Photosynthesis in flashing light in soybean leaves grown in different conditions. I. Photosynthetic induction state and regulation of ribulose-1,5-bisphosphate carboxylase activity

The photosynthetic induction state under conditions of different lightfleck frequencies or durations, or different shade periods was studied in soybean leaves in order to examine how it might limit utilization of sunflecks in leaf canopies. Induction following an increase in photon flux density (PFD) from strongly limiting to saturating PFDs exhibited two phases; a fast-inducing one, requiring about 1 min and a slow one, requiring up to 60 min for completion. Transfer of fully induced leaves to low light resulted in a rapid decrease in the fast-inducing component, a slower decrease in the slow-inducing component and an even slower decrease in stomatal conductance. Therefore, the decreases in extent of induction appeared to be due to biochemical factors and not to stomatal closure. Under flashing light regimes consisting of 1-s lightflecks given at different frequencies for long periods, a constant induction state was achieved, the measure of induction state increased with the frequency of the lightflecks. This constant induction state also depended on the growth conditions, with shade leaves having a higher value than those grown at high light at any particular lightfleck frequency. The measure of induction state was mostly lower in flashing light as compared to constant light of the same mean PFD, particularly in leaves with a low light saturation point and in short lightflecks. Initial activities of ribulose-1,5-bisphosphate carboxylase (rubisco) were also higher in continuous light and were highly correlated with the measure of induction state. The rapid decrease in extent of induction of soybean leaves during shade periods is an important limitation to the ability of the leaves to respond to light increases similar to those occurring with sunflecks. At least part of the limitation on carbon assimilation during sunflecks due to photosynthetic induction is based on regulation of rubisco activity.     

Photosynthesis in flashing light in soybean leaves grown in different conditions. II. Lightfleck utilization efficiency

Leaves of soybean plants grown in contrasting light and nutrient availability conditions were exposed to constant and to flashing light regimes with lightflecks of different frequencies, durations and photon flux density (PFD). The lightfleck characteristics were selected to be representative of the range of variation found for sunflecks in a soybean canopy. CO₂ fixation rates were measured using a fast-response gas-exchange apparatus. The net CO₂ fixation due to 1-s-duration lightflecks was 1·3 times higher than predicted from steady-state measurements in constant light at the lightfleck and background PFD. This lightfleck utilization efficiency (LUE) was somewhat higher at a high than at a low frequency of one second lightflecks. LUE in flashing light with very short lightflecks (0·2s) and single 1 s lightflecks was as high as 2, but decreased sharply with increasing duration of lightflecks. This decrease occurred because CO₂ fixation rates during lightflecks were constrained by carbon metabolism and induction limitations, and because the contribution of post-illumination CO₂ fixation to total CO₂ fixation decreased with increased duration of lightflecks. LUE increased with increased PFD during the lightflecks, particularly in leaves from plants grown in high-light, high-nutrient conditions. Saturation PFDs were much higher in flashing light than in constant light. Only small differences in LUE were apparent between leaves from the three growth conditions.     

Leaf gas exchange of beech (Fagus sylvatica L.) seedlings in lightflecks: effects of fleck length and leaf temperature in leaves grown in deep and partial shade

Summary Responses of leaf gas exchange in shade and half-shade grown seedlings of the European beech, Fagus sylvatica L., to constant light conditions indicate different phases of photosynthetic induction: an immediate, a fast and a subsequent slow phase. The slow phase has both biochemical and stomatal components. The higher the induction, the higher the lightfleck utilization efficiency (LUE) attributable to a lightfleck. LUE can be higher than 100% compared to a theoretical instantaneous response. Lightfleck quantum yield (total carbon gain attributable to a lightfleck per incident quantum density in the fleck) is highest in short pulses of light. Post-illumination carbon gain initially increases with fleck length, levelling off above 20 s. The lower the induction, the longer carbon is fixed post-illuminatively (up to 84 s) but the less carbon is gained. Shade leaves are induced much faster than partial shade leaves. They utilize series of lightflecks to become fully induced, while half-shade (and sun) leaves depend on continuous high light. Half-shade leaves lose induction faster in low light between lightflecks. High as well as low temperatures strongly delay induction in half-shade but not in shade leaves. In general, shade leaves are much better adapted to the dynamic light environment of the forest understorey; however, their water-use efficiency during induction is lower.Dedicated to Prof. O. L. Lange on the occasion of his 65th birthday     

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     

The role of enzyme activation state in limiting carbon assimilation under variable light conditions

The mechanisms regulating transient photosynthesis by soybean (Glycine max) leaves were examined by comparing photosynthetic rates and carbon reduction cycle enzyme activities under flashing (saturating 1 s lightflecks separated by low photon flux density (PFD) periods of different durations) and continuous PFD. At the same mean PFD, the mean photosynthetic rates were reduced under flashing as compared to continuous light. However, as the duration of the low PFD period lengthened, the CO₂ assimilation attributable to a lightfleck increased. This enhanced lightfleck CO₂ assimilation was accounted for by a greater postillumination CO₂ fixation occurring after the lightfleck. The induction state of photosynthesis, ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), fructose 1,6-bisphosphatase (FBPase) and ribulose 5-phosphate kinase (Ru5P kinase) activities all responded similarly and were all lower under flashing as compared to constant PFD of the same integrated mean value. However, the fast phase of induction and FBPase and Ru5P kinase activities were reduced more than were the slow phase of induction and rubisco activity. This was consistent with the role of the former enzymes in the fast induction component that limited RuBP regeneration. Competition for reducing power between carbon metabolism and thioredoxin-mediated enzyme activation may have resulted in lower enzyme activation states and hence lower induction states under flashing than continuous PFD, especially at low lightfleck frequencies (low mean PFD).Abbreviations FBPase fructose 1,6-bisphosphatase (EC 3.1.3.11) - LUE lightfleck use efficiency - P-glycerate 3-phosphoglycerate - PICF post-illumination CO₂ fixation - Ru5P kinase ribulose 5-phosphate kinase (EC 2.7.1.19) - RuBP ribulose 1,5-bisphosphate - rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) - SBpase sedoheptulose 1,7-bisphosphatase (EC 3.1.3.37)     

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

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

亚热带常绿阔叶林下富贵草 （Pachysandra Terminalis）对模拟光斑的光合响应（英文）

 以亚热带常绿阔叶林下一种常见的灌木富贵草(Pachysandra terminalis）为研究对象,利用气体交换和叶绿素荧光技术研究了其对模拟光斑的光合响应。在同样辐射通量（非光抑制）的情况下,光合诱导过程的快速组分时间内光斑可以提高富贵草对光斑的利用能力（光斑诱导的碳同化量可高出对照48%）。叶绿素荧光测量结果表明：1）光斑与光斑之间的暗期发生了qN弛豫过程;2）暗期之后的光期光化学能量转换效率提高。这两个原因可能是快速组分时间内光斑诱导富贵草的碳同化量提高的主要原因之一。强光光斑簇可以诱导富贵草光抑制     

Stomatal dynamics and its importance to carbon gain in two rainforest Piper species

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

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

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

Acclimation of photosynthesis to lightflecks in tomato leaves: interaction with progressive shading in a growing canopy

Plants in natural environments are often exposed to fluctuations in light intensity, and leaf‐level acclimation to light may be affected by those fluctuations. Concurrently, leaves acclimated to a given light climate can become progressively shaded as new leaves emerge and grow above them. Acclimation to shade alters characteristics such as photosynthetic capacity. To investigate the interaction of fluctuating light and progressive shading, we exposed three‐week old tomato (Solanum lycopersicum ) plants to either lightflecks or constant light intensities. Lightflecks of 20 s length and 1000 μmol m^?2 s^?1 peak intensity were applied every 5 min for 16 h per day, for 3 weeks. Lightfleck and constant light treatments received identical daily light sums (15.2 mol m^?2 day^?1). Photosynthesis was monitored in leaves 2 and 4 (counting from the bottom) during canopy development throughout the experiment. Several dynamic and steady‐state characteristics of photosynthesis became enhanced by fluctuating light when leaves were partially shaded by the upper canopy, but much less so when they were fully exposed to lightflecks. This was the case for CO₂‐saturated photosynthesis rates in leaves 2 and 4 growing under lightflecks 14 days into the treatment period. Also, leaf 2 of plants in the lightfleck treatment showed significantly faster rates of photosynthetic induction when exposed to a stepwise change in light intensity on day 15. As the plants grew larger and these leaves became increasingly shaded, acclimation of leaf‐level photosynthesis to lightflecks disappeared. These results highlight continuous acclimation of leaf photosynthesis to changing light conditions inside developing canopies.     

Characteristics of transient photosynthesis in Quercus serrata seedlings grown under lightfleck and constant light regimes

Photosynthetic-induction response and light-fleck utilization were investigated for the current-year seedlings of Quercus serrata, a deciduous tree found in temperate regions of Japan. The tree seedlings were grown under three light regimes: a constant low photosynthetic photon flux density (PFD) regime of 50 mol m^–2 s^–1, a constant high PFD regime of 500 mol m^–2 s^–1, and a lightfleck regime with alternated low (lasting 5 s) and high (lasting 35 s) PFD. The photosynthetic-induction response following a sudden increase of PFD from 50 to 500 mol m^–2 s^–1 exhibited two phases: an initial fast increase complete within 3–5 s, and a second slow increase lasting for 15–20 min. Induction times required to reach 50% and 90% of steady-state assimilation rates were significantly shorter in leaves from the constant low PFD than those from the high PFD regime. During the first 60–100 s, the ratio of observed integrated CO₂ uptake to that predicted by assuming that a steady-state assimilation would be achieved instantaneously after the light increase was significantly higher for leaves from the low PFD regime than from the high PFD regime. Lightfleck utilization was examined for various durations of PFD of 500 mol m^–2 s^–1 on a background PFD of 50 mol m^–2 s^–1. Lightfleck utilization efficiency was significantly higher in low PFD leaves than in the high PFD leaves for 5-s and 10-s lightflecks, but showed no difference among different light regimes for 100-s lightflecks. The contribution of post-illumination CO₂ fixation to total carbon gain decreased markedly with increasing lightfleck durations, but exhibited no significant difference among growth regimes. Photosynthetic performances of induction response and lightfleck utilization in leaves from the lightfleck regime were more similar to those in leaves from the low PFD regime. It may be the total daily PFD rather than PFD dynamics in light regimes that affects the characteristics of transient photosynthesis in Q. serrata seedlings.     

Light use efficiency of California redwood forest understory plants along a moisture gradient

We investigated photosynthesis of five plant species growing in the understory at three sites (1,170-, 1,600- and 2,100-mm annual moisture inputs), along the geographical range of coastal California redwood forest, to determine whether greater inputs of rain and fog at northern sites enhance photosynthetic utilization of fluctuating light. Measurements of understory light environment and gas exchange were carried out to determine steady state and dynamic photosynthetic responses to light. Leaf area index ranged from 4.84 at the 2,100-mm site to 5.98 at the 1,170-mm site. Maximum rates of net photosynthesis and stomatal conductance (g) did not vary appreciably within species across sites. Photosynthetic induction after a change from low to high light was significantly greater in plants growing in lower light conditions regardless of site. Photosynthetic induction also increased with the rate of g in diffuse light, prior to the increase to saturating light levels. Post-illumination CO₂ assimilation was the largest factor contributing to variation in C gain during simulated lightflecks. The duration of post-illumination photosynthetic activity, total CO₂ assimilation per light received, and light use efficiency during simulated lightflecks increased significantly with moisture inputs in four out of five species. Increasing leaf N concentration with increasing moisture inputs in three out of five species, coupled with changes in leaf N isotopic composition with the onset of the summer fog season suggest that natural N deposition increases with rain and fog inputs and contributes to greater utilization of fluctuating light availability in coastal California redwood forests.     

Modelling photosynthesis in fluctuating light with inclusion of stomatal conductance, biochemical activation and pools of key photosynthetic intermediates

Photosynthetic carbon gain in rapidly fluctuating light is controlled by stomatal conductance, activation of ribulose-1,5-bisphosphate carboxylase-oxygenase, a fast induction step in the regeneration of ribulose-1,5-bisphosphate, and the build-up of pools of photosynthetic intermediates that allow post-illumination CO₂ fixation. Experimental work over recent years has identified and characterised these factors. A physiologically-based dynamic model is described here that incorporates these factors and allows the simulation of carbon gain in response to any arbitrary sequence of light levels. The model output is found to conform well to previously reported plant responses of Alocasia macrorrhiza (L.) G. Don. observed under widely differing conditions. The model shows (i) responses of net assimilation rate and stomatal conductance to constant light levels and different CO₂ concentrations that are consistent with experimental observations and predictions of a steady-state model; (ii) carbon gain to continue after the end of lightflecks, especially in uninduced leaves; (iii) carbon gain to be only marginally reduced during low-light periods of up to 2 s; (iv) a fast-inducing component in the regeneration of ribulose-1,5-bisphosphate to be limiting for up to 60 s after an increase in light in uninduced leaves: the duration of this limitation lengthens with increasing CO₂ concentration and is absent at low CO₂ concentration; (v) oxygen evolution to exceed CO₂ fixation during the first few seconds of a lightfleck, but CO₂ fixation to continue after the end of the lightfleck whereas oxygen evolution decreases to low-light rates immediately. The model is thus able to reproduce published responses of leaves to a variety of perturbations. This provides good evidence that the present formulation of the model includes the essential rate-determining factors of photosynthesis under fluctuating light conditions. Received: 27 January 1997 / Accepted: 15 April 1997     

Modeling dynamic understory photosynthesis of contrasting species in ambient and elevated carbon dioxide

Dynamic responses of understory plants to sunflecks have been extensively studied, but how much differences in dynamic light responses affect daily photosynthesis (A_day) is still the subject of active research. Recent models of dynamic photosynthesis have provided a quantitative tool that allows the critical assessment of the importance of these sunfleck responses on A_day. Here we used a dynamic photosynthesis model to assess differences in four species that were growing in ambient and elevated CO₂. We hypothesized that Liriodendron tulipifera, a species with rapid photosynthetic induction gain and slow induction loss, would have the least limitations to sunfleck photosynthesis relative to the other three species (Acer rubrum, Cornus florida, Liquidambar styraciflua). As a consequence, L. tulipifera should have the highest A_day in an understory environment, despite being the least shade tolerant of the species tested. We further hypothesized that daily photosynthetic enhancement by elevated CO₂ would differ from enhancement levels observed during light-saturated, steady-state measurements. Both hypotheses were supported by the model results under conditions of low daily photosynthetic photon flux density (PFD; <3% of the above-canopy PFD). However, under moderate PFD (10-20% of the above-canopy PFD), differences in dynamic sunfleck responses had no direct impact on A_day for any of the species, since stomatal and photosynthetic induction limitations to sunfleck photosynthesis were small. Thus, the relative species ranking in A_day under moderate PFD closely matched their rankings in steady-state measurements of light-saturated photosynthesis. Similarly, under elevated CO₂, enhancement of modeled A_day over A_day at ambient CO₂ matched the enhancement measured under light saturation. Thus, the effects of species-specific differences in dynamic sunfleck responses, and differences in elevated CO₂ responses of daily photosynthesis, are most important in marginal light environments.     

Photosynthetic responses to short-term and long-term light variation in <Emphasis Type="Italic">Pinus sylvestris</Emphasis> and <Emphasis Type="Italic">Salix dasyclados</Emphasis>

Pinus sylvestris and Salix dasyclados, which differ in leaf longevity, were compared with respect to four aspects of photosynthetic light use and response: high light acclimation, photoinhibition resistance and recovery, lightfleck exposure and use and chloroplast acclimation across leaves. The first two aspects were examined using seedlings under controlled conditions and the other two were tested using trees in the field. When exposed to high light, shade leaves of Pinus acclimated completely, achieving the same photosynthetic capacities as sun leaves, whereas shade leaves of Salix did not reach sun leaf capacities although the absolute magnitude of their acclimation was larger. Shade leaves of Pinus were also more resistant to photoinhibition than those of Salix. Much of the direct light supplied within the canopy was in the form of rapid fluctuations, lightflecks, for Pinus and Salix alike. They exploited short lightflecks with similar efficiency. The greater proportion of diffuse light in the canopy for Pinus than Salix seems to lead to a lesser degree of differential intra-leaf acclimation of chloroplasts, in turn leading to lower efficiency of photosynthesis under unilateral light as reflected by a lower convexity, rate of bending, of the light–response curve. The differences in light use and responses are discussed in relation to possible differences in characteristics of the long and short-lived leaf.     

RELATIONS BETWEEN SUNFLECK SEQUENCES AND PHOTOINHIBITION OF PHOTOSYNTHESIS IN A TROPICAL RAIN FOREST UNDERSTORY HERB

We report the photosynthetic characteristics of a C₃ shade plant native to the tropical rain forest understory. It was shown that Elatostema repens Lour. (Hall) f. (Urticaceae) presents a large light adjustment capacity. The effects of several lightfleck sequences on photoinhibition of photosynthesis and carbon gain are analyzed. Photoinhibition is measured both as a decrease in leaf net CO₂ uptake in limiting light (shown to be linearly correlated to quantum yield of O₂ evolution measured at saturating CO₂) and as a decrease of the ratio of variable fluorescence (Fv) to maximum fluorescence (Fmax) measured in liquid nitrogen. It is shown that lightflecks (from 10 to 30 min in duration) of 700 μmol m^–2 s^–1 (high light) induce photoinhibition, and that the effects of those successive high light periods are additive; there is apparently no recovery from photoinhibition during the low light periods (from 10 to 45 min in duration). In contrast, the Fv/Fmax ratio, though decreasing similarly to quantum yield of net CO₂ uptake on leaves submitted to a continuous illumination of 700 μmol m^–2 s^–1, is only decreased a little on leaves submitted to lightfleck sequences of the same photon flux density. Lightflecks of 250 μmol m^–2 s^–1 are not photoinhibitory. Compared to the control maintained under light growth condition (40 μmol m^–2 s^–1) carbon gain is increased on leaves submitted to lightflecks; this gain remains high throughout the light cycles on leaves submitted to nonphotoinhibitory lightflecks and to the photoinhibitory lightflecks followed by the shortest low light period. In the other cases, carbon gain, higher than that of the control at the beginning of the treatments, decreases and becomes lower than the control carbon gain. Finally, the relevance of photoinhibition in the tropical rain forest understory environment is discussed.     

Photosynthetic responses to variable light: a comparison of species from contrasting habitats

Photosynthetic responses to variable light were compared for species from habitats differing in light availability and dynamics. Plants were grown under the same controlled conditions and were analysed for the kinetics of photosynthetic induction when photon flux density (PFD) was increased from 25 to 800 mol m^-2s^-1. Gas exchange techniques were used to analyse the two principal components of induction, opening of stomata and activation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). On average, 90% of the final photosynthetic rate was attained after 7 min for obligate shade plants (two species), 18 min for fast-growing sun plants (seven species from productive habitats) and 32 min for slow-growing sun plants (nine species from unproductive habitats). The rapidity of response of the shade plants was explained by stomata remaining more open in the low-light period prior to induction. This was also observed in two species of deciduous trees, which therefore resembled shade plants rather than other fast-growing sun plants. The slow response of the slow-growing sun plants was the result of lower rates of both Rubisco activation and stomatal opening, the latter being more important for the final phase of induction. The lower rate of Rubisco activation was confirmed by direct, enzymatic measurements of representative plants. With increasing leaf age, the rate of stomatal opening appeared to decrease but the rate of Rubisco activation was largely conserved. Representative species were also compared with respect to the efficiency of using light-flecks relative to continuously high light. The shade plants and the slow-growing sun plants had a higher efficiency than the fast-growing sun plants. This could be related to the presence of a higher electron transport capacity relative to carboxylation capacity in the former group, which seems to be associated with their lower photosynthetic capacities. Representative species were also compared with respect to the ability to maintain the various induction components through periods of low light. Generally, the fast-growing sun plants were less able than the other two categories to maintain the rapidly reversible component. Thus, although the rate of induction appears to be related to the ecology of the plant, other aspects of photosynthetic dynamics, such as the efficiency of using lightflecks and the ability to maintain the rapidly reversible component, seem rather to be inversely related to the photosynthetic capacity.     

Photosynthetic sunfleck utilization potential of understory saplings growing under elevated CO<Subscript>2</Subscript> in FACE

Few studies have evaluated elevated CO₂ responses of trees in variable light despite its prevalence in forest understories and its potential importance for sapling survival. We studied two shade-tolerant species (Acer rubrum, Cornus florida) and two shade-intolerant species (Liquidambar styraciflua, Liriodendron tulipifera) growing in the understory of a Pinus taeda plantation under ambient and ambient+200 ppm CO₂ in a free air carbon enrichment (FACE) experiment. Photosynthetic and stomatal responses to artificial changes in light intensity were measured on saplings to determine rates of induction gain under saturating light and induction loss under shade. We expected that growth in elevated CO₂ would alter photosynthetic responses to variable light in these understory saplings. The results showed that elevated CO₂ caused the expected enhancement in steady-state photosynthesis in both high and low light, but did not affect overall stomatal conductance or rates of induction gain in the four species. Induction loss after relatively short shade periods (<6 min) was slower in trees grown in elevated CO₂ than in trees grown in ambient CO₂ despite similar decreases in stomatal conductance. As a result leaves grown in elevated CO₂ that maintained induction well in shade had higher carbon gain during subsequent light flecks than was expected from steady-state light response measurements. Thus, when frequent sunflecks maintain stomatal conductance and photosynthetic induction during the day, enhancements of long-term carbon gain by elevated CO₂ could be underestimated by steady-state photosynthetic measures. With respect to species differences, both a tolerant, A. rubrum, and an intolerant species, L. tulipifera, showed rapid induction gain, but A. rubrum also lost induction rapidly (c. 12 min) in shade. These results, as well as those from independent studies in the literature, show that induction dynamics are not closely related to species shade tolerance. Therefore, it cannot be concluded that shade-tolerant species necessarily induce faster in the variable light conditions common in understories. Although our study is the first to examine dynamic photosynthetic responses to variable light in contrasting species in elevated CO₂, studies on ecologically diverse species will be required to establish whether shade-tolerant and -intolerant species show different photosynthetic responses in elevated CO₂ during sunflecks. We conclude that elevated CO₂ affects dynamic gas exchange most strongly via photosynthetic enhancement during induction as well as in the steady state. Received: 1 April 1999 / Accepted: 16 August 1999     

Photosynthetic Responses to Dynamic Light Environments by Hawaiian Trees : Time Course of CO(2) Uptake and Carbon Gain during Sunflecks

Gas exchange responses to rapid changes in light were studied in a C₃ tree, Claoxylon sandwicense Muell-Arg and a C₄ tree, Euphorbia forbesii Sherff that are native to the understory of a mesic Hawaiian forest. When light was increased to 500 micromoles per meter per second following a 2 hour preexposure at 22 micromoles per meter per second, net CO₂ uptake rates and stomatal conductance gradually increased for over 1 hour in C. sandwicense but reached maximum values within 30 minutes in E. forbesii. Calculation of the intercellular CO₂ pressures indicated that the primary limitation to CO₂ uptake during this induction was nonstomatal in both species. The photosynthetic response to simulated sunflecks (lightflecks) was strongly dependent on the induction state of the leaf. Total CO₂ uptake during a lightfleck was greater and the response was faster after exposure of the leaf to high light than when the leaf had been exposed only to low light for the previous 2 hours. During a series of lightflecks, induction resulted in increased CO₂ uptake in successive lightflecks. Significant postillumination CO₂ fixation was evident and contributed substantially to the total carbon gain, especially for lightflecks of 5 to 20 seconds' duration.