After 2 months, we transferred experimental branches to opposite light conditions by relocating the enclosures. Leaf mortality was considerably higher under shade, both before and after the transfer. LH branches reversed the pattern of mortality by increasing new leaf production after the transfer. Rates of photosynthesis at light saturation, light compensation points, and dark respiration rates of transferred branches matched those of controls for the new light treatment, indicating rapid photochemical acclimation. The post-expansion acclimation of sun and shade foliage occurred with little modification of leaf structure. High photosynthetic plasticity was reflected in an almost immediate ability to respond to significant changes in light. This response did not depend on the initial light environment, but was determined by exposure to new light conditions. Stigmaphyllon responded rapidly to light changes through the functional adjustment of already expanded foliage and an increase in leaf production in places with high opportunity for carbon gain. Received: 24 April 1998 / Accepted: 11 May 1999     

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.     

Quantum Requirements of Photosynthetic Electron Transport In Sitka Spruce from Different Light Environments

Quantum requirements of photosynthetic electron transport have been measured in shoots of Picea sitchensis (Bong.) Carr. (Sitka spruce) from different levels in a forest canopy and in shoots from plants grown in contrasting light environments in controlled environment chambers. Neutral density filters were used to obtain very low photon flux densities. The light absorbed by the chloroplast suspensions was calculated from measurements of the transmittance of the suspensions. The shoots from the top of the forest canopy (“sun” shoots) had lower quantum requirements for photosystems I and II than the shoots from the bottom of the forest canopy (“shade” shoots). High light grown plants and “sun” shoots had higher rates of electron transport at light saturation than low light grown plants and “shade” shoots. Thus a higher potential for electron transport was found to exist in “sun” shoots than in “shade” shoots at both high and low photon flux densities.     

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     

Carbon uptake and respiration in above-ground parts of a Larix decidua × leptolepis tree

Summary Shade needles of hybrid larch (Larix decidua × leptolepis) had the same rates of photosynthesis as sun needles per dry weight and nitrogen, and a similar leaf conductance under conditions of light saturation at ambient CO₂ (A_max). However, on an area basis, A_max and specific leaf weight were lower in shade than in sun needles. Stomata of sun needles limited CO₂ uptake at light saturation by about 20%, but under natural conditions of light in the shade crown, shade needles operated in a range of saturating internal CO₂ without stomatal limitation of CO₂ uptake. In both needle types, stomata responded similarly to changes in light, but shade needles were more sensitive to changes in vapor pressure deficit than sun needles. Despite a high photosynthetic capacity, the ambient light conditions reduced the mean daily (in summer) and annual carbon gain of shade needles to less than 50% of that in sun needles. In sun needles, the transpiration per carbon gain was about 220 mol mol^–1 on an annual basis. The carbon budget of branches was determined from the photosynthetic rate, the needle biomass and respiration, the latter of which was (per growth and on a carbon basis) 1.6 mol mol^–1 year^–1 in branch and stem wood. In shade branches carbon gains exceeded carbon costs (growth + respiration) by only a factor of 1.6 compared with 3.5 in sun branches. The carbon balance of sun branches was 5 times higher per needle biomass of a branch or 9 times higher on a branch length basis than shade branches. The shade foliage (including the shaded near-stem sun foliage) only contributed approximately 23% to the total annual carbon gain of the tree.     

Somatic and genetic factors in sun and shade population differentiation in Plantago lanceolata and Anthoxanthum odoratum

Differences between individuals collected from sun and shade populations could result from either somatic or genetic differences, particularly in populations of perennial plants. Our objective in this study was to separate somatic from genetic differences. We collected Anthoxanthum odoratum and Plantago lanceolata from sun and shade populations and made measurements on both vegetatively propagated clones and seed progeny from each clone. The parental populations differed in a wide range of physiological and morphological traits. However, only photosynthetic capacity was significantly different between both the original sun and shade populations and their seed progeny. In both species, plants from the sun population had higher photosynthetic capacities than those from the shade population when grown in a common environment. This demonstrates that there was genetic differentiation between the sun and shade populations. Photosynthetic capacity of parents and offspring also differed, suggesting a somatic effect. Since many of the original clones were virus-infected, but all but one of the offspring were virus-free, this might have been a result of virus infection. However, in spite of the fact that the parents and offspring clones were propagated vegetatively so that the plants were at the same developmental stage at the time of measurement, we cannot rule out the possibility that differences in age of cell lines could also have been a factor.     

Sun-shade adaptability of the red mangrove,Rhizophora mangle (Rhizophoraceae): Changes through ontogeny at several levels of biological organization

Rhizophora mangle L., the predominant neotropical mangrove species, occupies a gradient from low intertidal swamp margins with high insolation, to shaded sites at highest high water. Across a light gradient, R. mangle shows properties of both “light-demanding” and “shade-tolerant” species, and defies designation according to existing successional paradigms for rain forest trees. The mode and magnitude of its adaptability to light also change through ontogeny as it grows into the canopy. We characterized and compared phenotypic flexibility of R. mangle seedlings, saplings, and tree modules across changing light environments, from the level of leaf anatomy and photosynthesis, through stem and whole-plant architecture. We also examined growth and mortality differences among sun and shade populations of seedlings over 3 yr. Sun and shade seedling populations diverged in terms of four of six leaf anatomy traits (relative thickness of tissue layers and stomatal density), as well as leaf size and shape, specific leaf area (SLA), leaf internode distances, disparity in blade–petiole angles, canopy spread: height ratios, standing leaf numbers, summer (July) photosynthetic light curve shapes, and growth rates. Saplings showed significant sun/shade differences in fewer characters: leaf thickness, SLA, leaf overlap, disparity in bladepetiole angles, standing leaf numbers, stem volume and branching angle (first-order branches only), and summer photosynthesis. In trees, leaf anatomy was insensitive to light environment, but leaf length, width, and SLA, disparities in bladepetiole angles, and summer maximal photosynthetic rates varied among sun and shade leaf populations. Seedling and sapling photosynthetic rates were significantly depressed in winter (December), while photosynthetic rates in tree leaves did not differ in winter and summer. Seasonal and ontogenetic changes in response to light environment are apparent at several levels of biological organization in R. mangle, within constraints of its architectural baiiplan. Such variation has implications for models of stand carbon gain, and suggest that response flexibility may change with plant age.     

Sun/shade adaptations of the photosynthetic apparatus of <Emphasis Type="Italic">Hoya carnosa</Emphasis>, an epiphytic CAM vine,in a subtropical rain forest in northeastern Taiwan

Most past work on the ecophysiology of the Crassulacean acid metabolism (CAM) plant, Hoya carnosa, in the lab and in situ in Australia indicates that this epiphytic vine is better adapted to shaded, not exposed, locations, although a recent study of this species in Taiwan presents findings that run counter to this conclusion. Thus, photosynthetic characteristics of shaded and exposed individuals of H. carnosa were compared in situ in a subtropical rain forest in northeastern Taiwan in order to determine whether this CAM epiphyte is better adapted to the shade or the sun. Although leaves of shade plants had much greater chlorophyll concentrations than did those of sun plants, chlorophyll a/b ratios did not differ between the two groups of plants. Fluorescence measurements revealed some ability of leaves to acclimate to both shade and sun, although some evidence for photoinhibition (photoprotection) was observed in more exposed plants. Despite the latter, both exposed and shaded plants exhibited CAM, measured as diel fluctuations in leaf acidity, and CAM was more consistently found in the exposed plants. Furthermore, some evidence for more CAM at higher light availabilities was found. Overall, the results of this investigation reveal that H. carnosa in this subtropical rain forest in Taiwan exhibits adaptations to both high and low light levels, which should prove adaptive for an epiphytic vine with leaves on the same individual exposed to a wide range of exposure and shade in the host tree canopy.     

The ratio of leaf to total photosynthetic area influences shade survival and plastic response to light of green-stemmed leguminous shrub seedlings

Different plant species and organs within a plant differ in their plastic response to light. These responses influence their performance and survival in relation to the light environment, which may range from full sunlight to deep shade. Plasticity, especially with regard to physiological features, is linked to a greater capacity to exploit high light and is usually low in shade-tolerant species. Among photosynthetic organs, green stems, which represent a large fraction of the total photosynthetic area of certain species, are hypothesized to be less capable of adjustment to light than leaves, because of biomechanical and hydraulic constraints. The response to light by leaves and stems of six species of leguminous, green-stemmed shrubs from dry and high-light environments was studied by growing seedlings in three light environments: deep shade, moderate shade and sun (3, 30 and 100 % of full sunlight, respectively). Survival in deep shade ranged from 2 % in Retama sphaerocarpa to 74 % in Ulex europaeus. Survival was maximal at moderate shade in all species, ranging from 80 to 98 %. The six species differed significantly in their ratio of leaf to total photosynthetic area, which influenced their light response. Survival in deep shade increased significantly with increasing ratio of leaf to total photosynthetic area, and decreased with increasing plasticity in net photosynthesis and dark respiration. Responses to light differed between stems and leaves within each species. Mean phenotypic plasticity for the variables leaf or stem specific mass, chlorophyll content, chlorophyll a/b ratio, and carotenoid to chlorophyll ratio of leaves, was inversely related to that of stems. Although mean plasticity of stems increased with the ratio of leaf to total photosynthetic area, the mean plasticity of leaves decreased. Shrubs with green stems and a low ratio of leaf to total photosynthetic area are expected to be restricted to well-lit habitats, at least during the seedling stage, owing to their inefficient light capture and the low plasticity of their stems.     

Natural selection on light response curve parameters in the herbaceous annual, <Emphasis Type="Italic">Impatiens capensis</Emphasis>

We tested for genetic variation in light response curves and their acclimation to sun versus shade in recombinant inbred lines (RILs) of the annual species Impatiens capensis derived from a cross between sun and shade populations. We exposed replicates of 49 RILs to experimentally manipulated light levels (open versus shade) in a greenhouse and measured photosynthetic light response curves, height, biomass, and reproduction. Plants were taller in the shade treatment, but we were unable to detect differences between light treatments (i.e., acclimation) in the maximal rate of photosynthesis, the light compensation point, or the quantum efficiency of photosynthesis. Genotypic selection analyses indicated that higher maximal rates of carbon assimilation and higher light compensation points (typical of sun-acclimated light curves) were favored by natural selection in both light treatments. Thus, it appears that the pattern of selection on photosynthetic parameters may not depend on light environment in this species.M. Shane Heschel and John R. Stinchcombe contributed equally to the paper.     

Differences in light usage among three fern species of genus <Emphasis Type="Italic">Blechnum</Emphasis> of contrasting ecological breadth in a forest light gradient

In Chilean evergreen temperate forest, fern species of the genus Blechnum occur in diverse microhabitats in a light gradient. We hypothesized that differences in the habitat preferences of three co-occurring Blechnum species would be associated with differences in the magnitude of responses of light capture [chlorophyll (Chl) content] and use (photosynthetic capacity and performance) to light availability. We measured the abundance, chlorophyll content, photosynthetic capacity (A), and photosynthetic performance (chlorophyll fluorescence of photosystems I and II) of juvenile individuals of each species growing in different light levels in the field. While Blechnum magellanicum covers a broad light environments range, B. mochaenum is restricted to shade, and B. penna-marina occupies full sun sites. Despite significant interspecific differences in average total chlorophyll content, this trait did not differ among species along the light gradient. There was significant interspecific variation in both the mean value and the plasticity of Chl a:Chl b ratio and A to light availability. While B. penna-marina showed a flatter reaction norm (lower response) of Chl a:Chl b ratio to light availability than its two congeners, B. mochaenum showed a lower response of A to light availability. B. penna-marina and B. magellanicum individuals from open sites had higher light saturation points of the electron transport rate (ETR) of both photosystems (ETR_LSP I and II) and photochemical quenching (qL and NA) than the shade restricted B. mochaenum. Additionally, non-photochemical quenching values for both photosystems (NPQ and ND) were higher in ferns species occurring in shaded sites. The adjustment of the photosynthetic capacity and performance to light availability appears to be an important mechanism of acclimation in these three Blechnum species that differ in their habitat preferences across a light gradient.     

Growth, biomass allocation and photosynthesis of invasive and native Hawaiian rainforest species

Growth, biomass allocation, and photosynthetic characteristics of seedlings of five invasive non-indigenous and four native species grown under different light regimes were studied to help explain the success of invasive species in Hawaiian rainforests. Plants were grown under three greenhouse light levels representative of those found in the center and edge of gaps and in the understory of Hawaiian rainforests, and under an additional treatment with unaltered shade. Relative growth rates (RGRs) of invasive species grown in sun and partial shade were significantly higher than those for native species, averaging 0.25 and 0.17 g g⁻¹ week⁻¹, respectively, while native species averaged only 0.09 and 0.06 g g⁻¹ week⁻¹, respectively. The RGR of invasive species under the shade treatment was 40% higher than that of native species. Leaf area ratios (LARs) of sun and partial-shade-grown invasive and native species were similar but the LAR of invasive species in the shade was, on average, 20% higher than that of native species. There were no differences between invasive and native species in biomass allocation to shoots and roots, or in leaf mass per area across light environments. Light-saturated photosynthetic rates (Pmax) were higher for invasive species than for native species in all light treatments. Pmax of invasive species grown in the sun treatment, for example, ranged from 5.5 to 11.9 μmol m⁻² s⁻¹ as compared with 3.0−4.5 μmol m⁻² s⁻¹ for native species grown under similar light conditions. The slope of the linear relationship between Pmax and dark respiration was steeper for invasive than for native species, indicating that invasive species assimilate more CO₂ at a lower respiratory cost than native species. These results suggest that the invasive species may have higher growth rates than the native species as a consequence of higher photosynthetic capacities under sun and partial shade, lower dark respiration under all light treatments, and higher LARs when growing under shade conditions. Overall, invasive species appear to be better suited than native species to capturing and utilizing light resources, particularly in high-light environments such as those characterized by relatively high levels of disturbance. Received: 30 December 1997 / Accepted: 1 September 1998     

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.     

Photosynthetic responses in developing and year-old Douglas-fir needles during new shoot development

Summary A transient decline in photosynthetic rate and several correlates of photosynthetic function in year-old shade needles coincided with shoot elongation in 15 fullsib 8-year-old Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] saplings. In year-old needles and current year needles collected from May to November from branchlets with a single terminal bud and at least 4 needle age classes, chlorophyll (chl) content, photosynthetic rate and non-photochemical quenching of chl fluorescence declined during the period of flushing of the new shoots and recovered as shoot elongation slowed. Developing shade needles did not achieve the same oxygen evolution rate per unit area as the year-old needles, but did develop a higher quantum yield (estimated from chl a fluorescence). In short, in shade branchlets shoot development occurred at a cost of photosynthetic function in year-old needles. In year-old sun needles collected from the upper portions of the same trees, total protein concentration increased prior to, and decreased during, flushing. The concentration of ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) rose and decreased more than chlorophyll-binding proteins. In general, protein concentration in needles reflected age class rather than sun or shade environment. A specific decline in Rubisco in year-old sun needles during the period of new shoot elongation strengthens the hypothesis that degradation of this photosynthetic protein contributes to development of the new shoot.     

Acclimation of photosynthesis to irradiance and spectral quality in British plant species: chlorophyll content, photosynthetic capacity and habitat preference

Twenty-two common British angiosperms were examined for their ability to acclimate photosynthetically to sun and shade conditions. Plants were grown under low irradiance, far-red enriched light (50 μmol m^?2 s^?1), selected to mimic as closely as possible natural canopy shade, and moderately high light of insufficient irradiance to induce photoinhibitory or photoprotective responses (300 μmol m^?2 s^?1). Light-and CO²-saturated photosynthetic rates of oxygen evolution (P_max) and chlorophyll content were measured. Large variation was found in both parameters, and two ‘strategies’ for long-term acclimation were identified: firstly a change in chlorophyll per unit leaf area which was found to correlate positively with photosynthetic capacity, and secondly changes in chlorophyll alb ratio and P_max, indicative of alterations at the chloroplast level, which were not associated with a change in chlorophyll content per unit leaf area. Combinations of these two strategies may occur, giving rise to the observed diversity in photosynthetic acclimation. The extent and nature of photosynthetic acclimation were compared with an index of shade association, calculated from the association each species has with woodland. It was found that the greatest flexibility for change at the chloroplast level was found in those species possessing an intermediate shade association, whilst acclimation in ‘sun’ species proceeded by a change in chlorophyll content; obligate shade species showed little capacity for acclimation at either the chloroplast or leaf level. A framework for explaining the variation between plant species in leaf-level photosynthetic capacity, in relation to the natural light environment, is presented. This is the first time the potential for light acclimation of photosynthesis in different plant species has been satisfactorily linked to habitat distribution.     

Contribution of intercellular reflectance to photosynthesis in shade leaves

The potential contribution of intercellular light reflectance to photosynthesis was investigated by infiltrating shade leaves with mineral oil. Infiltration of leaves of Hydrophyllum canadense and Asarum canadense with mineral oil decreased adaxial leaf reflectance but increased transmittance. As a result of the large increase in transmittance, infiltration caused a decrease in absorptance of 25% and 30% at 550 and 750 nm, respectively. Thus, intercellular reflectance increased absorptance in these species by this amount. In a comparison of sun and shade leaves of Acer saccharum and Parthenocissus quinquefolia, oil infiltration decreased absorptance more in shade than in sun leaves. This difference suggests that the higher proportion of spongy mesophyll in shade leaves may increase internal light scattering and thus absorptance. The importance of the spongy mesophyll in increasing internal reflectance was also evident in comparisons of the optics of Populus leaves and in the fluorescence yield of oil-infiltrated leaves of several sun and shade species. Oil infiltration decreased the quantum yield of fluorescence (F_o) by 39–52% for shade leaves but only 21–25% for sun leaves. We conclude that the greater proportion of spongy parenchyma in shade leaves increased intercellular light scattering and thus absorptance. Direct measurements with fibre-optic light probes of the distribution of light inside leaves of Hydrophyllum canadense confirmed that oil infiltration decreased the amount of back-scattered light and that most of the light scattering for this species occurred from the middle of the palisade layer to the middle of the spongy mesophyll. We were not, however, able to assess the potential contribution of reflectance from the internal abaxial epidermis to total internal light scattering in these experiments. Using a mathematical model to compare the response of net photosynthesis (O₂, flux) to incident irradiance for control leaves of H. canadense and theoretical leaves with no intercellular reflectance, we calculated that intercellular reflectance caused a 1.97-fold increase in photosynthesis at 20 μmol m^?2s^?1 (incident photon flux density). This enhancement of absorption and photosynthesis by inter-cellular reflectance, without additional production and maintenance of photosynthetic pigments, may maintain shade leaves above the photosynthetic light compensation point between sunflecks and maintain the light induction state during protracted periods of low diffuse light.