Light adaptation/acclimation of photosynthesis and the regulation of ribulose-1,5-bisphosphate carboxylase activity in sun and shade plants

The consequences of light adaptation and acclimation of photosynthesis on photosynthetic nitrogen use efficiency (NUE), particularly as it relates to the efficiency of ribulose-1,5-bisphosphate carboxylase (Rubisco) use in photosynthetic CO₂ assimilation, was studied in the sun species Glycine max and the shade species Alocasia macrorrhiza. Both G. max and A. macrorrhiza were found to possess the capacity for light acclimation of CO₂ assimilation, but over distinctly different ranges of photon flux density (PFD). For each species, light acclimation of photosynthesis had little effect on the rate of photosynthesis per unit Rubisco protein or the light response of Rubisco carbamylation and CA 1P metabolism. In contrast, photosynthesis per unit Rubisco protein was significantly higher in G. max than in A. macrorrhiza, due in part to a lower total (fully carbamylated) molar activity (activity per unit enzyme) of A. macrorrhiza Rubisco than that of G. max. Comparison of the light response of Rubisco regulatory mechanisms between G. max and A. macrorrhiza indicated some degree of adaptation, such that carbamylation was higher and CA 1P levels lower at lower PFDs in the shade species than the sun species. However, this adjustment was not sufficient for Rubisco in low light grown A. macrorrhiza to be fully active at the growth PFD. Photosynthesis in A. macrorrhiza appeared to become RuBP regeneration-limited at lower PFDs than G. max, and this was probably the determinant of the light saturated rate of photosynthesis in the shade species. The low efficiency of Rubisco use in A. macrorrhiza was a major contributing factor to its five- to sixfold lower photosynthetic NUE than G. max. Shade species such as A. macrorrhiza appear to make far from maximal use of Rubisco protein N.     

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

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

Scaling sun and shade photosynthetic acclimation of Alocasia macrorrhiza to whole-plant performance – II. Simulation of carbon balance and growth at different photon flux densities

A whole-plant carbon balance model incorporating a light acclimation response was developed for Alocasia macrorrhiza based on empirical data and the current understanding of light acclimation in this species. The model was used to predict the relative growth rate (RGR) for plants that acclimated to photon flux density (PFD) by changing their leaf type, and for plants that produced only sun or shade leaves regardless of PFD. The predicted RGR was substantially higher for plants with shade leaves than for those with sun leaves at low PFD. However, the predicted RGR was not higher, and in fact was slightly lower, for plants with sun leaves than for those with shade leaves at high PFD. The decreased leaf area ratios (LARs) of the plants with sun leaves counteracted their higher photosynthetic capacities per unit leaf area (A_max). The model was manipulated by changing parameters to examine the sensitivity of RGR to variation in single factors. Overall, RGR was most sensitive to LAR and showed relatively little sensitivity to variation in A_max or maintenance respiration. Similarly, RGR was relatively insensitive to increases in leaf life-span beyond those observed. Respiration affected RGR only at low PFD, whereas A_max was moderately important only at high PFD.     

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 significance of differences in the mechanisms of photosynthetic acclimation to light, nitrogen and CO2 for return on investment in leaves

1. We report changes in photosynthetic capacity of leaves developed in varying photon flux density (PFD), nitrogen supply and CO₂ concentration. We determined the relative effect of these environmental factors on photosynthetic capacity per unit leaf volume as well as the volume of tissue per unit leaf area. We calculated resource-use efficiencies from the photosynthetic capacities and measurements of leaf dry mass, carbohydrates and nitrogen content.
2. There were clear differences between the mechanisms of photosynthetic acclimation to PFD, nitrogen supply and CO₂. PFD primarily affected volume of tissue per unit area whereas nitrogen supply primarily affected photosynthetic capacity per unit volume. CO₂ concentration affected both of these parameters and interacted strongly with the PFD and nitrogen treatments.
3. Photosynthetic capacity per unit carbon invested in leaves increased in the low PFD, high nitrogen and low CO₂ treatments. Photosynthetic capacity per unit nitrogen was significantly affected only by nitrogen supply.
4. The responses to low PFD and low nitrogen appear to function to increase the efficiency of utilization of the limiting resource. However, the responses to elevated CO₂ in the high PFD and high nitrogen treatments suggest that high CO₂ can result in a situation where growth is not limited by either carbon or nitrogen supply. Limitation of growth at elevated CO₂ appears to result from internal plant factors that limit utilization of carbohydrates at sinks and/or transport of carbohydrates to sinks.     

Regulation of Ribulose-1,5-Bisphosphate Carboxylase Activity in Alocasia macrorrhiza in Response to Step Changes in Irradiance

The regulation of ribulose-1,5-bisphosphate (RuBP) carboxylase (Rubisco) activity and pool sizes of RuBP and P-glycerate were examined in the tropical understory species Alocasia macrorrhiza following step changes in photon flux density (PFD). Previous gas exchange analysis of this species following a step increase in PFD from 10 to 500 micromoles quanta per square meter per second suggested that the increase in photosynthetic rate was limited by the rate of increase of Rubisco activity for the first 5 to 10 minutes. We demonstrate here that the increase in photosynthetic rate was correlated with an increase in both the activation state of Rubisco and the total k_cat (fully activated specific activity) of the enzyme. Evidence presented here suggests that a change in the pool size of the naturally occurring tight binding inhibitor of Rubisco activity, 2-carboxyarabinitol 1-phosphate, was responsible for the PFD-dependent change in the total k_cat of the enzyme. RuBP pool size transiently increased after the increase in PFD, indicating that photosynthesis was limited by the capacity for carboxylation. After 5 to 10 minutes, RuBP pool size was again similar to the pool size at low PFD, presumably because of the increased activity of Rubisco. Following a step decrease in PFD from 500 to 10 micromoles quanta per square meter per second, Rubisco activity declined but at a much slower rate than it had increased in response to a step increase in PFD. This slower rate of activity decline than increase was apparently due to the slower rate of 2-carboxyarabinitol 1-phosphate synthesis than degradation and, to a lesser degree, to slower deactivation than activation. RuBP pool size initially declined following the decrease in PFD, indicating that RuBP regeneration was limiting photosynthesis. As Rubisco activity decreased, RuBP slowly increased to its original level at high PFD. The slow rate of activity loss by Rubisco in this species suggests a biochemical basis for the increased efficiency for CO₂ assimilation of successive lightfleck use by species such as A. macrorrhiza.     

Scaling sun and shade photosynthetic acclimation of Alocasia macrorrhiza to whole-plant performance – I. Carbon balance and allocation at different daily photon flux densities

We determined the carbon allocation patterns and construction costs of Alocasia macrorrhiza plants grown at different photon flux densities (PFD) as well as the whole-plant carbon gain of these plants at different daily PFDs. Growth at high PFD resulted in thicker leaves with a higher leaf mass per unit area, and increased biomass allocation to petioles and roots, as compared to growth at low PFD. Increased allocation to petioles may have been necessary to support the heavier leaves, whereas increased allocation to roots may have been necessary to supply sufficient water for the higher transpiration rates in high PFD. Root biomass was highly correlated with the daily, whole-plant transpiration rate. Tissue construction costs per unit dry mass were unchanged by acclimation, but, since the mass per unit areas of leaves, roots and petioles all increased, construction costs per unit leaf area were much higher for plants grown at high PFD. On a per unit leaf area basis, daily whole-plant carbon gain measured at high daily PFD was higher in high- than in low-PFD-grown plants. However, on a per unit leaf mass basis, low-PFD-grown plants had a daily carbon gain at least as high as that of high-PFD-grown plants at high daily PFD. At low daily PFD, low-PFD-grown plants maintained an advantage over high-PFD-grown plants in terms of carbon gain because of their larger leaf area ratios. Thus, in terms of carbon gain, low-PFD-grown plants performed better than sun plants at low PFD and as well as high-PFD-grown plants at high PFD, despite their lower photosynthetic capacities per unit area. For high-PFD-grown plants, the higher construction costs per unit leaf area resulted in lower leaf area ratios, which counteracted the advantage of higher photosynthetic rates per unit leaf area.     

Improving models of photosynthetic thermal acclimation: Which parameters are most important and how many should be modified?

Photosynthetic temperature acclimation could strongly affect coupled vegetation–atmosphere feedbacks in the global carbon cycle, especially as the climate warms. Thermal acclimation of photosynthesis can be modelled as changes in the parameters describing the direct effect of temperature on photosynthetic capacity (i.e., activation energy, E_a; deactivation energy, H_d; entropy parameter, ΔS) or the basal value of photosynthetic capacity (i.e., photosynthetic capacity measured at 25°C). However, the impact of acclimating these parameters (individually or in combination) on vegetative carbon gain is relatively unexplored. Here we compare the ability of 66 photosynthetic temperature acclimation scenarios to improve the ability of a spatially explicit canopy carbon flux model, MAESTRA, to predict eddy covariance data from a loblolly pine forest. We show that: (1) incorporating seasonal temperature acclimation of basal photosynthetic capacity improves the model's ability to capture seasonal changes in carbon fluxes and outperforms acclimation of other single factors (i.e., E_a or ΔS alone); (2) multifactor scenarios of photosynthetic temperature acclimation provide minimal (if any) improvement in model performance over single factor acclimation scenarios; (3) acclimation of E_a should be restricted to the temperature ranges of the data from which the equations are derived; and (4) model performance is strongly affected by the H_d parameter. We suggest that a renewed effort be made into understanding whether basal photosynthetic capacity, E_a, H_d and ΔS co‐acclimate across broad temperature ranges to determine whether and how multifactor thermal acclimation of photosynthesis occurs.     

Light acclimation, CO2 response and long-term capacity of underwater photosynthesis in three terrestrial plant species

To characterize underwater photosynthetic performance in some terrestrial plants, we determined (i) underwater light acclimation (ii) underwater photosynthetic response to dissolved CO₂, and (iii) underwater photosynthetic capacity during prolonged submergence in three species that differ in submergence tolerance: Phalaris arundinacea, Rumex crispus (both submergence-tolerant) and Arrhenatherum elatius (submergence-intolerant). None of the species had adjusted to low irradiance after 1 week of submergence. Under non-submerged (control) conditions, only R. crispus displayed shade acclimation. Submergence increased the apparent quantum yield in this species, presumably because of the enhanced CO₂ affinity of the elongated leaves. In control plants of the grass species P. arundinacea and A. elatius, CO₂ affinities were higher than for R. crispus. The underwater photosynthetic capacity of R. crispus increased during 1 month of submergence. In P. arundinacea photosynthesis remained constant during 1 month of submergence at normal irradiance; at low irradiance a reduction in photosynthetic capacity was observed after 2 weeks, although there was no tissue degeneration. In contrast, underwater photosynthesis of the submergence-intolerant species A. elatius collapsed rapidly under both irradiances, and this was accompanied by leaf decay. To describe photosynthesis versus irradiance curves, four models were evaluated. The hyperbolic tangent produced the best goodness-of-fit, whereas the rectangular hyperbola (Michaelis-Menten model) gave relatively poor results.     

Ecophysiological studies of sonoran Desert plants

Summary Photosynthetic responses to light and temperature are compared for two genetically related chromosomal races of Machaeranthera gracilis. The ancestral foothills race occurs in cooler, more mesic environments, while the derived desert race occurs in more arid environments. The desert race exhibited greater rates of net photosynthesis at all levels of irradiance than did plants of the foothills race. This enhancement is due primarily to a greater quantum yield and leaf density. High light pretreatment significantly increased the photosynthetic capacity of the desert race, with little or no effect on the foothills race. Furthermore, the leaf density of the desert race was affected proportionately more than the foothills race by light pretreatment. The desert race also possessed higher stomatal and mesophyll conductances to CO₂. Both races exhibited enhanced photosynthetic capacities when grown in a warm thermoperiod (35/25 C), relative to a cool thermoperiod (25/15 C), concomitant with slight increases in leaf density. There was a lack of thermal acclimation for both photosynthesis and respiration of the two races. The greater photosynthetic capactity of the desert race is suggested as an evolutionary modification which could account for invasion into and survival in the sonoran Desert.     

Photosynthetic characteristics and chloroplast ultrastructure of C3 and C4 tree species grown in high- and low-light environments

Plants of the C₄ tree species, Euphorbia forbesii, Sherff and the C₃ tree species, Claoxylon sandwicense Muell-Arg., were grown in a full sun and a shade environment designed to simulate the understory of their native Hawiian forest habitat. When grown under shade conditions, both species exhibited a photosynthetic light response typical of shade plants with low light compensation points and low dark respiration rates. E. forbesii, however, exhibited greater acclimation of light saturated photosynthetic rates and no evidence of photoinhibition in high light. In contrast, quantum yields for CO₂ uptake and chlorophyll contents were reduced in the high-light as compared to the low-light grown C. sandwicense plants. Both species exhibited similar changes in the intercellular CO₂ response curves and chloroplast whole-chain electron transport capacities, suggesting that the underlying mechanisms of light acclimation are similar. Chloroplasts of E. forbesii exhibited large changes in ultrastructure, with much greater thylakoid membrane development in low than high light. In contrast, C. sandwicense exhibited different starch contents, but otherwise similar membrane development in high and low light. The results show that E. forbesii possesses a very flexible photosynthetic apparatus which may account for its ability to survive in the understory of shaded forests.Abbreviations g_s = stomatal conductance - HL = high light - LL = low light - P_i = intercellular CO₂ partial pressure - PFD = photon flux density     

RESPONSE OF LEAF ANATOMY AND PHOTOSYNTHETIC CAPACITY IN ALOCASIA MACRORRHIZA (ARACEAE) TO A TRANSFER FROM LOW TO HIGH LIGHT

Alocasia macrorrhiza plants were grown in 1% and 20% full sunlight, and their leaf anatomical and physiological parameters were measured. Total leaf thickness was 41% greater and mesophyll thickness was 52% greater in high-light leaves than in low-light leaves. This increase in thickness resulted from both increased cell size and number. Maximum leaf photosynthetic capacity was also 66% greater in high- than in low-light leaves. When low-light plants were transferred to high light, the thickness of mature leaves did not increase but the thickness of the first leaf to expand after the transfer was significantly greater than that of the low-light leaves. Thus, only leaves that were still expanding at the time of transfer developed leaf thickness greater than plants remaining in low light. Fully mature leaves showed no change in photosynthetic capacity in response to transfer. Leaves that had just completed expansion at the time of low- to high-light transfer were able to develop slightly higher maximum photosynthetic capacities than older leaves. However, full photosynthetic acclimation to the new light environment did not occur until the second new leaf expanded after transfer. These results are discussed in relation to the timing and mechanisms of whole plant acclimation to increased light.     

Low-temperature photosynthetic performance of a C4 grass and a co-occurring C3 grass native to high latitudes

The photosynthetic performance of C₄ plants is generally inferior to that of C₃ species at low temperatures, but the reasons for this are unclear. The present study investigated the hypothesis that the capacity of Rubisco, which largely reflects Rubisco content, limits C₄ photosynthesis at suboptimal temperatures. Photosynthetic gas exchange, chlorophyll a fluorescence, and the in vitro activity of Rubisco between 5 and 35 °C were measured to examine the nature of the low‐temperature photosynthetic performance of the co‐occurring high latitude grasses, Muhlenbergia glomerata (C₄) and Calamogrostis canadensis (C₃). Plants were grown under cool (14/10 °C) and warm (26/22 °C) temperature regimes to examine whether acclimation to cool temperature alters patterns of photosynthetic limitation. Low‐temperature acclimation reduced photosynthetic rates in both species. The catalytic site concentration of Rubisco was approximately 5.0 and 20 µmol m^?2 in M. glomerata and C. canadensis, respectively, regardless of growth temperature. In both species, in vivo electron transport rates below the thermal optimum exceeded what was necessary to support photosynthesis. In warm‐grown C. canadensis, the photosynthesis rate below 15 °C was unaffected by a 90% reduction in O₂ content, indicating photosynthetic capacity was limited by the capacity of P_i‐regeneration. By contrast, the rate of photosynthesis in C. canadensis plants grown at the cooler temperatures was stimulated 20–30% by O₂ reduction, indicating the P_i‐regeneration limitation was removed during low‐temperature acclimation. In M. glomerata, in vitro Rubisco activity and gross CO₂ assimilation rate were equivalent below 25 °C, indicating that the capacity of the enzyme is a major rate limiting step during C₄ photosynthesis at cool temperatures.     

Photosynthetic and fatty acid acclimation of four phytoplankton species in response to light intensity and phosphorus availability

Photosynthetic acclimation of phytoplankton to lower irradiation can be met by several strategies such as increasing the affinity for light or increasing antenna size and stacking of the thylakoids. The latter is reflected by a higher proportion of polyunsaturated fatty acids (PUFAs). Additionally, photosynthetic capacity (P_max), respiratory losses, and proton leakage can be reduced under low light. Here we consider the effect of light intensity and phosphorus availability simultaneously on the photosynthetic acclimation and fatty acid composition of four phytoplankters. We studied representatives of the Chlorophyceae, Cryptophyceae and Mediophyceae, all of which are important components of plankton communities in temperate lakes. In our analysis, excluding fatty acid composition, we found different acclimation strategies in the chlorophytes Scenedesmus quadricauda, Chlamydomonas globosa, cryptophyte Cryptomonas ovata and ochrophyte Cyclotella meneghiniana. We observed interactive effects of light and phosphorus conditions on photosynthetic capacity in S. quadricauda and Cry. ovata. Cry. ovata can be characterized as a low light-acclimated species, whereas S. quadricauda and Cyc. meneghiniana can cope best with a combination of high light intensities and low phosphorus supply. Principal component analyses (PCA), including fatty acid composition, showed further species-specific patterns in their regulation of P_max with PUFAs and light. In S. quadricauda and Cyc. meneghiniana, PUFAs negatively affected the relationship between P_max and light. In Chl. globosa, lower light coincided with higher PUFAs and lower P_max, but PCA also indicated that PUFAs had no direct influence on P_max. PUFAs and P_max were unaffected by light in Cry. ovata. We did not observe a general trend in the four species tested and concluded that, in particular, the interactive effects highlight the importance of taking into account more than one environmental factor when assessing photosynthetic acclimation to lower irradiation.     

Photosynthetic responses of tree seedlings in grass and under shrubs in early-successional tropical old fields, Costa Rica

Only recently have studies addressed the effect of early-colonizing vegetation on tree seedling survival and growth during secondary succession in tropical old fields, and few studies have elucidated the physiological responses of tree seedlings to different vegetational communities. We compared growth and various photosynthetic parameters for seedlings of four rain-forest tree species, Cedrela tonduzii, Inga punctata, Ocotea whitei, and Tapirira mexicana, growing in areas of pasture grass and shrubs in early-successional abandoned pasture in Costa Rica; in addition, we made measurements for two species in forest gaps. We tested the general hypothesis that early-colonizing shrubs facilitate growth of forest tree seedlings. Specifically, we measured microclimate, growth, CO₂ assimilation, stomatal conductance, photosystem II quantum yield (KPSII), and xanthophyll pigment pools for all seedlings. Photosynthetic flux density (PFD) was higher under grass than shrubs or forest gaps, but was highly variable in each growth environment. For three of the four species, height growth was greatest in the grass compared to the shrubs and forest gaps; growth was similar below grass and shrubs for O. whitei. Photosynthetic capacity, apparent quantum yield, and stomatal conductance did not vary across habitats, but light compensation point and PFD at light saturation tended to be higher in the grass compared to forest and shrub growth environments. Water use efficiency differed across growth environments for three of the species. For plants in ambient PFD and dark-adapted plants, the efficiency of excitation energy transfer through PSII was lowest for plants in the grass compared to shrubs and forest gaps and also differed across species. Measurement of steady-state responses of KPSII to increasing PFD indicated a significant effect of growth environment at low PFD for all species and significant effects at high PFD only for I. punctata. All species exhibited a high degree of midday xanthophyll de-epoxidation in the different growth environments. Xanthophyll pigment pool size on an area basis was highest in the grass compared to shrubs and forest gaps for all four species. The results suggest that shrubs do not provide a facilitative effect for growth or photosynthesis for ~1.5-year-old seedlings of these four species. We conclude that site differences in success of tree seedlings during succession are a result of complex interactions of facilitation and competition and are not simply based on physiological responses to PFD.     

Do fern gametophytes have the capacity for irradiance acclimation?

Ferns present two alternant generations: sporophyte and gametophyte. In the present work we address the question of whether fern gametophytes have the potential to acclimate to different irradiances as vascular plants do. We studied the gametophytes of three different fern species belonging to the Aspleniaceae family with different ecological requirements (Asplenium trichomanes, Asplenium scoloprendrium and Ceterach officinarum). Fern spores were germinated and the gametophytes cultivated under photon flux density (PFD) of 10, 50 or 100 μmol m⁻² s⁻¹. From the early stages of spore germination (the formation of the 5-celled germinal filament), photosynthetic apparatus acclimates showing the typical patterns of photochemical responses to high or low PFD. In agreement with the photochemical pattern of acclimation, higher contents of xanthophyll cycle pigments and α-tocopherol was observed in plants grown under high PFD. The α/β-carotene ratio, used as indicator of the acclimation of the photosynthetic apparatus, also sustained the initial hypothesis except for A. trichomanes. We conclude that fern gametophytes display a complete array of photosynthetic and photoprotective traits that allow an effective acclimation to PFD.     

Photosynthetic acclimation in the context of structural constraints to carbon export from leaves

The potential role of foliar carbon export features in the acclimation of photosynthetic capacity to differences and changes in light environment was evaluated. These features included apoplastic vs. symplastic phloem loading, density of loading veins, plasmodesmatal frequency in intermediary cells, and the ratio of loading cells to sieve elements. In initial studies, three apoplastic loaders (spinach, pea, Arabidopsis thaliana) exhibited a completely flexible photosynthetic response to changing light conditions, while two symplastic loaders (pumpkin, Verbascum phoeniceum), although able to adjust to different long-term growth conditions, were more limited in their response when transferred from low (LL) to high (HL) light. This suggested that constraints imposed by the completely physical pathway of sugar export might act as a bottleneck in the export of carbon from LL-acclimated leaves of symplastic loaders. While both symplastic loaders exhibited variable loading vein densities (low in LL and high in HL), none of the three apoplastic loaders initially characterized exhibited such differences. However, an additional apoplastic species (tomato) exhibited similar differences in vein density during continuous growth in different light environments. Furthermore, in contrast to the other apoplastic loaders, photosynthetic acclimation in tomato was not complete following a transfer from LL to HL. This suggests that loading vein density and loading cells per sieve element, and thus apparent loading surface capacity, play a major role in the potential for photosynthetic acclimation to changes in light environment. Photosynthetic acclimation and vein density acclimation were also characterized in the slow-growing, sclerophytic evergreen Monstera deliciosa. This evergreen possessed a lower vein density during growth in LL compared to HL and exhibited a more severely limited potential for photosynthetic acclimation to increases in light environment than the rapidly-growing, mesophytic annuals.     

Photosynthetic responses of Miconia species to canopy openings in a lowland tropical rainforest

We examined the photosynthetic acclimation of three tropical species of Miconia to canopy openings in a Costa Rican rainforest. The response of photosynthesis to canopy opening was very similar in Miconia affinis, M. gracilis, and M. nervosa, despite differences in growth form (trees and shrubs) and local distributions of plants (understory and gap). Four months after the canopy was opened by a treefall, photosynthetic capacity in all three species had approximately doubled from closed canopy levels. There were no obvious signs of high light damage after treefall but acclimation to the gap environment was not immediate. Two weeks after treefall, A_max, stomatal conductance, apprarent quantum efficiency, and dark respiration rates had not changed significantly from understory values. The production of new leaves appears to be an important component of light acclimation in these species. The only variables to differ significantly among species were stomatal conductance at A_max and the light level at which assimilation was saturated. M. affinis had a higher stomatal conductance which may reduce its water use efficiency in gap environments. Photosynthesis in the more shade-tolerant M. gracilis saturated at lower light levels than in the other two species. Individual plant light environments were assessed after treefall with canopy photography but they explained only a small fraction of plant variation in most measures of photosynthesis and growth. In conclusion, we speculate that species differences in local distribution and in light requirements for reproduction may be more strongly related to species differences in carbon allocation than in carbon assimilation.     

Photosynthetic Characteristics and the Ratios of Chlorophyll, β-Carotene,and the Components of the Xanthophyll Cycle Upon a Sudden Increase in Growth Light Regime in Several Plant Species*

Among three species, Gossypium hirsutum, Rhizophora mangle, and Monstera deliciosa, which were transferred from low to high growth PFD, only small decreases in the efficiency of photochemical energy conversion were observed in those plants which exhibited an increase in photosynthetic capacity. Leaves of plants which showed no increase in photosynthetic capacity experienced a continuous decrease in photochemical efficiency, accompanied by a more pronounced loss of chlorophyll than that observed in the former group. In all species marked increases in the xanthophyll/β-carotene ratio resulted from small increases in lutein, and several-fold increases in the sum of the three components of the xanthophyll cycle, zeaxanthin, antheraxanthin, and violaxanthin. A strong increase in the level of zeaxanthin was only partially matched by a decrease of violaxanthin to zero, and was further paralleled by a decrease in β-carotene. Antiparallel changes in the sum of zeaxanthin + antheraxanthin + violaxanthin and β-carotene between morning and evening were observed in all species. These diel changes were overlaid on a net increase in β-carotene as well as total carotenoid content in those plants in which photosynthetic capacity increased. In those, however, which exhibited no photosynthetic acclimation upon transfer to high light, a decrease in both β-carotene and total carotenoid content was observed. Rhizophora mangle grown at 100 % seawater exhibited a particularly high capacity for increasing the level of zeaxanthin in response to high light.     

The dynamics of photosynthetic acclimation to changes in light quanlity and quality in three Australian rainforest tree species

Photosynthetic acclimation was studied in seedlings of three subtropical rainforest species representing early (Omalanthus populifolius), middle (Duboisia myoporoides) and late (Acmena ingens) successional stages in forest development. Changes in the photosynthetic characteristics of pre-existing leaves were observed following the transfer of plants between deep shade (1–5% of photosynthetically active radiation (PAR), selectively filtered to produce a red/far-red (R/FR) ratio of 0.1) and open glasshouse (60% PAR and a R/FR ratio of 1.1–1.2), and vice versa. The extent and rate of response of the photosynthetic characteristics of each species to changes in light environment were recorded in this simulation of gap formation and canopy closure/overtopping. The light regimes to which plants were exposed produced significant levels of acclimation in all the photosynthetic parameters examined. Following transfer from high to low light, the light-saturated rate of photosynthesis was maintained near pre-transfer levels for 7 days, after which it decreased to levels which closely approximated those in leaves which had developed in low light. The decrease in photosynthetic capacity was associated with lower apparent quantum yields and stomatal conductances. Dark respiration was the parameter most sensitive to changes in light environment, and responded significantly during the first 4–7 days after transfer. Acclimation of photosynthetic capacity to increases in irradiance was significant in two of the three species studied, but was clearly limited in comparison with that of new leaves produced in the high light conditions. This limitation was most pronounced in the early-successional-stage species, O. populifolius. It is likely that structural characteristics of the leaves, imposed at the time of leaf expansion, are largely responsible for the limitations in photosynthetic acclimation to increases in irradiance.