Photosynthesis provides all the chemical energy and the carbon compounds needed for all other growth processes. It is thus the central process governing the primary productivity of all green plants. The ultimate substrate for this process is Carbon dioxide (CO2). The ability of leaf photosynthesis to respond to increased CO2 concentration depends on various external and internal limits to CO2 uptake, their dependence on CO2 concentration, and their interactions with other environmental factors. It is convenient to consider CO2 uptake into the leaf in terms of a series of conductances associated with the leaf boundary layer, the stomata, the intercellular air space, the cell wall and intracellular liquid phase. Photosynthetic enhancement by increased CO2 concentration depends strongly on other environmental factors. For C3 plants, the enhancement is least at low temperatures and increases continuously with increasing temperature. Stomatal opening is the result of an accumulation of potassium salts in the guard cells.

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.

Summary Shade tolerance can be defined as the light level at which plants can survive and possibly grow. This light level is referred to as the whole‐plant light compensation point (LCP). The LCP depends on multiple leaf and architectural traits. We are still uncertain how often interspecific trait differences allow species to specialize for separate light niches, as observed between shade‐tolerant species and light‐demanding species. Alternatively, trait plasticity may allow many species to grow in similar light conditions. We measured leaf and architectural traits of up to 1.5‐year‐old seedlings of 15 sympatric Psychotria shrub species grown at three light levels. We used a 3D plant model to estimate the impacts of leaf traits, architectural traits and plant size on the whole‐plant light compensation point (LCPplant). Plant growth rates were estimated from destructive harvests and allometric relationships. At lower light levels, plants of all species achieved a lower leaf light compensation point (LCPleaf). The light interception efficiency (LIE), an index of self‐shading, decreased with increasing plant size and was therefore lower in high‐light treatments where plants grew more rapidly. When corrected for size, LIE was lower in the low‐light treatment, possibly as a result of lower investments in woody support. Species did not show trade‐offs in growth under low‐ and high‐light conditions, because species with the greatest plasticity in LCPplant and underlying traits (LCPleaf and LIE) achieved the highest growth rates at lower light levels. Synthesis. The interspecific differences in LCPplant did not result in a growth or survival trade‐off between low‐ and high‐light conditions. Instead, these differences were more than offset by the greater plasticity in LCPplant in some species, which was driven by greater plasticity in both leaves and architecture. The most plastic species achieved the fastest growth at different light levels. The results show that plasticity largely neutralizes the separation of light niches amongst species in this forest understorey genus and imply that differential preferences of species for either gaps or forest understorey occur in later life phases or are driven by other stress factors than low light alone.

Background and AimsThe objective of this study was to develop a simplified approach to simulate three-dimensional (3D) grapevine canopies and model light interception.Methods and ResultsA re-sampling procedure was developed to generate a set of allometric parameters based on their sample mean and distribution derived from shoots of field-grown vines grown in response to variable amounts of irrigation and N fertiliser. A large number of 3D models of grapevine canopies was reconstructed with the plant architecture model YPLANT based on re-sampled allometric parameters. Approximately 80% of intercepted light by the canopies was shown to be captured by 20-30% of the leaves, with deficit-irrigated and N-stressed plants having a greater proportion of leaf area exposed to high and moderate light intensity throughout the day compared with that of non-stressed vines. The amount of daily absorbed light increased with increasing leaf area per vine and was highly affected by row direction.ConclusionsModel predictions agreed with previously reported and measured amounts and patterns of light interception.Significance of the StudyThe approach described here simplifies previous attempts to represent the 3D architecture of a grapevine canopy, provides a good approximation to conditions vines may experience in the field and may be of practical use for field research and/or optimisation of canopy management practices in commercial production systems.

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 CO2 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.

• Plant light interception efficiency is a crucial determinant of carbon uptake by individual plants and by vegetation. Our aim was to identify whole-plant variables that summarize complex crown architecture, which can be used to predict light interception efficiency. • We gathered the largest database of digitized plants to date (1831 plants of 124 species), and estimated a measure of light interception efficiency with a detailed three-dimensional model. Light interception efficiency was defined as the ratio of the hemispherically averaged displayed to total leaf area. A simple model was developed that uses only two variables, crown density (the ratio of leaf area to total crown surface area) and leaf dispersion (a measure of the degree of aggregation of leaves). • The model explained 85% of variation in the observed light interception efficiency across the digitized plants. Both whole-plant variables varied across species, with differences in leaf dispersion related to leaf size. Within species, light interception efficiency decreased with total leaf number. This was a result of changes in leaf dispersion, while crown density remained constant. • These results provide the basis for a more general understanding of the role of plant architecture in determining the efficiency of light harvesting.

The study of how plants make use of sunflecks, relatively short duration but high-intensity patches of light in the understory and shaded tree canopy, has been of interest for decades. The potential ecological significance of sunflecks was recognized early in the 20th century (Allee 1926, Evans 1956), and the first few studies attempting to quantify the contribution of sunflecks to carbon gain in understories were undertaken at

The influence of salinity on the productivity of four plant species occurring or potentially occurring in Suisun Marsh was investigated with laboratory measurements of growth, photosynthetic responses and competitive interacti~ns, and field measurements of water relations, growth and carbohydrate reserves. Special emphasis was placed on the responses of Scirpus robustus and its competitors because of its importance as a waterfowl food source. The results showed that photosynthesis and. growth were much more reduced by increased salinity in Scirpus than in Spartina fo1iosa, Salicornia virginica or ~Cotula cornup i f oLia . However, at low salinities .Scirpus had the highest growth rates. Competition studies showed a strong shift in competitive advantage from Scirpus at low salinities to Salicornia at high salinities. Field studies showed that at most sites where Scirpus occurs, there is a strong seasonal change in salinity from nearly fresh water in the spring to high sun~er salinity levels. The rapid growth of Scirpus at the low salinities present during the spring may give it a competitive advantage over Salicornia. Scirpus also exhibited considerable tolerance to high salinities--defined here as an ability to survive even though growth was completely stopped. This tolerance may be important, since high summer salinities could prevent encroachment of less salt tolerant but highly competitive and less desirable species such as Scirpus acutus or Typha latifolia. We conclude that management plans designed to enhance Scirpus productivity should put as much weight on maintaining an annual cycle of salinity as reducing the yearly average salinity. VIATER RESOURCES CENfER ARCHIVES \ UNIVERSITY OF. CALifORNIA I ~KEJ:EY __ ._f

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.

P hysiological ecologists have long been concerned with photosynthesis, which incorporates carbon to provide plants with all their energy and structural building blocks. This carbon gain is an important aspect of plant performance in natural environments. The acquisition and use of other resources, such as nitrogen and water, are tightly linked to photosynthetic performance. For example, energy from photosynthesis is employed for nitrogen acquisition and reduction, and photosynthetic capacity is strongly coupled to leaf nitrogen content (Chapin et al., p. 49, this issue). Photosynthesis is also closely related to water movement in a plant. To facilitate absorption of carbon dioxide for use in photosynthesis, a leaf must have wet cell surfaces. As a consequence, transpiration uses nearly all of the water taken up

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.

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.

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.

4th International Workshop on Functional-Structural Plant Models, 7-11 june 2004 –Montpellier, France Edited by C. Godin et al., pp. 298-301 Invited talk Evaluating shrub architectural performance in sun and shade environments with the 3-D model Y-plant: are there optimal strategies? Robert W. Pearcy, Hiroyuki Muraoka, Fernando Valladares Section of evolution and Ecology University of California, Davis, CA 95616 Institute for Basin Ecosystem Studies, Gifu University, 1-1 Yanagido, 501–1193 Gifu, Japan Centro de Ciencias Medioambientales, C.S.I.C., Serrano 115 dpdo. 28006 Madrid, Spain

Four endemic Hawaiian Euphorbia species range in habitat from open arid coastal strand to shaded mesic forest and in growth-form from small prostrate shrubs to trees. As shown in the present study, these large differences in habitat and growth-form are paralleled by equally large differences in maximal photosynthetic rate (13.7 to 37.1 μmol CO2 m-2s-1), dark respiration rate (0.7 to 4.1 μmol CO2 m-2s-1), light level for saturation of photosynthesis (0.9 to over 2.0 mmol m-2s-1), light compensation point (0.01 to 0.11 mmol m-2s-1), leaf conductance to CO2 (1.7 to 4.9 mm s-1), and mesophyll conductance to CO2 (3.7 to 8.5 mm s-1). A principal consequence of this differentiation is that the capacity for photosynthesis at high light levels is higher in open site species, such as E. celastroides and E. degeneri, and at low light levels is higher in shade species, such as E. forbesii. E. hillebrandii, a species from intermediate semiopen habitats, exhibits an intermediate photosynthetic capacity at both high and low light levels. Despite this remarkable diversity, all four species exhibit the distinguishing physiological features of C4 photosynthesis.

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 (N area ) was influenced more by light availability than by leaf age in the gap saplings. In contrast, in understory saplings, N area 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 N area with leaf age depends on architecture and self-shading regimes that respond to changes in light regimes.

We used Y-plant, a computer-based model of crown architecture, to examine the implications of leaf reorientation resulting from petiole bending in Psychotria limonensis (Rubiaceae) seedlings. During this reorientation process, bending of the petioles of lower leaves that are potentially self-shaded by the upper leaves rotates the lamina around the stem's orthotropic axis so that self-shading is reduced. Simulations of daily light capture and assimilation revealed a 66% increase in daily C gain due to reorientation of the leaves as compared to simulations where the leaves remained in their characteristic opposite decussate pattern set by the phyllotaxy. This was due to enhanced carbon (C) gain of the lower leaves because of the reduction of shading by upper developing leaves in the same vertical plane. The light signal for this movement was experimentally examined by placing leaf-shaped filters above already fully expanded leaves and following the resulting shade-avoiding movements. The filters were either neutral density shade cloth that reduced the photon flux density (PFD) but did not alter the red to far red ratio (R:FR) or a film that reduced the PFD equivalently but also reduced the R:FR. Leaf reorientation was much more rapid and complete under the low R:FR as compared to the high R:FR indicating involvement of a phytochrome photosensory system that detected the presence of a shading leaf. Plants in gaps were found to lack a reorientation response indicating that the reorientation is specific to the shaded understory environment.

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.