Higher Light-saturated Photosynthetic Rate Research Articles

Advantages of growth and competitive ability of the invasive plant Solanum rostratum over two co-occurring natives and the effects of nitrogen levels and forms.

Atmospheric nitrogen (N) deposition has often been considered as a driver of exotic plant invasions. However, most related studies focused on the effects of soil N levels, and few on those of N forms, and few related studies were conducted in the fields. In this study, we grew Solanum rostratum, a notorious invader in arid/semi-arid and barren habitats, and two coexisting native plants Leymus chinensis and Agropyron cristatum in mono- and mixed cultures in the fields in Baicheng, northeast China, and investigated the effects of N levels and forms on the invasiveness of S. rostratum. Compared with the two native plants, S. rostratum had higher aboveground and total biomass in both mono- and mixed monocultures under all N treatments, and higher competitive ability under almost all N treatments. N addition enhanced the growth and competitive advantage of the invader under most conditions, and facilitated invasion success of S. rostratum. The growth and competitive ability of the invader were higher under low nitrate relative to low ammonium treatment. The advantages of the invader were associated with its higher total leaf area and lower root to shoot ratio compared with the two native plants. The invader also had a higher light-saturated photosynthetic rate than the two native plants in mixed culture (not significant under high nitrate condition), but not in monoculture. Our results indicated that N (especially nitrate) deposition may also promote invasion of exotic plants in arid/semi-arid and barren habitats, and the effects of N forms and interspecific competition need to be taken into consideration when studying the effects of N deposition on invasion of exotic plants.

Predicting ecosystem productivity based on plant community traits.

With the rapid accumulation of plant trait data, major opportunities have arisen for the integration of these data into predicting ecosystem primary productivity across a range of spatial extents. Traditionally, traits have been used to explain physiological productivity at cell, organ, or plant scales, but scaling up to the ecosystem scale has remained challenging. Here, we show the need to combine measures of community-level traits and environmental factors to predict ecosystem productivity at landscape or biogeographic scales. We show how theory can extend the production ecology equation to enormous potential for integrating traits into ecological models that estimate productivity-related ecosystem functions across ecological scales and to anticipate the response of terrestrial ecosystems to global change.

Multi-Stemmed Habit in Trees Contributes Climate Resilience in Tropical Dry Forest

Understanding how environmental adaptations mediate plant and ecosystem responses becomes increasingly important under accelerating global environmental change. Multi-stemmed trees, for example, differ in form and function from single-stemmed trees and may possess physiological advantages that allow for persistence during stressful climatic events such as extended drought. Following the worst drought in Hawaii in a century, we examined patterns of stem abundance and turnover in a Hawaiian lowland dry forest (LDF) and a montane wet forest (MWF) to investigate how multi-stemmed trees might influence site persistence, and how stem abundance and turnover relate to key functional traits. We found stem abundance and multi-stemmed trees to be an important component for climate resilience within the LDF. The LDF had higher relative abundance of multi-stemmed trees, stem abundance, and mean stem abundance compared to a reference MWF. Within the LDF, multi-stemmed trees had higher relative stem abundance (i.e., percent composition of stems to the total number of stems in the LDF) and higher estimated aboveground carbon than single-stemmed trees. Stem abundance varied among species and tree size classes. Stem turnover (i.e., change in stem abundance between five-year censuses) varied among species and tree size classes and species mean stem turnover was correlated with mean species stem abundance per tree. At the plot level, stem abundance per tree is also a predictor of survival, though mortality did not differ between multiple- and single-stemmed trees. Lastly, species with higher mean stem abundance per tree tended to have traits associated with a higher light-saturated photosynthetic rate, suggesting greater productivity in periods with higher water supply. Identifying the traits that allow species and forest communities to persist in dry environments or respond to disturbance is useful for forecasting ecological climate resilience or potential for restoration in tropical dry forests.

Intraspecific variation in stomatal traits, leaf traits and physiology reflects adaptation along aridity gradients in a South African shrub.

Trait-environment relationships are commonly interpreted as evidence for local adaptation in plants. However, even when selection analyses support this interpretation, the mechanisms underlying differential benefits are often unknown. This study addresses this gap in knowledge using the broadly distributed South African shrub Protea repens. Specifically, the study examines whether broad-scale patterns of trait variation are consistent with spatial differences in selection and ecophysiology in the wild. In a common garden study of plants sourced from 19 populations, associations were measured between five morphological traits and three axes describing source climates. Trait-trait and trait-environment associations were analysed in a multi-response model. Within two focal populations in the wild, selection and path analyses were used to test associations between traits, fecundity and physiological performance. Across 19 populations in a common garden, stomatal density increased with the source population's mean annual temperature and decreased with its average amount of rainfall in midsummer. Concordantly, selection analysis in two natural populations revealed positive selection on stomatal density at the hotter, drier site, while failing to detect selection at the cooler, moister site. Dry-site plants with high stomatal density also had higher stomatal conductances, cooler leaf temperatures and higher light-saturated photosynthetic rates than those with low stomatal density, but no such relationships were present among wet-site plants. Leaf area, stomatal pore index and specific leaf area in the garden also co-varied with climate, but within-population differences were not associated with fitness in either wild population. The parallel patterns of broad-scale variation, differences in selection and differences in trait-ecophysiology relationships suggest a mechanism for adaptive differentiation in stomatal density. Densely packed stomata may improve performance by increasing transpiration and cooling, but predominately in drier, hotter climates. This study uniquely shows context-dependent benefits of stomatal density--a trait rarely linked to local adaptation in plants.

Differential responses in photosynthesis, growth and biomass yields in two mulberry genotypes grown under elevated CO2 atmosphere

This study was aimed to examine the responses of two mulberry genotypes (Morus alba L.), which include a drought tolerant (DT) Selection-13 (S13) and a drought susceptible (DS) Kanva-2 (K2) grown under elevated atmospheric CO2 concentration ([CO2]) of 550μmolmol−1. Although both genotypes exhibited positive responses to elevated CO2, S13 showed higher light saturated photosynthetic rates (A′) and apparent quantum efficiency (AQE), suggesting better Rubisco carboxylation. Increased water use efficiency (WUEi) in elevated CO2 grown S13 (ES13) was due to reduced stomatal conductance (gs) and transpiration (E). Elevated CO2 significantly increased chlorophyll a fluorescence characteristics including maximum quantum yield of primary photochemistry (FV/FM) and performance index (PIABS) suggesting an improved photosystem-II efficiency in both genotypes compared to their respective controls. Even though ES13 showed superior photosynthetic performance, accumulation of soluble and insoluble sugars (starch) were significantly low compared to elevated CO2 grown K2 (EK2), demonstrating higher sink capacity in ES13, which in turn resulted in better biomass yields. We conclude that S13 could be a potential genotype for mulberry-based short rotation forestry (SRF) to mitigate increasing atmospheric [CO2] as well as for the production of carbon neutral renewable bio-energy.

Comparisons of plastic responses to irradiance and physiological traits by invasive Eupatorium adenophorum and its native congeners

To explore the traits contributing to invasiveness of Eupatorium adenophorum and to test the relationship between plasticity of these traits and invasiveness, we compared E. adenophorum with its two native congeners at four irradiances (10%, 23%, 40%, and 100%). The invader showed constantly higher performance (relative growth rate and total biomass) across irradiances than its native congeners. Higher light-saturated photosynthetic rate (Pmax), respiration efficiency (RE), and nitrogen (PNUE) and water (WUE, at 40% and 100% irradiances only) use efficiencies contributed directly to the higher performance of the invader. Higher nitrogen allocation to, stomatal conductance, and the higher contents of leaf nitrogen and pigments contributed to the higher performance of the invader indirectly through increasing Pmax, RE, PNUE and WUE. The invader had consistently higher plasticity only in carotenoid content than its native congeners in ranges of low (10–40%), high (40–100%) and total (10–100%) irradiances, contributing to invasion success in high irradiance by photo protection. In the range of low irradiances, the invader had higher plasticity in some physiological traits (leaf nitrogen content, nitrogen contents in bioenergetics, carboxylation and in light-harvesting components, and contents of leaf chlorophylls and carotenoids) but not in performance, while in the ranges of high or total irradiances, the invader did not show higher plasticity in any variable (except Car). The results indicated that the relationship between invasiveness and plasticity of a specific trait was complex, and that a universal generalization about the relationship might be too simplistic.

A quicker return energy-use strategy by populations of a subtropical invader in the non-native range: a potential mechanism for the evolution of increased competitive ability

Summary 1. Many studies testing the evolution of increased competitive ability hypothesis have focused on whether plants from invasive populations of a species show reduced allocation to defence and increased allocation to growth than plants from native populations. But few have attempted to understand ecophysiological mechanisms by which decreased allocation to defence may increase growth. 2. Previously, we found that invasive Ageratina adenophora plants increase nitrogen allocation to photosynthesis and reduce allocation to cell walls compared with native Ageratina plants, suggesting a shift from defence to growth in invasive populations. Here, carrying this work forward, we measured construction costs and benefits associated with photosynthesis at light saturation to leaves. We hypothesized that invasive Ageratina populations might employ a quicker return energyuse strategy by increasing light-saturated photosynthetic rates and photosynthetic energy-use efficiency (PEUE) and by decreasing leaf construction costs. 3. Faster-growing plants from invasive populations (China and India) had significantly higher leaf nitrogen concentrations and specific leaf areas than plants from native populations (Mexico). Inconsistent with our prediction, leaf construction costs were not significantly different between plants from invasive and native populations, but higher light-saturated photosynthetic rates and in turn higher PEUE resulted in a significantly shorter payback time of construction costs, which allowed plants from invasive populations to grow faster. 4. Synthesis. Our results indicate that Ageratina plants from populations in non-native ranges have a distinct quick return energy-use strategy, a high PEUE and a short payback time but not lower construction costs, which might provide a mechanistic explanation for the commonly observed increase in growth when plants are introduced to new parts of the world. To our knowledge, this is the first study to compare energy-use strategy for plants from invasive and native populations of a noxious invasive species. We cannot exclude some alternative hypotheses for these patterns, such as founder effects, but these ecophysiological differences might provide mechanistic insight for how the evolution of decreased allocation to defence may increase growth and competitive ability.

Invasive and native tall forms of Spartina alterniflora respond differently to nitrogen availability

The objective of this study was to compare various plant traits of native and invasive conspecifics of the tall form of Spartina alterniflora to test the genetic shift hypothesis, which predicts that due to genetic differences invasive plants may have different performance. We conducted a controlled experiment with six populations of the tall form of S. alterniflora: three transported from its introduced range in Jiangsu Province, China, and three from its native range in Georgia, USA. Plants were grown in a greenhouse and subjected to one of two levels of nitrogen availability for several months. Growth, biomass accumulation, biomass allocation, and photosynthetic and morphological traits, as well as the corresponding trait plasticities, were measured and compared between the populations. Nitrogen addition significantly affected individual traits of both invasive Jiangsu and native Georgia S. alterniflora populations, with invasive populations having stronger responses to nitrogen addition in RGR, total biomass, total leaf number (TLN), total leaf area (TLA) and maximum culm height (MxCH) than native populations according to the results of both reaction norms and the relative distance plasticity index (RDPI). Invasive S. alterniflora was more vigorous than its native conspecific as shown by greater total biomass and higher light-saturated photosynthetic rate (Pmax). Several morphological traits (MxCH, TLN and LAR) and most of the biomass allocation traits of invasive populations were found to differ from native populations. Our results indicated probable genetic shifts in plant traits in the introduced populations relative to the native populations of the tall form of S. alterniflora. Such genetic shifts may play a vital role in their success as invasive plants.

The effects of cleared larch canopy and nitrogen supply on gas exchange and leaf traits in deciduous broad-leaved tree seedlings

To understand the leaf-level responses of successional tree species to forest gap formation and nitrogen deposition, we performed canopy clearing and nitrogen-amendment treatments in larch plantations and investigated the changes in the light-use characteristics and the leaf structure of the invading deciduous broad-leaved tree seedlings. We hypothesized that the responses of the tree seedlings to clearing and nitrogen input would reflect specific traits in the shoot development that would be related to the species-specific successional characteristics. The gap phase species Magnolia hyporeuca Siebold et Zucc. and the mid-late successional tree species Quercus mongolica Fischer ex Ledeb. var. crispula (Blume) Ohashi., which grow in or near the forest gaps, had higher light-saturated photosynthetic rates (Psat), enhanced mesophyll surface area (Smes) and increased leaf mass per area (LMA) under both the clearing treatment and the clearing with nitrogen-amendment treatment. These two species therefore increased their Psat via an increase in Smes and LMA. The LMA values of the late successional tree species Prunus ssiori F. Schmidt and Carpinus cordata Blume, which grow in the forest understory, were enhanced by the clearing treatment. However, they displayed lesser responses to the clearing treatment under which there were no marked increases in Psat or Smes values in the second year. These results indicate distinct and varied responses to disturbance regimes among the four seral tree seedlings. The Psat value largely increased in line with the increase in Smes value during the second year in M. hyporeuca and Q. mongolica. The nitrogen supply accelerated the change in LMA and increased the Smes value in the leaves of Q. mongolica.

Growth, biomass allocation, morphology, and photosynthesis of invasive Eupatorium adenophorum and its native congeners grown at four irradiances

Eupatorium adenophorum is one of the more noxious invasive plants worldwide. However, the mechanisms underlying its invasiveness are still not well elucidated. In this study, we compared the invader with its two native congeners (E. heterophyllum and E. japonicum) at four irradiances in terms of growth, biomass allocation, morphology, and photosynthesis. The higher light-saturated photosynthetic rate (Pmax) and total leaf area of the invader may contribute to its higher relative growth rate (RGR) and total biomass compared with its native congeners. Total biomass and RGR increased significantly with the increase of Pmax and total leaf area. The higher support organ mass fraction and the lower root mass fraction of the invader may also contribute to its higher RGR and biomass through increasing carbon assimilation and reducing respiratory carbon loss, respectively. The higher growth rate of the invader increased its total leaf area, ramet number, and crown area. These traits may help the invader to form dense monoculture, outshading native plant species. However, consistently higher leaf area ratio, specific leaf area, and leaf mass fraction were not found across irradiances for the invader compared with its native congeners. Higher plasticity in response to irradiance was also not found for the invader. The invader retained advantages over the natives across irradiances, while its performance decreased with lower irradiance. The results indicate that the invader may be one of the few super invaders. Reducing irradiance may inhibit its invasions.

Photosynthesis, nitrogen allocation and specific leaf area in invasive Eupatorium adenophorum and native Eupatorium japonicum grown at different irradiances

The mechanisms underlying biological invasions are still not well elucidated. In this study, ecophysiological traits of invasive Eupatorium adenophorum and native E. japonicum were compared at 10 irradiances in field. I hypothesized that the invader may allocate a higher fraction of leaf nitrogen (N) to photosynthesis and have higher light-saturated photosynthetic rate (P(max)) and specific leaf area (SLA) than E. japonicum. The invader had a significantly higher ability to acclimate to high irradiance than E. japonicum, while it showed a similar shade-tolerant ability. The invader indeed allocated a higher fraction of leaf N to photosynthesis than E. japonicum, which, with its high leaf N content (N(A)), resulted in a higher N content in photosynthesis (N(P)), contributing to its higher biochemical capacity for photosynthesis and P(max). However, the invader had a significantly lower SLA than E. japonicum, contributing to its higher P(max) but increasing its area-based leaf construction cost. The abilities to acclimate to a wider range of irradiance and to allocate a higher fraction of leaf N to photosynthesis, and the higher P(max), N(A), N(P) and leaf area ratio may contribute to the invasion of the invader. High SLA is not always necessary for invasive species.

Nitrogen allocation and partitioning in invasive and native Eupatorium species

There is a trade-off between nitrogen (N) allocation to photosynthesis and to defence. Invasive species may reduce N allocation to defence because of the absence of natural enemies. Thus, I hypothesised that invasive species may allocate a higher fraction of total leaf N to photosynthesis and have higher light-saturated photosynthetic rate (P(max)) and photosynthetic N-use efficiency (PNUE) than closely related native species. To test these hypotheses, invasive Eupatorium adenophorum and native E. chinense and E. heterophyllum were compared in a limestone shrub. Unlike expectation, the invader did not allocate a higher fraction of leaf N to photosynthesis than the natives. However, it was more efficient in photosynthetic N partitioning than the natives. It partitioned a higher fraction of the photosynthetic N to carboxylation and showed higher use efficiency of the photosynthetic N, while the natives partitioned a higher fraction of the photosynthetic N to light-harvesting components. Total leaf N content was not significantly different among the three studied invasive and native species. For the invader, the higher fraction of leaf N allocated to carboxylation resulted in the higher N content in carboxylation and in both carboxylation and bioenergetics, which led to higher P(max), and therefore to higher PNUE, water-use efficiency, respiration efficiency and apparent quantum yield. These physiological advantages of the invader and its higher leaf area ratio may contribute to its invasiveness.

Optimizing the distribution of resources between enzymes of carbon metabolism can dramatically increase photosynthetic rate: a numerical simulation using an evolutionary algorithm.

The distribution of resources between enzymes of photosynthetic carbon metabolism might be assumed to have been optimized by natural selection. However, natural selection for survival and fecundity does not necessarily select for maximal photosynthetic productivity. Further, the concentration of a key substrate, atmospheric CO(2), has changed more over the past 100 years than the past 25 million years, with the likelihood that natural selection has had inadequate time to reoptimize resource partitioning for this change. Could photosynthetic rate be increased by altered partitioning of resources among the enzymes of carbon metabolism? This question is addressed using an "evolutionary" algorithm to progressively search for multiple alterations in partitioning that increase photosynthetic rate. To do this, we extended existing metabolic models of C(3) photosynthesis by including the photorespiratory pathway (PCOP) and metabolism to starch and sucrose to develop a complete dynamic model of photosynthetic carbon metabolism. The model consists of linked differential equations, each representing the change of concentration of one metabolite. Initial concentrations of metabolites and maximal activities of enzymes were extracted from the literature. The dynamics of CO(2) fixation and metabolite concentrations were realistically simulated by numerical integration, such that the model could mimic well-established physiological phenomena. For example, a realistic steady-state rate of CO(2) uptake was attained and then reattained after perturbing O(2) concentration. Using an evolutionary algorithm, partitioning of a fixed total amount of protein-nitrogen between enzymes was allowed to vary. The individual with the higher light-saturated photosynthetic rate was selected and used to seed the next generation. After 1,500 generations, photosynthesis was increased substantially. This suggests that the "typical" partitioning in C(3) leaves might be suboptimal for maximizing the light-saturated rate of photosynthesis. An overinvestment in PCOP enzymes and underinvestment in Rubisco, sedoheptulose-1,7-bisphosphatase, and fructose-1,6-bisphosphate aldolase were indicated. Increase in sink capacity, such as increase in ADP-glucose pyrophosphorylase, was also indicated to lead to increased CO(2) uptake rate. These results suggest that manipulation of partitioning could greatly increase carbon gain without any increase in the total protein-nitrogen investment in the apparatus for photosynthetic carbon metabolism.

In situ field measurements of photosynthetic rates of tropical tree species: a test of the functional group hypothesis

We examined photosynthetic characteristics of 21 tree species from a Panamanian forest differing in successional status. We hypothesized that functional guilds of species, grouped by successional status, would differ in photosynthetic performance and that pioneers would be more sensitive to seasonality and more variable in response to light than intermediate or shade tolerants. Steady-state leaf-level photosynthesis (A) was measured in situ on eight trees per species. Light response curves were generated by fitting a hyperbolic model to these data. Average light saturated photosynthetic rates (Amax) were then calculated for each species. Variability of light, photosynthesis, and leaf characteristics were quantified using coefficients of variation (CV). Significant differences were detected among species and functional groups for A, Amax, and leaf N concentration. Functional group explained 46% of the observed variation in A. Pioneers exhibited higher light-saturated photosynthetic rates than intermediates; both were higher than shade tolerants. Intermediates were the most seasonally plastic group and had the highest leaf N concentration. Shade tolerants were found in lower, more variable light environments than pioneers. A strong positive correlation between diameter growth rate and photosynthetic rate (r2= 0.55, p = 0.004) was observed across species. Our results tend to confirm the hypothesis that physiological traits can be used to differentiate among functional groups of plants. However, no evidence was found for higher plasticity of pioneer compared with shade-tolerant species.Key words: tropical trees, physiological plasticity, photosynthesis, nitrogen, growth, tropical succession.

Sex-specific physiology and source-sink relations in the dioecious plant Silene latifolia.

Differences in reproductive demands between the sexes of dioecious plants could cause divergence in physiology between the sexes. We found that the reproductive effort of female Silene latifolia plants increased to more than twice that of male plants or female plants that were prevented from setting fruit by lack of pollination after 4 weeks of flowering. Whole-plant source/sink ratios of pollinated females were significantly lower than those of males or unpollinated females because of investment in fruit. We hypothesized that these differences in source/sink ratio between the sexes and within females, depending on pollination, would lead to differences in leaf photosynthetic rates. Within females, we found that photosynthetic capacity was consistent with measurement of whole-plant source/sink ratio. Females that were setting fruit had 30% higher light-saturated photosynthetic rates by 28 days after flowering than females that were not setting fruit. Males, however, had consistently higher photosynthetic rates than females from 10 days after flowering onwards. Males also had approximately twice the dark respiration rates of fruiting females. We found that female reproductive structures are longer-lived and contribute more carbon to their own support than male reproductive structures. Despite the higher rates of leaf dark respiration and lower calyx photosynthetic rates, males fix more carbon than do females. We conclude that females have a sink-regulated mechanism of photosynthesis that allows them to respond to variations in fruit set. This mechanism is not, however, sufficient to explain why male S. latifolia plants have higher rates of photosynthesis, higher source/sink ratios, and lower reproductive allocation, but fail to grow larger than female plants.

Photosynthetic Gas Exchange Responses of Australian Tropical Forest Trees in Canopy, Gap and Understory Micro-Environments

The photosynthetic responses of three tropical tree species were investigated in the contrasting micro-environments of the canopy, a gap and the understory of a tropical forest in north-eastern Australia. Measurements were made with a small transportable field gas exchange apparatus on the evergreen species Argyrodendron peralatum (F.M. Bailey) Edlin ex I.H. Boas and Castanospermum apstrale (Cunn.) Frazer and Cook, which regenerate in the understory and the deciduous gap species Toona australis (F. Muell.) Harms. Acclimation to the prevailing light environment was evident in all species, with the canopy leaves having higher light saturated photosynthetic rates, dark respiration rates and stomatal conductances than the understory leaves. For A. peralatum, leaves in the gap were generally intermediate between those in the canopy and understory. There was no evidence for acclimation to the differences in temperature or leaf-air vapour pressure deficits between the canopy and understory leaves. Determinations of the daily courses of CO2 uptake showed that although the daily photon flux density (PFD) in the understory was only 3% of that at the canopy top, leaf carbon gain was nearly 10% of that measured for the canopy leaf. In the understory, photosynthesis during sunflecks accounted for about one third of the daily carbon gain. There was no evidence that temperatures or leaf-air vapour pressure deficits significantly limited CO2 uptake in the canopy. Key-words: Photosynthesis, stomatal conductance, acclimation, light environment, tropical forest, understory, canopy, Argyrodendron peralatum, Toona australis, Castanospermum australe * Current address: Professor R.W. Pearcy, Department of Botany, University of California, Davis, California 95616, USA. Introduction Tropical forests consist of a mosaic of contrasting micro-environments including those of the canopy, the understory and gaps created by tree falls. These micro-environments have been characterized in several forests (Akoi, Yabuki & Koyama, 1975; Odum, Drewry & Kline, 1970; Allen, Lemon & Muller, 1972) but to date relatively little attention has been given the physiological responses to them. Gas exchange responses have been reported for saplings and herbs in understories (Bjorkman, Ludlow & Morrow, 1972; Pearcy & Calkin, 1983) and for shrubs in gaps vs understory habitats (Lebron, 1979; Walters & Field, 1987) but there have been no measurements reported for the canopy. Tree species that germinate in the understory but ultimately reach the' canopy are typically exposed to this wide range of environments and thus the capacity to acclimate photosynthetic responses to these different conditions may be an important characteristic. Laboratory studies have shown that most, but apparently not all, tropical forest tree species can acclimate their photosynthetic apparatus to highand lowlight environments (Langenheim et al., 1984; Oberbauer & Strain, 1984; Fetcher et al., 1986; Pearcy & Francheschi, 1986). However, there have been few field studies in contrasting tropical forest micro-environments where other environmental stresses may influence the extent of acclimation to light. In temperate forests, large spatial and temporal variations in photosynthetic capacity have been reported (Fuchs, Schulze & Fuchs, 1977; Schulze, Fuchs & Fuchs, 1977ab). In this study, the photosynthetic gas exchange of tree species in an Australian tropical forest were investigated in canopy, understory and gap microenvironments in order to examine the contrasts in the physiological and environmental controls of photosynthetic CO2 exchange in these environments. Three species were selected for study. Argyrodendron peralatum (F.M. Bailey) Edlin ex I.H. Boas is a canopy-emergent evergreen having sclerophyllous trifoliate leaves with leaflets 4-6 cm wide and 15-25 cm long. Seedlings of this This content downloaded from 207.46.13.122 on Thu, 19 May 2016 04:52:06 UTC All use subject to http://about.jstor.org/terms

Photosynthetic responses of C3 and C4 species from cool shaded habitats in Hawaii.

The C4 species, Euphorbia forbesii, and the C3 species, Claoxylon sandwicense, occupy cool, shaded habitats in Hawaii. Both of these species exhibit the photosynthetic characteristics of typical shade plants: low light-saturated photosynthetic rates, low dark respiration rates, low light levels for saturation of photosynthesis, and low light compensation points. In addition, the quantum yields of the two species are similar at leaf temperatures near 22°C, reflecting a significant increase in the quantum yield of E. forbesii over that of C4 species from open habitats. C. sandwicense has a lower dark respiration rate than E. forbesii. Hence, since the quantum yields of the two species are similar at cool temperatures, C. sandwicense has a higher photosynthetic rate than E. forbesii at low incident photon flux densities. As a consequence, C. sandwicense should have a greater carbon gain than E. forbesii under the diffuse radiation conditions of their native habitat. However, since E. forbesii has a higher light-saturated photosynthetic rate than C. sandwicense, E. forbesii may have a greater carbon gain than C. sandwicense during sunflecks.

Phytoplankton photosynthetic response to vertical movement in a mixed layer

14C uptake was measured in experimental bottles suspended at a series of fixed depths (using subsamples from one sample depth), and was compared with estimates obtained from bottles provided with vertical movement similar to that which might occur in Langmuir circulations. In most experiments, the vertically cycled bottles gave estimates of integral photosynthesis 19 to 87% higher than estimates from the control series. Evidence from laboratory experiments detailing the change in photosynthesis over time at various irradiance levels shows that a higher lightsaturated photosynthetic rate under variable light conditions could account for the enhanced integral photosynthesis found in the bottles provided with vertical movement.