Some like it hot: the physiological ecology of C4 plant evolution.

The evolution of C4 photosynthesis requires an intermediate phase where photorespiratory glycine produced in the mesophyll cells must flow to the vascular sheath cells for metabolism by glycine decarboxylase. This glycine flux concentrates photorespired CO2 within the sheath cells, allowing it to be efficiently refixed by sheath Rubisco. A modest C4 biochemical cycle is then upregulated, possibly to support the refixation of photorespired ammonia in sheath cells, with subsequent increases in C4 metabolism providing incremental benefits until an optimized C4 pathway is established. 'Why' C4 photosynthesis evolved is largely explained by ancestral C3 species exploiting photorespiratory CO2 to improve carbon gain and thus enhance fitness. While photorespiration depresses C3 performance, it produces a resource (photorespired CO2) that can be exploited to build an evolutionary bridge to C4 photosynthesis. 'Where' C4 evolved is indicated by the habitat of species branching near C3-to-C4 transitions on phylogenetic trees. Consistent with the photorespiratory bridge hypothesis, transitional species show that the large majority of > 60 C4 lineages arose in hot, dry, and/or saline regions where photorespiratory potential is high. 'When' C4 evolved has been clarified by molecular clock analyses using phylogenetic data, coupled with isotopic signatures from fossils. Nearly all C4 lineages arose after 25 Ma when atmospheric CO2 levels had fallen to near current values. This reduction in CO2, coupled with persistent high temperature at low-to-mid-latitudes, met a precondition where photorespiration was elevated, thus facilitating the evolutionary selection pressure that led to C4 photosynthesis.

Sunflecks in trees and forests: from photosynthetic physiology to global change biology.

Sunflecks are brief, intermittent periods of high photon flux density (PFD) that can significantly improve carbon gain in shaded forest understories and lower canopies of trees. In this review, we discuss the physiological basis of leaf-level responses to sunflecks and the mechanisms plants use to tolerate sudden changes in PFD and leaf temperature induced by sunflecks. We also examine the potential effects of climate change stresses (including elevated temperatures, rising CO(2) concentrations and drought) on the ability of tree species to use sunflecks, and advocate more research to improve our predictions of seedling and tree carbon gain in future climates. Lastly, while we have the ability to model realistic responses of photosynthesis to fluctuating PFD, dynamic responses of photosynthesis to sunflecks are not accounted for in current models of canopy carbon uptake, which can lead to substantial overestimates of forest carbon fixation. Since sunflecks are a critical component of seasonal carbon gain for shaded leaves, sunfleck regimes and physiological responses to sunflecks should be incorporated into models to more accurately capture forest carbon dynamics.

The taxonomic distribution of C4 photosynthesis in Amaranthaceae sensu stricto.

C(4) photosynthesis evolved multiple times in the Amaranthaceae s.s., but the C(4) evolutionary lineages are unclear because the photosynthetic pathway is unknown for most species of the family. To clarify the distribution of C(4) photosynthesis in the Amaranthaceae, we determined carbon isotope ratios of 607 species and mapped these onto a phylogeny determined from matK/trnK sequences. Approximately 28% of the Amaranthaceae species use the C(4) pathway. C(4) species occur in 10 genera-Aerva, Amaranthus, Blutaparon, Alternanthera, Froelichia, Lithophila, Guilleminea, Gomphrena, Gossypianthus, and Tidestromia. Aerva, Alternanthera, and Gomphrena contain both C(3) and C(4) species. In Aerva, 25% of the sampled species are C(4). In Alternanthera, 19.5% are C(4), while 89% of the Gomphrena species are C(4). Integration of isotope and matK/trnK data indicated C(4) photosynthesis evolved five times in the Amaranthaceae, specifically in Aerva, Alternanthera, Amaranthus, Tidestromia, and a lineage containing Froelichia, Blutaparon, Guilleminea, Gomphrena pro parte, and Lithophila. Aerva and Gomphrena are both polyphyletic with C(3) and C(4) species belonging to distinct clades. Alternanthera appears to be monophyletic with C(4) photosynthesis originating in a terminal sublineage of procumbent herbs. Alpine C(4) species were also identified in Alternanthera, Amaranthus, and Gomphrena, including one species (Gomphrena meyeniana) from 4600 m a.s.l.

Crown architecture in sun and shade environments: assessing function and trade-offs with a three-dimensional simulation model.

Sun and shade environments place markedly different constraints on the photosynthetic performance of plants. Leaf-level photosynthetic responses to sun and shade have been extensively investigated, whereas there has been much less research on the functional role of crown architecture in these environments. This paper focuses on the role of architecture in maximizing light capture and photosynthesis in shaded understories and in minimizing exposure to excess radiation in open high light environments. Understanding these contrasting roles of architecture is facilitated by application of a three-dimensional structural-functional model, Y-plant. Surveys of understory plants reveal a diversity of architectures but a strong convergence at only modest light-capture efficiencies because of significant self-shading. Simulations with Psychotria species revealed that increasing internode lengths would increase light-capture efficiencies and whole plant carbon gain. However, the costs of the additional required biomechanical support was high, which, in terms of relative growth rates, would override the advantage provided by higher light-capture efficiencies. In high light environments, leaf angles and self-shading provide structural photoprotection, minimizing potential damage from photoinhbition. Simulations reveal that without these structural protections photoinhibition of photosynthesis is likely to be much greater with daily carbon gain significantly reduced.

Photoinhibition and drought in Mediterranean woody saplings: scaling effects and interactions in sun and shade phenotypes.

Interacting effects of high light and drought on the performance of sun and shade phenotypes were experimentally undertaken following survival, chlorophyll fluorescence and gas exchange in 2-year-old saplings of four Mediterranean trees (Quercus ilex and Q. coccifera as water-saving species, and Pistacia lentiscus and P. terebinthus as water-spending species). Half of the saplings were grown in full sunlight and the other half in the shade (6% sunlight). Half of each combination of species-phenotype was exposed to high light during a simulated late-summer drought. Light absorptance and gas exchange were scaled up to the whole plant with the 3-D geometrical model, Y-Plant. Quercus species were more plastic and tolerated high light and water stress better than Pistacia species, surviving longer and in drier soils, and exhibiting a less pronounced photoinhibition. There was no evidence of disadvantage for shade phenotypes under high light with increasing drought. By contrast, shade phenotypes survived longer despite larger initial decreases in photochemical efficiency and higher sensitivity to drought than sun phenotypes. The enhanced control of transpiration during drought in water-saving versus water-spending species (and also in shade versus sun phenotypes in three out of the four species) allowed extended survival. Photoinhibition reduced whole crown carbon gain in high light by c. 3% and affected significantly more the shaded leaves of a given plant (reducing their carbon gain by up to 7%) than those exposed to direct sunlight. Despite this apparently minor impact, whole plant carbon gain reduction by photoinhibition negatively correlated with survival and drought tolerance. The implications for succession and forest regeneration in arid environments, particularly under a global change scenario, are discussed.

A functional analysis of the crown architecture of tropical forest Psychotria species: do species vary in light capture efficiency and consequently in carbon gain and growth?

The crown architectures of 11 Psychotria species native to Barro Colorado Island, Panama were reconstructed from field measurements of leaf and branch geometry with the three-dimensional simulation model Y-plant. The objective was to assess the role of species differences in architecture in light capture and carbon gain in their natural understory environment. When species were grouped according to their putative light environment preference, the shade tolerant species were found to have a small but significantly higher efficiency of light capture for both diffuse and direct light as compared to the light demanding species. Within each grouping, however, there were few significant differences in light capture efficiency among species. The lower efficiencies of light demanding species was due to slightly higher self-shading and slightly lower angular efficiencies. Simulations of whole plant assimilation showed that light demanding species had greater daily assimilation in both direct and diffuse light due to the significantly greater light availability in the sites where light demanding species were found, as compared to those where shade tolerant species occurred. Among light demanding species, the above ground relative growth rate measured over a 1-year period by applying allometric equations for mass versus linear dimensions, was positively correlated with diffuse PFD and with mean daily assimilation estimated from Y-plant. For the shade tolerant plants, there was no significant correlation between RGR and mean daily assimilation or with any measure of light availability, probably because they occurred over a much narrower range of light environments. Overall, the results reveal a strong convergence in light capture efficiencies among the Psychotria species at lower values than previously observed in understory plants using similar approaches. Constraints imposed by other crown functions such as hydraulics and biomechanical support may place upper limits on light capture efficiency.

Functional correlates of leaf demographic response to gap release in saplings of a shade-tolerant tree, Elateriospermum tapos.

For a shade-tolerant SE Asian tropical tree, Elateriospermum tapos (Euphorbiaceae), we studied field-established saplings in gaps and the shaded understory to test the hypothesis that differences in leaf demography and leaf life span under contrasting light regimes should be functionally correlated with architecture, self-shading and nitrogen distribution within the sapling crown. Rates of leaf production and net leaf gain were greater for saplings in gaps than those in the understory. Median leaf life span was approximately 26 months in the gap saplings, while it was estimated to be greater than 38 months in the understory saplings. Consequently, gap saplings had a greater standing leaf number and experienced greater degrees of self-shading than understory saplings. Light availability at individual leaves, estimated by a combination of canopy photos and a three-dimensional architecture model, were negatively correlated with leaf age in gap saplings but not so in understory saplings. Leaf nitrogen content per unit area (Narea) was influenced more by light availability than by leaf age in the gap saplings. In contrast, in understory saplings, Narea was neither correlated with light availability nor with leaf age, and did not decrease significantly before 38 months in leaf age. We conclude that saplings of this shade-tolerant species apparently prolong their leaf life span in the shaded understory through slower rates of leaf production, lower standing number of leaves and lower degrees of self shading than in gap, and that the rate of decline of Narea with leaf age depends on architecture and self-shading regimes that respond to changes in light regimes.

Leaf display and photosynthesis of tree seedlings in a cool-temperate deciduous broadleaf forest understorey.

To examine a possible convergence in leaf photosynthetic characteristics and leaf display responses to light environment in seedlings of three canopy and two shrub tree species in understorey of cool-temperate deciduous broadleaf forest, relationships between light environment, leaf orientation and leaf light-photosynthetic response were measured. Light capture of the seedlings (17-24 individuals with 2-12 leaves for each species) was assessed with a three dimensional geometric modeling program Y-plant. Leaf photosynthetic characteristics of the five species were found to have acclimated to the understorey light environment, i.e., low light compensation point and high apparent quantum yield. In addition, light-saturated photosynthetic rates were higher in seedlings inhabiting microsites with higher light availability. Efficiencies of light capture and carbon gain of the leaf display were evaluated by simulating the directionalities of light capture and daily photosynthesis for each seedling using hemispherical canopy photography. The results showed that most of the seedlings orientated their leaves in a way to increase the daily photosynthesis during the direct light periods (sunflecks) rather than maximize daily photosynthesis by diffuse light. Simulations also showed that daily photosynthesis would increase only 10% of that on actual leaf display when the leaves orientated to maximize the diffuse light interception. Simulations in which leaf orientations were varied showed that when the leaf display fully maximized direct light interception, the time that leaves were exposed to excessive photon flux density of >800 mumol photons m(-2) s(-1) were doubled. The understorey seedlings studied responded to the given light environments in a way to maximize the efficiency of acquisition and use of light during their short (approximately 3 month) seasonal growth period.

Convergence in light capture efficiencies among tropical forest understory plants with contrasting crown architectures: a case of morphological compensation.

Leaf and crown characteristics were examined for 24 tree and herbaceous species of contrasting architectures from the understory of a lowland rainforest. Light-capture efficiency was estimated for the crowns of the different species with a three-dimensional geometric modeling program. Causal relationships among traits affecting light absorption at two hierarchical levels (leaf and whole crown) were quantified using path analysis. Light-capture and foliage display efficiency were found to be very similar among the 24 species studied, with most converging on a narrow range of light absorption efficiencies (ratio of absorbed vs. available light of 0.60-0.75). Exceptionally low values were found for the climber vines and, to a lesser extent, for the Bromeliad Aechmea magdalenae. Differences in photosynthetic photon flux density (PFD) absorbed per unit leaf area by individual plants were mostly determined by site to site variation in PFD and not by the differences in crown architecture among individuals or species. Leaf angle, and to a lesser extent also supporting biomass, specific leaf area, and internode length, had a significant effect on foliage display efficiency. Potential constraints on light capture such as the phyllotactic pattern were generally offset by other compensatory adjustments of crown structure such as internode length, arching stems, and plagiotropy. The variety of shoot morphologies capable of efficiently capturing light in tropical forest understories is greater than initially thought, extending over species with very different phyllotactic patterns, crown architectures, leaf sizes, and morphologies.

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 = 71o) 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 = 5o). 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.

Photosynthetic responses to dynamic light under field conditions in six tropical rainforest shrubs occuring along a light gradient.

We examined in the field the photosynthetic utilization of fluctuating light by six neotropical rainforest shrubs of the family Rubiaceae. They were growing in three different light environments: forest understory, small gaps, and clearings. Gas exchange techniques were used to analyse photosynthetic induction response, induction maintenance during low-light periods, and lightfleck (simulated sunfleck) use efficiency (LUE). Total daily photon flux density (PFD) reaching the plants during the wet season was 37 times higher in clearings than in the understory, with small gaps exhibiting intermediate values. Sunflecks were more frequent, but shorter and of lower intensity in the understory than in clearings. However, sunflecks contributed one-third of the daily PFD in the understory. Maximum rates of net photosynthesis, carboxylation capacity, electron transport, and maximum stomatal conductance were lower in understory species than in species growing in small gaps or clearings, while the reverse was true for the curvature factor of the light response of photosynthesis. No significant differences were found in the apparent quantum yield. The rise of net photosynthesis during induction after transfer from low to high light varied from a hyperbolic shape to a sigmoidal increase. Rates of photosynthetic induction exhibited a negative exponential relationship with stomatal conductance in the shade prior to the increase in PFD. Leaves of understory species showed the most rapid induction and remained induced longer once transferred to the shade than did leaves of medium- or high-light species. LUE decreased rapidly with increasing lightfleck duration and was affected by the induction state of the leaf. Fully induced leaves exhibited LUEs up to 300% for 1-s lightflecks, while LUE was below 100% for 1-80 s lightflecks in uninduced leaves. Both induced and uninduced leaves of understory species exhibited higher LUE than those of species growing in small gaps or clearings. However, most differences disappeared for lightflecks 10 s long or longer. Thus, understory species, which grew in a highly dynamic light environment, had better capacities for utilization of rapidly fluctuating light than species from habitats with higher light availability.

A three-dimensional crown architecture model for assessment of light capture and carbon gain by understory plants.

A model simulating the three-demensional crown architecture of a plant was developed with the objective of assessing the light capture and whole-plant carbon gain consequences of leaf display in understory plants. This model uses geometrical measurements taken in the field to reconstruct the projected image of a plant so that light absorption from any direction can be assessed. The photon flux density (PFD) from a given direction was estimated from the canopy openness derived from hemispherical canopy photographs and equations simulating the daily course of direct and diffuse PFD. For diffuse PFD, the directional fluxes and absorbed PFD were integrated over 160 different directions representing 8 azimuth classes and 20 elevation angle classes. Direct PFD absorption was determined for the time that a solar track on a given day intersected a canopy gap. Assimilation rate was simulated for the sunlit and shaded parts of leaves separately and then summed to give the whole-plant carbon gain. Comparisons of simulations for a tropical forest edge species, Clidemia octona, and an understory species, Conostegia cinnamomea, illustrate the operation of the model and show that the edge species is more efficient at capturing side light while the understory species is slightly more efficient at capturing light from directly above, the predominant light direction in this environment. Self-shading within Conostegia crown and steep leaf angles in the Clidemia crown reduced light capture efficiencies for light from directly above. Whole-plant daily carbon gain was much higher in the forest edge site, mostly because of the additional PFD available in this site. However, simulations for both species in the understory light environment show that the higher light capture efficiencies of the understory species in this environment conferred a 27% advantage in carbon gain in this environment.

Photosynthetic gas exchange response of poplars to steady-state and dynamic light environments.

The steady-state and dynamic photosynthetic response of two poplar species (Populus tremuloides and P. fremontii) to variations in photon flux density (PFD) were observed with a field portable gas exchange system. These poplars were shown to be very shade intolerant with high light saturation (800 to 1300 µmol photons m-2 s-1) and light compensation (70 to 100 µmol m-2 s-1) points. Understory poplar leaves showed no physiological acclimation to understory light environments. These plants become photosynthetically induced quickly (10 min). Activation of Rubisco was the primary limitation for induction, with stomatal opening playing only a minor role. Leaves maintained high stomatal conductances and stomata were unresponsive to variations in PFD. Leaves were very efficient at utilizing rapidly fluctuating light environments similar to those naturally occurring in canopies. Post-illumination CO2 fixation contributed proportionally more to the carbon gain of leaves during short frequent lightflecks than longer less frequent ones. The benefits of a more dynamic understory light environment for the carbon economy of these species are discussed.

Effect of leaf flutter on the light environment of poplars.

The dynamics of the canopy light environment for two poplar species (Populus tremuloides Michx., and P. fremontii Wats.) were characterized with an array of photocells in fixed positions within the canopy or attached directly to leaves and using a data logger that recorded photon flux density (PFD) at frequencies from 1 to 20 Hz. The majority of sunflecks were short in duration (<1 s) with a similar short interval between sunflecks. Sunflecks contribute as much as 90% of the total daily PFD in the lower canopy. Leaf flutter may cause high frequency (3 to 5 Hz) variations of PFD in poplar canopies. The amount of light intercepted by a fluttering leaf at the top of the canopy decreased with increasing flutter, whereas a fluttering lower canopy leaf showed no such trend. When leaves fluttered at the top of the canopy the understory light environment showed an increased number of shorter sunflecks. Leaf flutter may increase mean PFD for understory leaves. It also creates a canopy light environment that is more dynamic temporally and more evenly distributed spatially. The potential benefits of these changes in light dynamics are discussed.

Stomatal dynamics and its importance to carbon gain in two rainforest Piper species : I. VPD effects on the transient stomatal response to lightflecks.

The effects of leaf-air vapor pressure deficit (VPD) on the transient and steady-state stomatal responses to photon flux density (PFD) were evaluated in Piper auritum, a pioneer tree, and Piper aequale, a shade tolerant shrub, that are both native to tropical forests at Los Tuxtlas, Veracruz, México. Under constant high-PFD conditions, the stomata of shade-acclimated plants of both species were sensitive to VPD, exhibiting a nearly uniform decrease in gs as VPD increased. Acclimation of P. auritum to high light increased the stomatal sensitivity to VPD that was sufflcient to cause a reduction in transpiration at high VPD's. At low PFD, where gs was already reduced, there was little additional absolute change with VPD for any species or growth condition. The stomatal response to 8-min duration lightflecks was strongly modulated by VPD and varied between the species and growth light conditions. In P. aequale shade plants, increased VPD had no effect on the extent of stomatal opening but caused the rate of closure after the lightfleck to be faster. Thus, the overall response to a lightfleck changed from hysteretic (faster opening than closure) to symmetric (similar opening and closing rates). Either high or low VPD caused gs not to return to the steady-state value present before the lightfleck. At high VPD the value after was considerably less than the value before whereas at low VPD the opposite occurred. Shade-acclimated plants of P. auritum showed only a small gs response to lightflecks, which was not affected by VPD. Under sunfleck regimes in the understory, the stomatal response of P. aequale at low VPD may function to enhance carbon gain by increasing the induction state. At high VPD, the shift in the response enhances water use efficiency but at the cost of reduced assimilation.

Stomatal dynamics and its importance to carbon gain in two rainforest Piper species : II. Stomatal versus biochemical limitations during photosynthetic induction.

The relative importance of biochemical and stomatal limitations on assimilation (A) during photosynthetic induction were compared in sun and shade plants of Piper auritum, a pioneer tree, and shade plants of Piper aequale, a shade tolerant shrub native to a Mexican tropical rainforest. For non-induced leaves, increases in A during induction depended on the dynamics of stomatal conductance (gs) and ribulose-1,5-bisphosphate carboxylase (RuBisCO) activation. At high leaf-air vapor pressure deficit (VPD), more of the limitation during induction was stomatal. Calculations of mesophyll conductance revealed longer time constants for shade than for sun plants. However, no differences in the time course of RuBisCO activity between sun- and shade-plants were found. We conclude on the basis of the similar RuBisCO responses that differences in induction can be accounted for by the differences in stomatal behavior. Differences in the time course of mesophyll conductance may be due to an artifact caused by stomatal patchiness. Experiments on induction loss of previously induced leaves revealed that under these circumstances biochemical limitations can be important. A more rapid induction loss was evident in sun as compared to shade plants. The rapid loss of induction in sum plants was not due to the decreases in gs and RuBisCO activity, which both occurred slowly. Instead, a limitation, probably in RuBP regeneration capacity, appeared to develop during the low light periods. This limitation was much smaller or absent in shade plants.

Dynamic stomatal behavior and its role in carbon gain during lightflecks of a gap phase and an understory Piper species acclimated to high and low light.

Steady-state and dynamic stomatal and assimilation responses to light transients were characterized in sun- and shade-acclimated plants of Piper auritum, a pioneer tree, and Piper aequale a shade-tolerant shrub from a tropical forest at Los Tuxtlas, Veracruz, México. Despite essentially identical steady-state responses of stomatal conductance to PFD of P. aequale and P. auritum shade plants, the dynamic responses to lightflecks were markedly different and depended on the growth regime. For both species from both growth environments, the increase in stomatal conductance occurring in response to a lightfleck continued long after the lightfleck itself so that the maximum stomatal conductance was not reached until 20-40 min after the lightfleck. Closing then occurred until stomatal conductance returned to near its original value before the lightfleck. Plants that were grown under light regimes similar to those of their natural habitat (high light for P. auritum and shade for P. aequale) had large maximum excursions of stomatal conductance and slower closing than opening responses. Plants grown under the opposite conditions had smaller excursions of stomatal conductance, especially in P. auritum, and more symmetrical opening and closing. The large and hysteretic response of stomatal conductance of P. aequale shade plants to a lightfleck was shown to improve carbon gain during subsequent lightflecks by 30-200%, depending on lightfleck duration. In contrast the very small stomatal response to lightflecks in P. auritum shade plants, resulted in no significant improvement in use of subsequent lightflecks.

Photosynthesis and respiration in Alocasia macrorrhiza following transfers to high and low light.

Photosynthetic capacities and respiration rates of Alocasia macrorrhiza leaves were measured for 4 weeks following reciprocal transfers between high (20% of full sun) and low (1% of full sun) light environments. Photosynthetic capacities and respiration rates of mature, high-light leaves were 1.7 and 4.5 times those of low-light leaves, respectively. Following transfer, respiration rates adjusted within 1 week to those characteristic of plants grown in the new environment. By contrast, photosynthetic capacities either did not adjust or changed only slowly following transfer. Most of the difference in respiration between high- and low-light leaves was related to the carbohydrate status as determined by the daily PFD and little was directly related to the maintenance costs of the photosynthetic apparatus. Leaf construction cost was directly proportional to maximum photosynthetic capacity. Consequently, although daily carbon gain per unit leaf area was the same for low-light and high to low-light transferred plants within a week after transfer, the carbon return per unit of carbon investment in the leaves remained lower in the high to low transfer plants throughout the 4 week measurement period. Conversely, in high-light, the low leaf construction cost of the low to high-light transferred plants resulted in carbon gain per unit investment just as high as that of the high-light plants.

Influence of sunflecks on the δ 13 C of Adenocaulon bicolor plants occurring in contrasting forest understory microsites.

Leaf characteristics and carbon isotope ratios (δ13C) of Adenocaulon bicolor were examined in the understory of a redwood forest along a gradient of microsites that differed in the amount of direct (sunfleck) photon flux density. Comparisons were made between plants that had been shaded from sunflecks with shadow bands but still received diffuse light, and adjacent plants that received both sunflecks and diffuse light. The δ13C of the shaded plants were 1.2‰ lower than predicted from the intercellular CO2 pressure (pi), probably because of recycling of respired CO2 in the understory. Plants receiving sunflecks had higher δ13C values because assimilation during sunflecks occurred at a lower pi than assimilation in diffuse light. The amount that their δ13C was higher was positively correlated with predicted direct photon flux density received by a plant. Leaf weight per unit area increased with increasing PFD. Although plants receiving sunflecks had greater leaf weights per unit area and photosynthetic capacities than those under shadow bands, there was no apparent acclimation of photosynthetic capacity to the differences in PFD among the microsites.