Contrasting Light Environments Research Articles

Phenotypic plasticity and insect herbivory of trees in contrasting light environments in a Mexican rainforest

Abstract Contrasting light environments in rainforests generates changes in the characteristics of the leaves and in the herbivore community. In the present study, we carried out a reciprocal transplant experiment under natural conditions to determine the plasticity of leaf characteristics of plant species that grow in contrasting light environments in a Neotropical forest. We further explored the relationship between these traits and insect herbivory. We found that six woody species differ markedly in the phenotypic plasticity of leaf features. The specific leaf area, chlorophyll content, carbon content, nitrogen content, and leaf thickness of the most light-demanding species were highest in gaps, but their carbon/nitrogen ratios were higher under closed canopies. The herbivores were more abundant in gaps (5.9%–14.8%) than under closed canopy habitats (3.4%–6.1%) and seemingly associated to the plasticity of the leaf traits. We observed 47% more herbivores in gaps than under closed canopies. Our results suggest that the phenotypic plasticity of leaf traits depends on the identity of the plant species and its wood density, while herbivory seems to be affected by plant defence, low nutritional quality, or herbivore tolerance.

Trade-off between plant resistance and tolerance to herbivory: Mechanical defenses outweigh chemical defenses.

Plant resistance includes mechanical and chemical defenses that reduce herbivory, whereas plant tolerance reduces the fitness impact of herbivory. Because defenses are costly and investing in both resistance and tolerance may be superfluous, trade-offs among them are expected. In forest ecosystems, the mechanical strengthening of leaves is linked both to shade adaptation and antiherbivore defenses, but it also compromises resource uptake, therefore limiting regrowth following damage, suggesting a trade-off between mechanical defenses and tolerance. We tested for the resistance-tolerance trade-off across 11 common tree species in a temperate rainforest and explored mechanistic explanations by measuring chemical and mechanical defenses. Herbivory damage was negatively associated with leaf toughness and fiber content, whereas there was no significant relationship between herbivory and secondary metabolites (flavonols, gallic acid, tannins, and terpenoids). We detected a resistance-tolerance trade-off, as expected. We found a negative relationship between mechanical defenses and tolerance, estimated as the survival ratio between experimentally damaged and undamaged seedlings. Tolerance and secondary metabolites showed no significant association. Results suggest that selective forces other than herbivory acting on defensive traits can favor a resistance-tolerance trade-off. Therefore, plant adaptation to contrasting light environments may contribute to the evolution of resistance-tolerance trade-offs.

Supplementing green light under strong sunlight improves growth and functional ingredients of ginger (Zingiber officinale Rosc.) in summer

The green light for ginger growth may not be as unimportant as often assumed, but still now the roles of green light in ginger remain poorly understood. To investigate this issue, the ‘Shandong No. 1’ginger cultivar was grown and treated with two contrasting light environments: sunlight (CK treatment), supplementing green light under strong sunlight (T1 treatment), and this study explored morphological alterations, photosynthetic responses and gene expression of ginger in response to supplementing green light under strong sunlight, based on ginger's own species characteristics. The results showed as follows. One hand, supplementing green light under strong sunlight promoted expression of gibberellin biosynthesis gene and altered the morphological characteristics of ginger. In response to supplementing green light under strong sunlight, ginger presented not only shade avoidance symptoms characteristics, but also promotion of number of shoots, and thus rendered higher crown density and greater crown volume. When exposed to supplementing green light under strong sunlight condition, the strategies implemented by ginger to deal with stem elongation and branching increase seemed to be a species-specific response. On the other hand, the positive effect of supplementing green light under strong sunlight on ginger also depends on the growth conditions. Previous research confirmed that ginger did not grow well under green light only treatment. In contrast, supplementing green light under strong sunlight resulted in higher crown density and greater crown volume, which played a role in shading for the inner leaves and young pseudostems under summer sunlight, although strong summer light posed a threat to ginger growth. In addition, supplementing green light under strong sunlight led to upregulation of chlorophyll metabolism gene and an increase in Chlorophyll a content. Also, the enhancement of green light radiation effectively increased expressions of photosynthetic genes and photosynthesis rate. These all contributed to the improvement of photosynthetic efficiency of ginger leaves, as a way to increase light energy consumption, and thus reduced chloroplast senescence caused by excess light energy in summer. Furthermore, higher photosynthesis rate contributed to accumulation of photosynthetic products, and supported the material requirements for increasing biomass and improving rhizome functional ingredients. Taken together, this study will lay the foundations of a better understanding of the importance of green light to ginger.

Sprouts of shoot-clipped oak (Quercus alba and Q. robur) germinants show morphological and photosynthetic acclimation to contrasting light environments

Sprouting by woody plants can increase species resilience to disturbance and foster regeneration during periods with little recruitment from seed. Though sprouting often plays a critical role in oak forest regeneration, there is little information available on sprouting capacity and sprout physiology at the seedling stage, particularly for new germinants. This study compared sprouting capacity and sprout photosynthesis of shoot-clipped germinants of two temperate oaks established in contrasting light environments. We studied the North American Quercus alba and the European Q. robur, both are in the section Quercus and appear to share similar biological and ecological requirements. Sprouting capacity for both species was enhanced under high light availability (29% more sprouts per plant), a response not previously noted for oak germinants. Seedling sprouts acclimated to high light with a 34% decrease in leaf area ratio, a 56% increase in leaf mass per area, and a 49% increase in the light-saturated maximum photosynthetic rate. Though both species appeared similarly adapted to shoot loss, a greater sprouting capacity (29% more sprouts per plant) and plant-level net photosynthesis (73% higher) was observed for Q. robur, regardless of light environment. As naturally regenerated oak seedlings in forest understories often experience disturbance or stress resulting in shoot loss or die-back, our results highlight the importance of the light environment during early plant development. Our comparison of temperate oaks from different continents should facilitate exchange of successful stand regeneration practices within the range of temperate oak forests.

Physiological and growth strategies of two Cariniana species in response to contrasting light availability

Cariniana legalis and C. estrellensis are tropical emergent trees of the Lecythidaceae family that occur naturally in different forest ecosystems and belong to different ecological groups. With the aim of providing information to better manage these two congeneric Cariniana species in secondary forest enrichment plantations, an experiment was conducted using potted plants for comparing the physiological and growth responses of young C. legalis and C. estrellensis plants under contrasting light radiation availabilities. Considering that these two congeneric species occur naturally in different forest ecosystems and belong to different ecological groups, we tested the hypothesis that young plants of C. legalis have more plastic responses of photosynthetic light response curves, anthocyanin content, growth and biomass allocation to contrasting light environments than C. estrellensis. Plants were grown for 105 days under three light conditions (26, 10 and 1 mol photons m−2 day-1), simulating the daily total photosynthetic photon flux density (PPFD) in large and small canopy gaps and in the forest understory, respectively. The photosynthetic responses to contrasting light availabilities were very similar for both species. Higher values for anthocyanin reflectance index (ARI), leaf mass per area (LMA), and changes in biomass allocation indicate that C. legalis has a greater capacity for acclimation to contrasting light environments than did C. estrellensis. In addition, a greater number of smaller leaves in C. legalis could be an advantage in the understory of forests subjected to constant changes in canopy openness through the opening and closing of small- and medium-sized canopy gaps. From a practical point of view, seedlings of C. estrellensis should be planted in more open forests with less dynamic changes in canopy gaps, and seedlings of C. legalis should be planted in forests with more fluctuations in the opening and closing of small and large canopy gaps.

Acclimation responses of macaw palm seedlings to contrasting light environments

The photosynthetic adjustments of macaw palm (Acrocomia aculeata) were evaluated in 30-day-old seedlings exposed to high and low light environments, and sudden transference from low to high light and comparisons were made with the hardening protocol used in nurseries. Furthermore, we evaluated the responses to long-term exposure (265 days) to high and low light environments. Macaw palm seedlings exhibited an efficient mechanism that maximized light capture under scarce conditions, and dissipated excess energy to avoid damaging to the photosystem II under high light. The seedlings showed low saturation irradiance but no photoinhibition when exposed to excess light. When grown under low light intensities, seedlings presented higher photochemical efficiency and minimized the respiratory costs with positive carbon balance at lower irradiance than hardened seedlings did. The hardening procedure did not appear to be an advantageous method during seedling production. Long-term exposure to either low or high light did not cause significant leaf anatomical adjustments. However, the low light seedlings showed higher leaf area and chlorophyll content than those exposed to higher light intensity did, which enabled shaded seedlings to maximize the captured light. Furthermore, the high non-photochemical dissipation allowed rapid acclimation to excessive light exposure. These responses allow macaw palm cultivation and establishment in very different light environments.

Light intensity-regulated stomatal development in three generations of Lepidium sativum

Stomata are of crucial importance for CO2 uptake and the water economy of terrestrial plants. They evolve during the early phases of leaf development according to genetically conserved information modulated in its expression by environmental conditions. Mature leaves experience the environment, e.g. light intensity, and signal to younger developing leaves, modifying stomatal density (SD) and probably other leaf traits as well. In addition, the parental environment can affect the offspring’s phenotypes in an adaptive manner. However, the transgenerational effect of light on SD and interactions with the offspring’s light environment have not been studied so far. Here we compare transgenerational (parental) and growth-light (environmental) effects on SD. We grew three generations of garden cress (Lepidium sativum) in two contrasting light environments in a full factorial design. Stomatal and pavement cell densities (SD, PCD), 13C abundance as a proxy of leaf internal CO2 concentration (Ci), and leaf area were analysed in cotyledons and first true leaves of parental plants and two generations of offspring. Our results indicate that SD of offspring reflects both current and maternal light intensities. The transgenerational effect of light on SD was much smaller than the influence of current growth light and the former’s manifestation was dependent on light experienced by offspring. The cotyledons’ SD was unresponsive to variations in Ci but retained its light sensitivity, though it was smaller than in true leaves. High light reduced amphistomy in favour of the lower (abaxial) leaf side. SD and PCD were linearly proportional irrespective of light level, indicating invariant portions of spacing and amplifying cell divisions, and causing lower sensitivity of stomatal index at high SD. We show that light-stimulated stomatal development in garden cress differs in cotyledons and true leaves, varies between adaxial and abaxial leaf sides, and retains transgenerational information about environmental clues.

Juvenile Coffee Leaves Acclimated to Low Light Are Unable to Cope with a Moderate Light Increase.

The understorey origin of coffee trees and the strong plasticity of Coffea arabica leaves in relation to contrasting light environments have been largely shown. The adaptability of coffee leaves to changes in light was tested under controlled conditions by increasing the illumination rate on C. arabica var. Naryelis seedlings acclimated to low light conditions and observing leaf responses at three different developmental stages (juvenile, growing and mature). Only mature leaves proved capable of adapting to new light conditions. In these leaves, different major mechanisms were found to contribute to maintaining a good photosynthetic level. With increased illumination, a high photosynthetic response was conserved thanks to fast nitrogen remobilization, as indicated by SPAD values and the photorespiration rate. Efficient photoprotection was accompanied by a great ability to export sucrose, which prevented excessive inhibition of the Calvin cycle by hexose accumulation. In contrast, in younger leaves, increased illumination caused photodamage, observable even after 9 days of treatment. One major finding was that young coffee leaves rely on the accumulation of chlorogenic acids, powerful antioxidant phenolic compounds, to deal with the accumulation of reactive oxygen species rather than on antioxidant enzymes. Due to a lack of efficient photoprotection, a poor ability to export sucrose and inadequate antioxidant protection, younger leaves seemed to be unable to cope with increased illumination. In these leaves, an absence of induced antioxidant enzyme activity was accompanied, in growing leaves, by an absence of antioxidant synthesis or, in juvenile leaves, inefficient synthesis of flavonoids because located in some epidermis cells. These observations showed that coffee leaves, at the beginning of their development, are not equipped to withstand quick switches to higher light levels. Our results confirm that coffee trees, even selected for full sunlight conditions, remain shade plants possessing leaves able to adapt to higher light levels only when mature.

Maternal light environment during seed development can affect seed quality of Euterpe edulis

The primary concern of this research was to investigate how the contrasting light environments of a native forest (NF) and banana intercrop (BI) affect seed quality of Euterpe edulis. The results from our study also led us to investigate a possible light-induced component of seed dormancy in this species. Mature fruit bunches were collected from both environments and evaluated for seed germination and viability of non-germinated seeds, seed mass, water content and vigor tests (germination rate index and seedling mass). Total phenolics in seed coats and the effects of seed coat extracts on germination of lettuce seeds were also evaluated to investigate a possible cause for the lack of germination of many viable seeds from BI. Results showed that seed vigor did not differ between the environments, but germination of seeds from BI was lower, despite viability of most of the non-germinated ones. Seed coats formed in BI had a greater concentration of phenolics than those that developed in NF. In contrast to seed coat extracts from NF, extracts prepared from BI seeds had a negative, non-osmotic effect on the germination of lettuce seeds. Our results indicate that the more open environment provided by BI does not affect seed vigor, but may add a light-induced component to seed dormancy of the species, which can make seeds produced in more open environments less suitable to generate new plants. Such finding might be particularly useful for the species management in agroforestry systems and for its conservation in the rainforest.

Depth-specific fluctuations of gene expression and protein abundance modulate the photophysiology in the seagrass Posidonia oceanica

Here we present the results of a multiple organizational level analysis conceived to identify acclimative/adaptive strategies exhibited by the seagrass Posidonia oceanica to the daily fluctuations in the light environment, at contrasting depths. We assessed changes in photophysiological parameters, leaf respiration, pigments, and protein and mRNA expression levels. The results show that the diel oscillations of P. oceanica photophysiological and respiratory responses were related to transcripts and proteins expression of the genes involved in those processes and that there was a response asynchrony between shallow and deep plants probably caused by the strong differences in the light environment. The photochemical pathway of energy use was more effective in shallow plants due to higher light availability, but these plants needed more investment in photoprotection and photorepair, requiring higher translation and protein synthesis than deep plants. The genetic differentiation between deep and shallow stands suggests the existence of locally adapted genotypes to contrasting light environments. The depth-specific diel rhythms of photosynthetic and respiratory processes, from molecular to physiological levels, must be considered in the management and conservation of these key coastal ecosystems.

Specific leaf area variations drive acclimation of Cistus salvifolius in different light environments

Cistus salvifolius L. is the most widely spread Cistus species around the Mediterranean basin. It colonizes a wide range of habitats growing from sea level to 1,800 m a.s.l., on silicolous and calcicolous soils, in sun areas as well as in the understory of wooded areas. Nevertheless, this species has been mainly investigated in term of its responsiveness to drought. Our aim was to understand which leaf traits allow C. salvifolius to cope with low-light environments. We questioned if biochemical and physiological leaf trait variations in response to a reduced photosynthetic photon flux density were related to leaf morphological plasticity, expressed by variations of specific leaf area (SLA) and its anatomical components (leaf tissue density and thickness). C. salvifolius shrubs growing along the Latium coast (41°43'N,12°18'E, 14 m a.s.l., Italy) in the open and in the understory of a Pinus pinea forest, were selected and the relationships between anatomical, gas exchange, chlorophyll (Chl) fluorescence, and biochemical parameters with SLA and PPFD variations were tested. The obtained results suggested long-term acclimation of the selected shrubs to contrasting light environments. In high-light conditions, leaf nitrogen and Chl contents per leaf area unit, leaf thickness, and Chl a/b ratio increased, thus maximizing net photosynthesis, while in shade photosynthesis, it was downregulated by a significant reduction in the electron transport rate. Nevertheless, the increased pigment-protein complexes and the decreased Chl a/b in shade drove to an increased light-harvesting capacity (i.e. higher actual quantum efficiency of PSII). Moreover, the measured vitality index highlighted the photosynthetic acclimation of C. salvifolius to contrasting light environments. Overall, our results demonstrated the morphological, anatomical, and physiological acclimation of C. salvifolius to a reduced light environment.

Phenotypic plasticity of lianas in response to altered light environment

AbstractThe growth, morphology and biomass allocation of 11 liana species (six light‐demanding and five shade‐tolerant) were investigated by growing plants in three contrasting light environments (i.e., field, forest edge and forest interior). Our objectives were to determine: (1) changes in plant traits at the species level; and (2) differences in light‐demanding and shade‐tolerant species in response to altered light environment. We found that all seedlings of liana species increased in total biomass, total leaf area, relative growth rate (RGR), net assimilation rate (NAR), height, basal diameter, root length, leaf number, root mass/total plant mass (RMR) and root‐to‐shoot dry biomass (R/S ratio), and decreased in leaf area ratio (LAR), specific leaf area (SLA), leaf size, stem mass‐to‐total plant mass ratio (SMR) and leaf mass‐to‐total plant mass ratio (LMR) with increasing light availability. Under the three light environments, the two types of species differed significantly in total biomass, total leaf area, RGR, NAR, LAR, SLA and leaf number, and not in leaf area. Only light‐demanding species differed significantly in height, root length, basal diameter, RMR, SMR, LMR and R/S ratio. The mean plasticity index of growth and biomass allocation were relatively higher than the morphological variables, with significant differences between the two groups. Our results showed that liana species respond differently to changing light environments and that light‐demanding species exhibit higher plasticity. Such differences may affect the relative success of liana species in forest dynamics.

Strong leaf morphological, anatomical, and physiological responses of a subtropical woody bamboo (Sinarundinaria nitida) to contrasting light environments

Dwarf bamboos are an important understory component of the lowland and montane forests in the subtropical regions of Asia and South America, yet little is known about their physiology and phenotypic plasticity in response to changing light environments. To understand how bamboo species adapt to different light intensities, we examined leaf morphological, anatomical, and physiological differentiation of Sinarundinaria nitida (Mitford) Nakai, a subtropical woody dwarf bamboo, growing in open and shaded natural habitats in the Ailao Mountains, SW China. Compared with leaves in open areas, leaves in shaded areas had higher values in leaf size, specific leaf area, leaf nitrogen, and chlorophyll concentrations per unit area but lower values in leaf thickness, vein density, stomatal density, leaf carbon concentration, and total soluble sugar concentration. However, stomatal size and leaf phosphorus concentration per unit mass remained relatively constant regardless of light regime. Leaves in the open habitat exhibited a higher light-saturated net photosynthetic rate, dark respiration rate, non-photochemical quenching, and electron transport rate than those in the shaded habitat. The results of this study revealed that the bamboo species exhibited a high plasticity of its leaf structural and functional traits in response to different irradiances. The combination of high plasticity in leaf morphological, anatomical, and physiological traits allows this bamboo species to grow in heterogeneous habitats.

The Effect of Pollen Source vs. Flower Type on Progeny Performance and Seed Predation under Contrasting Light Environments in a Cleistogamous Herb

Dimorphic cleistogamy is a specialized form of mixed mating system where a single plant produces both open, potentially outcrossed chasmogamous (CH) and closed, obligately self-pollinated cleistogamous (CL) flowers. Typically, CH flowers and seeds are bigger and energetically more costly than those of CL. Although the effects of inbreeding and floral dimorphism are critical to understanding the evolution and maintenance of cleistogamy, these effects have been repeatedly confounded. In an attempt to separate these effects, we compared the performance of progeny derived from the two floral morphs while controlling for the source of pollen. That is, flower type and pollen source effects were assessed by comparing the performance of progeny derived from selfed CH vs. CL and outcrossed CH vs. selfed CH flowers, respectively. The experiment was carried out with the herb Ruellia nudiflora under two contrasting light environments. Outcrossed progeny generally performed better than selfed progeny. However, inbreeding depression ranges from low (1%) to moderate (36%), with the greatest value detected under shaded conditions when cumulative fitness was used. Although flower type generally had less of an effect on progeny performance than pollen source did, the progeny derived from selfed CH flowers largely outperformed the progeny from CL flowers, but only under shaded conditions and when cumulative fitness was taken into account. On the other hand, the source of pollen and flower type influenced seed predation, with selfed CH progeny the most heavily attacked by predators. Therefore, the effects of pollen source and flower type are environment-dependant and seed predators may increase the genetic differences between progeny derived from CH and CL flowers. Inbreeding depression alone cannot account for the maintenance of a mixed mating system in R. nudiflora and other unidentified mechanisms must thus be involved.

Extreme variation in the reproductive phenology of the weed Ruellia nudiflora

SummaryTemporal variation in morphological and life‐history traits is directly linked to phenotypic plasticity in perennial plants. Phenotypic plasticity in reproductive phenology determines colonisation ability and population persistence in invasive weeds. We describe the reproductive phenology of Ruellia nudiflora (a cleistogamous weed) and assess the plasticity of its response to between‐year environmental variation in two contrasting light environments. We recorded the flower and fruit production of focal plants under two experimental levels of light intensity (open vs. shaded) over two years. Our results suggest that light differentially affects the phenology of reproductive structures, but not between‐year variation. Specifically, individual production of chasmogamous structures (flowers and fruit) ended earlier under shaded conditions than in the open, whilst production of cleistogamous fruits was not affected. At the population level, there was between‐year variation in the phenology of chasmogamous structures, which were bimodal and unimodal in the first and second year respectively. At the individual level, we found very low repeatability, suggesting high levels of phenotypic plasticity. Temporal variation in rainfall did not explain phenological variation. Although shade did not affect between‐year variation, it produced a briefer period of production of chasmogamous structures. This effect of shade may have some consequences for the genetic background of R. nudiflora's progeny, because only chasmogamous flowers can be out‐crossed, and this therefore might be an important factor to consider when designing management strategies.

Adjustments in epidermal UV‐transmittance of leaves in sun‐shade transitions

Epidermal UV transmittance (TUV ) and UV-absorbing compounds were measured in sun and shade leaves of Populus tremuloides and Vicia faba exposed to contrasting light environments under field conditions to evaluate UV acclimation potentials and regulatory roles of photosynthetically active radiation (PAR) and UV in UV-shielding. Within a natural canopy of P. tremuloides, TUV ranged from 4 to 98% and showed a strong nonlinear relationship with mid-day horizontal fluxes of PAR [photon flux density (PFD) = 6-1830 µmol m⁻² s⁻¹]; similar patterns were found for V. faba leaves that developed under a comparable PFD range. A series of field transfer experiments using neutral-density shade cloth and UV blocking/transmitting films indicated that PAR influenced TUV during leaf development to a greater degree than UV, and shade leaves of both species increased their UV-shielding when exposed to full sun; however, this required the presence of UV, with both UV-A and UV-B required for full acclimation. TUV of sun leaves of both species was largely unresponsive to shade either with or without UV. In most, but not all cases, changes in TUV were associated with alterations in the concentration of whole-leaf UV-absorbing compounds. These results suggest that, (1) moderate-to-high levels of PAR alone during leaf development can induce substantial UV-protection in field-grown plants, (2) mature shade leaves have the potential to adjust their UV-shielding which may reduce the detrimental effects of UV that could occur following sudden exposures to high light and (3) under field conditions, PAR and UV play different roles in regulating UV-shielding during and after leaf development.

Modeling the environmental response of leaf net photosynthesis in Pinus pinea L. natural regeneration

Environmental factors as incident light, temperature and soil water content mainly determine the dynamics of growth and survival of natural forest regeneration in Mediterranean forests. The complex interactions among these factors highlight the need for physiology-based models which describe seedling and sapling performance under current and changing climatic conditions. These models should be flexible enough to take into account the effect of growth light environment changes on physiological processes and, therefore, to assist in the decision-making process on different silvicultural management alternatives under climatic uncertainty. In the present work the net photosynthesic rate of Pinus pinea L. natural regeneration is modeled as a function of light irradiance using the non-rectangular hyperbola function. The model fit was carried out following a two-step procedure, where the parameters of the original function were expanded over the most influential factors: leaf temperature, soil moisture, global site factor and needle type. The developed model allows defining the optimal niche conditions for the natural regeneration of the species and permits identifying the most limiting conditions which prevent natural regeneration. Regeneration niche was assessed by using the model to simulate net CO2 assimilation of a seedling over twelve contrasting light environments during both a normal and an extremely dry vegetative period. The model predicts that the most favorable carbon balance would be found in mid-shaded expositions, while fully exposed plants at midday during summer would exhibit longer periods of negative assimilation rates, especially on severe dry years.

Variation in violaxanthin and lutein cycle components in two provenances of Acer grandidentatum L. exposed to contrasting light

The ability of ornamental plants to acclimate to contrasting light environments via photoprotection mechanisms such as the violaxanthin (V) and lutein (Lx) cycles determines their success in managed landscapes. This study investigated whether provenance of origin influenced V and Lx cycle components in Acer grandidentatum L. (bigtooth maple) exposed to contrasting light environments. Seven-year-old bigtooth maple plants from two provenances, Lost Maples State Natural Area, Texas (LMPTX) and Guadalupe Mountains, Texas (GMTX) were subjected to unshaded conditions and 48 % shade. Regardless of light treatment, β-carotene content of plants from the LMPTX provenance (1,181 μg g−1) was higher than those from GMTX (964 μg g−1). Lutein content declined from day 5 to 10, but returned to day 5 levels at the end of the experiment at day 15. In contrast, more α-carotene accumulated at day 10 than at day 5. Although the conversion state of the xanthophyll pigment pool was unchanged, zeaxanthin was higher in unshaded than shaded bigtooth maple plants. Chlorophyll a and b decreased from day 5 to 10 and then increased on day 15. Light treatment did not cause photosystem II efficiency and relative water content to differ between the provenances, suggesting that they effectively acclimated to contrasting light conditions.

Photosynthetic responses of four tropical tree species grown under gap and understorey conditions in a semi-deciduous forest

Leaf CO2 assimilation (A) as a function of photosynthetic photon flux density (Q) or intercellular CO2 concentration (Ci) and chlorophyll fluorescence measurements were carried out on four tropical woody species growing in forest gap and understorey (Bauhinia forficata Link. and Guazuma ulmifolia Lam. as pioneers, and Hymenaea courbaril L. and Esenbeckia leiocarpa Engl. as non-pioneers). Chlorophyll fluorescence indicated similar acclimation capacities of photochemical apparatus to contrasting light environments irrespective to plant species. Maximum CO2 assimilation and quantum yield derived from A/Q curves indicated higher photosynthetic capacity in pioneer than in non-pioneer species in forest gap. However, the differences among species did not show a straightforward relation with their successional status regarding data derived from A/Q curves under understorey conditions. Both successional groups are able to sustain positive carbon balance under contrasting natural light availabilities, modifying photochemical and biochemical photosynthetic traits with similar phenotypic plasticity capacity.

Light- and seasonal-induced plasticity in leaf morphology, N partitioning and photosynthetic capacity of two temperate deciduous species

Processes involved in leaf photosynthetic acclimation to light and throughout the growing season were investigated in two hardwood species ( Acer saccharum and Betula alleghaniensis), which differed in their level of shade-tolerance. For both species, variation in traits related to (i) leaf morphology ( LMA, leaf mass:area ratio), (ii) leaf N content ( N A, leaf nitrogen content on an area basis and N M, N concentration in leaf dry mass), (iii) leaf N partitioning among photosynthetic functions ( P r, N allocated to Rubisco, and P b, N allocated to bioenergetics), and (iv) leaf photosynthetic capacity ( V cmax, maximal carboxylation rates, and J max, maximal light-driven electron flow) were assessed at three different times during the growing season (early, mid- and late summer) and under four contrasting light regimes (40, 17, 6 and 2% of full sunlight). For both species, light-driven variation in most traits was greater than their seasonally driven variation. Furthermore, results showed for both species the pre-eminence of LMA changes in the light-driven acclimation of N A. Importance of N M to variation in N A was restricted to seasonal acclimation, especially for the less shade-tolerant species, B. alleghaniensis. Similarly, for both species, light-driven acclimation of leaf photosynthetic capacities was tightly related to variation in N A, which was related to LMA changes. However, variation in P r and P b better explained seasonally driven variation in V cmax and J max, specifically under lower light levels, where N A was low. Thus, the great variability observed for leaf activity in response to contrasting light environments was related to efficient morphological adjustments, regardless of species level of shade-tolerance. Finally, physiological adjustments were mainly involved in fine-scale changes observed during seasonally driven acclimation of leaves, when LMA was constrained to a slight range of variation.