Common-garden studies on adaptive radiation of photosynthetic physiology among Hawaiian lobeliads

Species in an adaptive radiation often occupy different habitats so that individuals of each species develop under different conditions. Showing that a radiation is adaptive thus requires evidence that taxa have diverged genetically and that each has an ecological advantage in using particular habitats or resources, taking into account both phenotypic plasticity and phylogenetic relationships among species. Here, we use a common-garden experiment to show that representative species of Hawaiian lobeliads have diverged adaptively in their leaf-level photosynthetic light responses. Across species, plants genetically shifted their photosynthetic physiology with native light regime in accord with theoretical predictions and exhibited adaptive crossover in net carbon gain—that is, species native to a given light regime outperformed others only under conditions similar to those they occupy in the field, with the rank order of species based on photosynthesis per unit leaf mass changing with light level. These findings make a powerful case for adaptation of photosynthetic light responses to native light regimes and, combined with our earlier field studies, provide the strongest demonstration to date for the evolution of divergent adaptations for energy capture in any group of closely related plants.     

Fluctuation of photosynthetic photon flux density within aMiscanthus sinensis canopy

Detailed measurements of diurnal variations in photosynthetic photon flux density (PPFD) were made at seven locations within the canopy of aMiscanthus sinensis grassland to evaluate the light conditions of microsites for heliophilic tree seedlings. Multiple regression analysis revealed that the short-term light fluctuation on a clear day was highly dependent on the wind speed and solar elevation angle, whereas on a cloudy day it was mainly determined by the PPFD incident from above the canopy. The relative PPFD at 40 cm aboveground varied from 0.065 to 0.252, depending on sky conditions and the sensor's position in relation to clumped patches ofM. sinensis. On a clear day, the proportion of PPFD readings above 100 μmol·m⁻²·s⁻¹ contributed by sunflecks ranged between 25.4% and 82.0%. Computer simulation showed that the contribution of sunflecks to the daily carbon gain ofQuercus serrata seedlings may range from 11% to 65%. The mean relative PPFD measured under diffuse light conditions was linearly related to the daily total PPFD and the daily carbon gain by single leaves ofQ. serrata seedlings. This suggests that the relative PPFD under diffuse light conditions provides an appropriate measure of site-specific light availability within a grass canopy.     

CHROMOSOME NUMBERS AND THEIR SYSTEMATIC IMPLICATIONS IN HAWAIIAN LOBELIOIDEAE (CAMPANULACEAE)

Chromosome numbers are reported for 20 collections of Hawaiian Lobelioideae (Campanulaceae), representing six genera, 13 species, and two interspecific hybrids. All are n = 14. Chromosome numbers are reported for the first time for eight species of Clermontia, Cyanea, Delissea, Lobelia, and Trematolobelia; the report for Delissea is the first for that genus. Additional determinations confirmed previously reported numbers in five other species of Brighamia, Clermontia, and Cyanea. Chromosome numbers are now known for all seven genera and 20 of the 110 species. All accepted counts are n = 14. It is suggested that all Hawaiian Lobelioideae share this number and are paleotetraploid. There is no evidence that the prolific speciation evident among these plants was accompanied by euploid or aneuploid change in chromosome number. The Hawaiian Lobelioideae, particularly the monophyletic lineage of 91 baccate species, offer further support for the generalization that change in chromosome number is an uncommon mode of speciation in insular floras.     

Physiological response to complex environments in annual Polygonum species of contrasting ecological breadth

Individual physiological response to complex environments is a major factor in the ecological breadth of species. This study compared individual patterns of both long-term and short-term response to controlled, multifactorial environments in four annual Polygonum species that differ in field distribution (P. cespitosum, P. hydropiper, P. lapathifolium, and P. persicaria). To test long-term response, instantaneous net photosynthetic rate and stomatal conductance were measured in situ on one full-sib replicate from five inbred lineages from each of five field populations per species, raised in all possible combinations of low or high light; dry, moist, or flooded soil; and poor or rich nutrient status. Short-term plastic adjustment to changes in light level was examined by switching individual plants of the four species from one of six multifactorial growth environments to the contrasting light environment, and measuring assimilation rates 1 h after transfer. The Polygonum species differed significantly in their patterns of long-term photosynthetic response to particular resources and resource combinations. The species known to have relatively broad ecological distributions (P. persicaria and P. lapathifolium) maintained high photosynthetic performance in a variety of moisture and nutrient environments when grown in high light, while the more narrowly distributed P. hydropiper maintained such functional levels only if given both high light and ample macronutrients. P. cespitosum, a species limited to shaded habitats, maintained low photosynthetic rates across the environmental range. Complex differences among the species in instantaneous water use efficiency (WUE) reflected their highly specific and to some extent independent patterns of photosynthetic and stomatal response to the multifactorial environments. The species also differed significantly in short-term physiological adjustment to changes in light level. Plants of P. persicaria and P. cespitosum reached 78% and 98%, respectively, of their maximum photosynthetic rates 1 h after transfer from low to high light, but P. hydropiper and P. lapathifolium plants reached only c. 60% of their maximum rates. When switched from high to low light, P. persicaria and P. cespitosum plants maintained 64–76% of their maximum rates, while P. hydropiper and P. lapathifolium plants decreased photosynthetic rates sharply to less than 50% of their maximum rates. These results indicate that the latter two species will be less able to maintain effective functional levels in variable light environments, a result consistent with their distributions in the field. Received: 23 May 1997 / Accepted: 3 March 1998     

Adaptive radiation of photosynthetic physiology in the Hawaiian lobeliads: light regimes, static light responses, and whole-plant compensation points

Six endemic genera/sections of lobeliads (Campanulaceae) occupy nearly the full range of light regimes on moist sites in the Hawaiian Islands, from open alpine bogs and seacliffs to densely shaded rainforest interiors. To determine whether this clade has undergone a corresponding adaptive radiation in photosynthetic adaptations, we studied the natural light habitats and physiological characteristics of 11 species representing each sublineage. Across species in the field, average photon flux density (PFD) varies from 2.3 to 30.0 mol · m(-2) · d(-1), and maximum assimilation rate (A(max)) ranges from 0.17 to 0.35 μmol CO(2) · g(-1) · s(-1). Across species, A(max), dark respiration rate (R), Michaelis-Menten constant (k), light compensation point, specific leaf area (SLA), maximum carboxylation rate (V(cmax)), maximum rate of electron transport (J(max)), photosynthesis at saturating CO(2) (A(satCO(2))), and carboxylation efficiency (α) all increase significantly and in tightly coupled fashion with PFD, in accord with classical economic theory. Area-based rates have a higher degree of physiological integration with each other and tighter coupling to PFD than the corresponding mass-based rates, despite the energetic importance of the latter. Area-based rates frequently show adaptive cross-over: high-light species outperform low-light species at high PFD and vice versa at low PFD. A(max)-mass has little relationship to leaf mass per unit area (LMA), leaf N content, or leaf lifespan individually, but a multiple regression explains 96% of the variance in A(max)-mass across species in terms of SLA, leaf N content, and average PFD. Instantaneous leaf compensation points range from 0.1 to 1.2% full sunlight, far lower than the ecological (whole-plant) compensation points (ECPs) of 1.1 to 29.0% sunlight calculated based on photosynthetic parameters, leaf longevity, and allocation to leaf vs. nonleaf tissue. The ECPs are much closer to the lower limits of PFD actually experienced by lobeliads, suggesting they may play an important role in restricting species distributions. Taken together, these data provide evidence for an adaptive radiation in photosynthetic traits that is strongly correlated with-and indeed may help determine-the light regime that each species inhabits.     

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

Summary 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.     

Origin, adaptive radiation and diversification of the Hawaiian lobeliads (Asterales: Campanulaceae)

The endemic Hawaiian lobeliads are exceptionally species rich and exhibit striking diversity in habitat, growth form, pollination biology and seed dispersal, but their origins and pattern of diversification remain shrouded in mystery. Up to five independent colonizations have been proposed based on morphological differences among extant taxa. We present a molecular phylogeny showing that the Hawaiian lobeliads are the product of one immigration event; that they are the largest plant clade on any single oceanic island or archipelago; that their ancestor arrived roughly 13 Myr ago; and that this ancestor was most likely woody, wind-dispersed, bird-pollinated, and adapted to open habitats at mid-elevations. Invasion of closed tropical forests is associated with evolution of fleshy fruits. Limited dispersal of such fruits in wet-forest understoreys appears to have accelerated speciation and led to a series of parallel adaptive radiations in Cyanea, with most species restricted to single islands. Consistency of Cyanea diversity across all tall islands except Hawai ;i suggests that diversification of Cyanea saturates in less than 1.5 Myr. Lobeliad diversity appears to reflect a hierarchical adaptive radiation in habitat, then elevation and flower-tube length, and provides important insights into the pattern and tempo of diversification in a species-rich clade of tropical plants.     

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

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

Carbon dioxide exchange of C3 and C4 tree species in the understory of a Hawaiian forest

Summary Field measurements of photosynthetic CO₂ exchange were made on saplings of a C₄ tree species, Euphorbia forbesii, and a C₃ tree species, Claoxylon sandwicense, in a shaded mesic forest on Oahu, Hawaii. Both species had light responses typical of those generally found in shade plants. Light saturated photosynthetic rates were 7.15 and 4.09 mol m² s¹ and light compensation points were 6.3 and 1.7 mol m² s¹ in E. forbesii and C. sandwicense, respectively. E. forbesii maintained a higher mesophyll conductance and a higher water use efficiency than C. sandwicense as is typically found in comparisons of C₄ and C₃ plants. Under natural light regimes, both species maintained positive CO₂ uptake rates over essentially the entire day because of low respiration rates and light compensation points. However, photosynthesis during sunflecks accounted for a large fraction of the daily carbon gain. The results show that the carbon-gaining capacity of E. forbesii is comparable to that of a C₃ species in a moderately cool, shaded forest environment. There appears to be no particular advantage or disadvantage associated with the C₄ photosynthetic pathway of E. forbesii in this environment.     

Natural selection on ecophysiological traits of a fern species in a temperate rainforest

Unlike other species of the genus Blechnum, the fern Blechnum chilense occurs in a wide range of habitats in Chilean temperate rainforest, from shaded forest understories to abandoned clearings and large gaps. We asked if contrasting light environments can exert differential selection on ecophysiological traits of B. chilense. We measured phenotypic selection on functional traits related to carbon gain: photosynthetic capacity (A _max), dark respiration rate (R _d), water use efficiency (WUE), leaf size and leaf thickness in populations growing in gaps and understorey environments. We assessed survival until reproductive stage and fecundity (sporangia production) as fitness components. In order to determine the potential evolutionary response of traits under selection, we estimated the genetic variation of these traits from clonally propagated individuals in common garden experiments. In gaps, survival of B. chilense was positively correlated with WUE and negatively correlated with leaf size. In contrast, survival in shaded understories was positively correlated with leaf size. We found positive directional fecundity selection on WUE in gaps population. In understories, ferns of lower R _d and greater leaf size showed greater fecundity. Thus, whereas control of water loss was optimized in gaps, light capture and net carbon balance were optimized in shaded understories. We found a significant genetic component of variation in WUE, R _d and leaf size. This study shows the potential for evolutionary responses to heterogeneous light environments in functional traits of B. chilense, a unique fern species able to occupy a broad successional niche in Chilean temperate rainforest.     

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

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

The presence and distribution of Antho-RFamide-like material in scyphomedusae

Summary The nervous systems of the scyphomedusae Chrysaora hysoscella, Cyanea capillata and Cyanea lamarckii (Phylum Cnidaria) were stained using an antiserum against the anthozoan neuropeptide Antho-RFamide. Staining was widespread in all three species. In Chrysaora, the antiserum revealed nerve nets in the subumrella and exumbrella ectoderm, in both faces of the oral lobes, and in the endoderm lining the subumbrella and exumbrella surfaces of the gastric cavity. The most prominent staining occurred in a dense plexus of neurons in the ectoderm at the base of the tentacles. This nerve net sent projections into the subumbrella ectoderm. For the most part, staining in the two species of Cyanea was similar to that in Chrysaora, with a few exceptions. These include the presence, in Cyanea, of an obvious tentacular nerve tract and nerve nets associated with clusters of cnidocytes in the tentacles. Radioimmunoassays of extracts from Chrysaora and Cyanea lamarkii revealed that both species contain large amounts of Antho-RFamide-like material (up to 55 nmol/animal). The results indicate that Antho-RFamide-like neuropeptides are widespread in scyphomedusae.     

Environmental characteristics,field water relations,and photosynthetic responses of C4 Hawaiian Euphorbia species from contrasting habitats

Summary 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 mol CO₂ m^-2s^-1), dark respiration rate (0.7 to 4.1 mol 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.     

Relative enhancement of photosynthesis and growth at elevated CO2 is greater under sunflecks than uniform irradiance in a tropical rain forest tree seedling

The survivorship of dipterocarp seedlings in the deeply shaded understorey of South‐east Asian rain forests is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain. To investigate the effect of elevated CO₂ upon photosynthesis and growth under sunflecks, seedlings of Shorealeprosula were grown in controlled environment conditions at ambient or elevated CO₂. Equal total daily photon flux density (PFD) (～7·7 mol m^?2 d^?1) was supplied as either uniform irradiance (～170 µmol m^?2 s^?1) or shade/fleck sequences (～30 µmol m^?2 s^?1/～525 µmol m^?2 s^?1). Photosynthesis and growth were enhanced by elevated CO₂ treatments but lower under flecked irradiance treatments. Acclimation of photosynthetic capacity occurred in response to elevated CO₂ but not flecked irradiance. Importantly, the relative enhancement effects of elevated CO₂ were greater under sunflecks (growth 60%, carbon gain 89%) compared with uniform irradiance (growth 25%, carbon gain 59%). This was driven by two factors: (1) greater efficiency of dynamic photosynthesis (photosynthetic induction gain and loss, post‐irradiance gas exchange); and (2) photosynthetic enhancement being greatest at very low PFD. This allowed improved carbon gain during both clusters of lightflecks (73%) and intervening periods of deep shade (99%). The relatively greater enhancement of growth and photosynthesis at elevated CO₂ under sunflecks has important potential consequences for seedling regeneration processes and hence forest structure and composition.     

Effects of high irradiances on photosynthesis,growth and crassulacean acid metabolism in the epiphyteKalanchoö uniflora

Summary Kalancho? uniflora was grown in the glasshouse with and without shading. Chlorophyll content, area/FW ratio and specific leaf area were higher in leaves of shaded as compared to unshaded plants. Light saturation curves and continuous gas exchange measurements showed that the apparent quantum yield and the light-saturated photosynthetic rate were higher in shaded plants. Shaded plants had lower “mesophyll resistances” than unshaded plants, indicating that the different photosynthetic capacities reflected different contents of ribulose biphosphate carboxylase-oxygenase. Highlight treatment of plants grown in the shade resulted in a decreased photosynthetic efficiency, showing that these plants were sensitive to photoinhibition. However, dry matter production was higher in unshaded than in shaded plants. Obviously the difference in irradiance between the two growth regimes did more than offset the differences in photosynthetic efficiency. Applying additional nutrients did not alter the effects of high PFDs. The results are discussed in respect to photosynthetic performence and plant distribution in the epiphytic habitat.     

Photosynthetic responses of Microstegium vimineum (Trin.) A. Camus, a shade-tolerant, C4 grass, to variable light environments

Microstegium vimineum (Trin.) A. Camus, a shade-tolerant C₄ grass, has spread throughout the eastern United States since its introduction in 1919. This species invades disturbed understory habitats along streambanks and surrounding mesic forests, and has become a major pest in areas such as Great Smoky Mountains National Park. The focus of this study was to characterize the photosynthetic induction responses of M. vimineum, specifically its ability to utilize low light and sunflecks, two factors that may be critical to invasive abilities and survival in the understory. In addition, we were curious about the ability of a grass with the C₄ photosynthetic pathway to respond to sunflecks. Plants were grown under 25% and 50% ambient sunlight, and photosynthetic responses to both steady-state and variable light were determined. Plants grown in both 25% and 50% ambient sun became 90% light saturated between 750–850 μmol m⁻² s⁻¹; however, plants grown in 50% ambient sun had significantly higher maximum steady-state photosynthetic rates (16.09 ± 1.37 μmol m⁻² s⁻¹ vs. 12.71 ± 1.18 μmol m⁻² s⁻¹). Both groups of plants induced to 50% of the steady-state rate in 3–5 min, while it took 10–13 min to reach 90% of maximum rates, under both flashing and steady-state light. For both groups of plants, stomatal conductance during induction reached maximum rates in 6–7 min, after which rates decreased slightly. Upon return to low light, rates of induction loss and stomatal closure were very rapid in both groups of plants, but were more rapid in those grown in high light. Rapid induction and the ability to induce under flashing light may enable this species to invade and dominate mesic understory habitats, while rapid induction loss due to stomatal closure may prevent excess water loss when low light constrains photosynthesis. The C₄ pathway itself does not appear to present an insurmountable barrier to the ability of this grass species to respond to sunflecks in an understory environment. Received: 21 February 1997 / Accepted: 10 October 1997     

Steady and dynamic photosynthetic responses of seedlings from contrasting successional groups under low‐light growth conditions

To test the hypothesis that leaf‐level photosynthetic‐related traits might confer late successionals a competitive advantage over early successionals in low‐light growth conditions, steady photosynthetic assimilation‐ and dynamic photosynthetic induction‐related traits were examined in low‐light‐grown seedlings with contrasting successional status. Compared with the early successionals, late successionals as a group significantly exhibited lower leaf gas exchange rates. While late successionals required a longer time to respond to simulated sunflecks, they had lower rates of induction losses after sunflecks. Such photosynthetic induction traits allowed late successionals to more effectively utilize subsequent sunflecks. It was observed that plants with lower gas exchange rates responded more slowly to simulated sunfelcks, but they had lower rates of induction losses after sunflecks. In addition, the rate of response to sunflecks was positively correlated with the rate of induction loss after sunflecks across the successional status of species. A principal components analysis (PCA) demonstrated that early and late successionals were separated along the first axis of the PCA, and that early successionals were grouped on the right and were associated with higher gas exchange rates, fast responses to sunflecks, and rapid rates of induction loss after sunflecks; late successionals held an opposite pattern. Overall, our results suggest that smaller respiratory carbon losses and lower metabolic costs give late successionals a competitive advantage in low‐light growth conditions, that late successionals have an advantage over early successionals in utilizing sunflecks, and thus that the successional status of species are mainly associated with the leaf‐level photosynthetic‐related traits.     

<Emphasis Type="Italic">Lantana camara</Emphasis> L.: a weed with great light-acclimation capacity

Plant invasions may be limited by low radiation levels in ecosystems such as forests. Lantana camara has been classified among the world’s 10 worst weeds since it is invading many different habitats all around the planet. Morphological and physiological responses to different light fluxes were analyzed. L. camara was able to acclimate to moderately shaded environments, showing a high phenotypic plasticity. Morphological acclimation to low light fluxes was typified by increasing leaf size, leaf biomass, leaf area index and plant height and by reduced stomatal density and leaf thickness. Plants in full sunlight produced many more inflorescences than in shaded conditions. Physiological acclimation to low radiation levels was shown to be higher stomatal conductance, higher net photosynthetic rates and higher efficiency of photosystem II (PSII). L. camara behaves as a facultative shade-tolerant plant, being able to grow in moderately sheltered environments, however its invasion could be limited in very shady habitats. Control efforts in patchy environments should be mainly directed against individuals in open areas since that is where the production of seeds would be higher and the progress of the invasion would be faster.     

Photosynthetic responses to light variation in rainforest species

Summary The dependence of net carbon gain during lightflecks (artificial sunflecks) on leaf induction state, lightfleck duration, lightfleck photosynthetic photon flux density (PFD), and the previous light environment were investigated in A. macrorrhiza and T. australis, two Australian rainforest species. The photosynthetic efficiency during lightflecks was also investigated by comparing observed values of carbon gain with predicted values based on steady-state CO₂ assimilation rates. In both species, carbon gain and photosynthetic efficiency increased during a series of five 30-or 60-s lightflecks that followed a long period of low light; efficiency was linearly related to leaf induction state.In fully-induced leaves of both species, efficiency decreased and carbon gain increased with lightfleck duration. Low-light grown A. macrorrhiza had greater efficiency than predicted based on steady-state rates (above 100%) for lightflecks less than 40 s long, whereas leaves grown in high light had efficiencies exceeding 100% only during 5-s lightflecks. The efficiency of leaves of T. australis ranged from 58% for 40-s lightflecks to 96% for 5-s lightflecks.In low-light grown leaves of A. macrorrhiza, photosynthetic responses to lightflecks below 120 mol m^-2 s^-1 were not affected significantly by the previous light level. However, during lightflecks at 530 mol m^-2 s^-1, net carbon gain and photosynthetic efficiency of leaves previously exposed to low light levels were significantly reduced relative to those of leaves previously exposed to 120 and 530 mol m^-2 s^-1.These results indicate that, in shade-tolerant species, net carbon gain during sunflecks can be enhanced over values predicted from steady-state CO₂ assimilation rates. The degree of enhancement, if any, will depend on sunfleck duration, previous light environment, and sunfleck PFD. In forest understory environments, the temporal pattern of light distribution may have far greater consequences for leaf carbon gain than the total integrated PFD.Supported by National Science Foundation Grant BSR 8217071 and USDA Grant 85-CRCR-1-1620     

Patterns of dynamic irradiance affect the photosynthetic capacity and growth of dipterocarp tree seedlings

In the deeply shaded understorey of S.E. Asian rain forests the growth and survival of dipterocarp seedlings is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain in understorey plants. To test the sensitivity of photosynthesis and growth to variation in the pattern of dynamic irradiance, dipterocarp tree seedlings (Shorea leprosula and Hopea nervosa) were grown for 370 days under shaded forest light treatments of equal total daily photosynthetic photon flux density (approximately 3.3 mol m(-2) day(-1)), but characterised by either long flecks (LF) or short flecks (SF). Seedling growth was more than 4-fold greater under LF, compared with SF, in both species. Variation in the relative growth rates (RGR) and light saturated rates of photosynthesis (A(max)) were strongly positively correlated with the mean duration of sunflecks. Variation in RGR was strongly correlated with greater unit leaf rate growth, indicating that photosynthetic carbon gain per unit leaf area was greater under LF. The accumulation of starch in leaves over the diurnal period was 117% greater in both species under LF, compared with SF. Greater carbon gain in seedlings under LF is likely to have resulted from the combination of (1) greater A(max) (S. leprosula 35%, H. nervosa 40%), (2) more efficient dynamic photosynthesis, and (3) greater incident photosynthetic quantum yield, compared with seedlings receiving the SF irradiance treatment. The pattern of dynamic irradiance received by seedlings may significantly impact their growth and survival to a previously unrecognised extent, with important consequences for regeneration processes and hence forest structure and composition.