Some like it hot: the physiological ecology of C4 plant evolution.

The evolution of C4 photosynthesis requires an intermediate phase where photorespiratory glycine produced in the mesophyll cells must flow to the vascular sheath cells for metabolism by glycine decarboxylase. This glycine flux concentrates photorespired CO2 within the sheath cells, allowing it to be efficiently refixed by sheath Rubisco. A modest C4 biochemical cycle is then upregulated, possibly to support the refixation of photorespired ammonia in sheath cells, with subsequent increases in C4 metabolism providing incremental benefits until an optimized C4 pathway is established. 'Why' C4 photosynthesis evolved is largely explained by ancestral C3 species exploiting photorespiratory CO2 to improve carbon gain and thus enhance fitness. While photorespiration depresses C3 performance, it produces a resource (photorespired CO2) that can be exploited to build an evolutionary bridge to C4 photosynthesis. 'Where' C4 evolved is indicated by the habitat of species branching near C3-to-C4 transitions on phylogenetic trees. Consistent with the photorespiratory bridge hypothesis, transitional species show that the large majority of > 60 C4 lineages arose in hot, dry, and/or saline regions where photorespiratory potential is high. 'When' C4 evolved has been clarified by molecular clock analyses using phylogenetic data, coupled with isotopic signatures from fossils. Nearly all C4 lineages arose after 25 Ma when atmospheric CO2 levels had fallen to near current values. This reduction in CO2, coupled with persistent high temperature at low-to-mid-latitudes, met a precondition where photorespiration was elevated, thus facilitating the evolutionary selection pressure that led to C4 photosynthesis.

The light environment and growth of C3 and C4 tree species in the understory of a Hawaiian forest.

The light environment in the understory of a Hawaiian forest containing a C4 tree species, Euphorbia forbesii, was characterized using photosynthetic photon flux density sensors connected to portable data acquisition systems and a strip chart recorder, and hemispherical "fisheye" photographs of the canopy. During July 1980, 86 µmol cm2 day1 was received in the understory of which approximately 40% was contributed by sunflecks. The understory received 2.4% of the light reaching the top of the canopy. Nearly all sunflecks had peak photon flux densities greater than 250 µmol m2 s1, but two-thirds were less than 0.5 min in length. The number of minutes of sunflecks received per day at any site was highly variable, depending on cloudiness and the overstory canopy structure. On a relatively clear day a 10-fold difference in the number of minutes of sunflecks was observed between sample sites. Estimates obtained from hemispherical photographs were used to calculate the annual mean potential number of minutes of sunflecks per day received by saplings of Euphorbia and a C3 tree species, Claoxylon sandwicense. The growth of saplings of both species was highly correlated with the estimates of the minutes of sunflecks and was similar for both species. Although C4 photosynthesis is usually found in plants native to high-light environments, it does not appear to confer any disadvantage in terms of growth to Euphorbia forbesii in the low-light conditions of the forest understory.

Temperature responses of growth and photosynthetic CO2 exchange rates in coastal and desert races of Atriplex lentiformis.

Comparative measurements of CO2 exchange and growth rates were made on Atriplex lentiformis (Torr.) Wats. plants from populations native to coastal as well as desert habitats in southern California. While both had similar CO2 exchange rates at moderate growth temperatures, the desert plants had a substantially greater capacity to acclimate to high growth temperatures indicating that clear ecotypic differences in acclimation capacity are present in this species. This large capacity for photosynthetic acclimation resulted in nearly equal CO2 exchange rates of the desert plants under the different day temperatures characteristic of the desert habitat during the summer and winter months. In contrast, the photosynthetic CO2 exchange rates of the coastal plants was markedly reduced by high growth temperatures. The large acclimation capacity of the desert plants may function to maintain high productivities during both the winter and summer months but would not be required in the coastal plants because of the moderate temperatures throughout the year in their native habitat.Relative growth rates (RGR) of the coastal and desert plants were similar at 23°C day/18°C night and 33°C day/25°C night growth temperatures. At 43°C day/30°C night temperatures, however, the RGR of the desert plants was higher than that of the coastal plants. Thus, the larger acclimation capacity of the desert plants is related to a greater ability to maintain high growth rates over a wide range of temperatures as compared to the coastal plants. Small differences in allocation patterns could account for differences in the comparative photosynthetic responses and growth rates in each temperature regime.

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 µmol photons m-2 s-1) and light compensation (70 to 100 µmol 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.

Effect of leaf flutter on the light environment of poplars.

The dynamics of the canopy light environment for two poplar species (Populus tremuloides Michx., and P. fremontii Wats.) were characterized with an array of photocells in fixed positions within the canopy or attached directly to leaves and using a data logger that recorded photon flux density (PFD) at frequencies from 1 to 20 Hz. The majority of sunflecks were short in duration (<1 s) with a similar short interval between sunflecks. Sunflecks contribute as much as 90% of the total daily PFD in the lower canopy. Leaf flutter may cause high frequency (3 to 5 Hz) variations of PFD in poplar canopies. The amount of light intercepted by a fluttering leaf at the top of the canopy decreased with increasing flutter, whereas a fluttering lower canopy leaf showed no such trend. When leaves fluttered at the top of the canopy the understory light environment showed an increased number of shorter sunflecks. Leaf flutter may increase mean PFD for understory leaves. It also creates a canopy light environment that is more dynamic temporally and more evenly distributed spatially. The potential benefits of these changes in light dynamics are discussed.

Stomatal dynamics and its importance to carbon gain in two rainforest Piper species : I. VPD effects on the transient stomatal response to lightflecks.

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 gs 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 gs 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 gs 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 gs 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.

Stomatal dynamics and its importance to carbon gain in two rainforest Piper species : II. Stomatal versus biochemical limitations during photosynthetic induction.

The relative importance of biochemical and stomatal limitations on assimilation (A) during photosynthetic induction were compared in sun and shade plants of Piper auritum, a pioneer tree, and shade plants of Piper aequale, a shade tolerant shrub native to a Mexican tropical rainforest. For non-induced leaves, increases in A during induction depended on the dynamics of stomatal conductance (gs) and ribulose-1,5-bisphosphate carboxylase (RuBisCO) activation. At high leaf-air vapor pressure deficit (VPD), more of the limitation during induction was stomatal. Calculations of mesophyll conductance revealed longer time constants for shade than for sun plants. However, no differences in the time course of RuBisCO activity between sun- and shade-plants were found. We conclude on the basis of the similar RuBisCO responses that differences in induction can be accounted for by the differences in stomatal behavior. Differences in the time course of mesophyll conductance may be due to an artifact caused by stomatal patchiness. Experiments on induction loss of previously induced leaves revealed that under these circumstances biochemical limitations can be important. A more rapid induction loss was evident in sun as compared to shade plants. The rapid loss of induction in sum plants was not due to the decreases in gs and RuBisCO activity, which both occurred slowly. Instead, a limitation, probably in RuBP regeneration capacity, appeared to develop during the low light periods. This limitation was much smaller or absent in shade plants.

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

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.

Influence of sunflecks on the δ 13 C of Adenocaulon bicolor plants occurring in contrasting forest understory microsites.

Leaf characteristics and carbon isotope ratios (δ13C) of Adenocaulon bicolor were examined in the understory of a redwood forest along a gradient of microsites that differed in the amount of direct (sunfleck) photon flux density. Comparisons were made between plants that had been shaded from sunflecks with shadow bands but still received diffuse light, and adjacent plants that received both sunflecks and diffuse light. The δ13C of the shaded plants were 1.2‰ lower than predicted from the intercellular CO2 pressure (pi), probably because of recycling of respired CO2 in the understory. Plants receiving sunflecks had higher δ13C values because assimilation during sunflecks occurred at a lower pi than assimilation in diffuse light. The amount that their δ13C was higher was positively correlated with predicted direct photon flux density received by a plant. Leaf weight per unit area increased with increasing PFD. Although plants receiving sunflecks had greater leaf weights per unit area and photosynthetic capacities than those under shadow bands, there was no apparent acclimation of photosynthetic capacity to the differences in PFD among the microsites.

Photosynthetic responses to light variation in rainforest species : I. Induction under constant and fluctuating light conditions.

Photosynthetic induction under constant and fluctuating light conditions was investigated in intact leaves of Alocasia macrorrhiza and Toona australis, two species native to Australian rainforests. When leaves were exposed to saturating light following a long period at low light intensity, an induction period of 25-40 min was required before steady-state photosynthesis was achieved. A long induction period was required regardless of plant growth conditions (high vs. low light) and ambient CO2 concentrations during mesurement. In low-light grown A. macrorrhiza, the initial slope of the relationship between assimilation and internal CO2 pressure increased 7-fold from 30 s following illumination to the end of the induction period. Both stomatal and carboxylation limitations play a role in photosynthetic induction, but carboxylation limitations predominate during the first 6-10 min. In both species, leaf induction state increased 2 to 3-fold during a sequence of five 30-or 60-s lightflecks separated by 2 min of low light. Rates of induction during 60-s lightflecks and during constant illumination were similar. Induction loss in low-light grown leaves of Alocasia macrorrhiza required more than 60 min of continuous exposure to low light conditions. These results suggest that, under forest understory conditions, leaves are at intermediate induction states for most of the day. The ability to utilize sunflecks may therefore be strongly influenced by the ability of leaves to maintain relatively high states of induction during long periods of low light.

Photosynthetic responses to light variation in rainforest species : II. Carbon gain and photosynthetic efficiency during lightflecks.

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 µmol m-2 s-1 were not affected significantly by the previous light level. However, during lightflecks at 530 µmol 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 µmol 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.

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.

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.

A three-dimensional crown architecture model for assessment of light capture and carbon gain by understory plants.

A model simulating the three-demensional crown architecture of a plant was developed with the objective of assessing the light capture and whole-plant carbon gain consequences of leaf display in understory plants. This model uses geometrical measurements taken in the field to reconstruct the projected image of a plant so that light absorption from any direction can be assessed. The photon flux density (PFD) from a given direction was estimated from the canopy openness derived from hemispherical canopy photographs and equations simulating the daily course of direct and diffuse PFD. For diffuse PFD, the directional fluxes and absorbed PFD were integrated over 160 different directions representing 8 azimuth classes and 20 elevation angle classes. Direct PFD absorption was determined for the time that a solar track on a given day intersected a canopy gap. Assimilation rate was simulated for the sunlit and shaded parts of leaves separately and then summed to give the whole-plant carbon gain. Comparisons of simulations for a tropical forest edge species, Clidemia octona, and an understory species, Conostegia cinnamomea, illustrate the operation of the model and show that the edge species is more efficient at capturing side light while the understory species is slightly more efficient at capturing light from directly above, the predominant light direction in this environment. Self-shading within Conostegia crown and steep leaf angles in the Clidemia crown reduced light capture efficiencies for light from directly above. Whole-plant daily carbon gain was much higher in the forest edge site, mostly because of the additional PFD available in this site. However, simulations for both species in the understory light environment show that the higher light capture efficiencies of the understory species in this environment conferred a 27% advantage in carbon gain in this environment.

Gas exchange characteristics and temperature relations of two desert annuals: A comparison of a winter-active and a summer-active species.

The gas exchange characteristics of two C3 desert annuals with contrasting phenologies, Geraea canescens T. & G. (winter-active) and Dicoria canescens T. & G. (summer-active), both Asteraceae, were determined for plants grown under a moderate (25°/15° C, day/night temperature) and a high (40°/27° C) growth temperature regime. Both species had high photosynthetic capacities; maximum net photosynthetic rates were 38 and 48 µmol CO2 m-2 s-1 for Geraea and Dicoria, respectively, and were not influenced by growth temperature regime. However, the temperature optima of net photosynthesis shifted from 26° C for Geraea and from 28° C for Dicoria when grown under the moderate temperature regime to 31° C for both species when grown under the high temperature regime. Although the shifts in temperature optima were smaller than those observed for many desert perennials, both species showed substantial increases in photosynthetic rates at high temperatures when grown at 40°/27° C. In general, the gas exchange characteristics of Geraea and Dicoria were very similar to each other and to those reported for other C3 desert annuals. Geraea and Dicoria experienced different seasonal patterns of change in several environmental variables. For Geraea, maximum daily air temperature (T a) increased from 24° to 41° C over its growing season while Dicoria experienced maximum T a at midseason (45° C). At points during their respective growing seasons when midday T a ranged between 35° and 40° C, leaf temperatures (T 1) of both species were below T a and, therefore, were closer to the photosynthetic temperature optima measured in the laboratory. Leaf conductances to water vapor (g 1) and water potentials (ψ) were high at these times, but later in their growing seasons Dicoria maintained high g 1 and ψ while Geraea showed large decreases in these quantities. The ability of Dicoria to successfully growth through the hot, dry summers of the California deserts may be related to its ability to acquire the available water in locally mesic habitats.

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

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

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

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

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

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 µmol m2 s1 and light compensation points were 6.3 and 1.7 µmol 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.

The functional ecology of shoot architecture in sun and shade plants of Heteromeles arbutifolia M. Roem., a Californian chaparral shrub.

The functional roles of the contrasting morphologies of sun and shade shoots of the evergreen shrub Heteromeles arbutifolia were investigated in chaparral and understory habitats by applying a three-dimensional plant architecture simulation model, YPLANT. The simulations were shown to accurately predict the measured frequency distribution of photosynthetic photon flux density (PFD) on both the leaves and a horizontal surface in the open, and gave reasonably good agreement for the more complex light environment in the shade. The sun shoot architecture was orthotropic and characterized by steeply inclined (mean = 71o) leaves in a spiral phyllotaxy with short internodes. This architecture resulted in relatively low light absorption efficiencies (E A) for both diffuse and direct PFD, especially during the summer when solar elevation angles were high. Shade shoots were more plagiotropic with longer internodes and a pseudo-distichous phyllotaxis caused by bending of the petioles that positioned the leaves in a nearly horizontal plane (mean = 5o). This shade-shoot architecture resulted in higher E A values for both direct and diffuse PFD as compared to those of the sun shoots. Differences in E A between sun and shade shoots and between summer and winter were related to differences in projection efficiencies as determined by leaf and solar angles, and by differences in self shading resulting from leaf overlap. The leaves exhibited photosynthetic acclimation to the sun and the shade, with the sun leaves having higher photosynthetic capacities per unit area, higher leaf mass per unit area and lower respiration rates per unit area than shade leaves. Despite having 7 times greater available PFD, sun shoots absorbed only 3 times more and had daily carbon gains only double of those of shade shoots. Simulations showed that sun and shade plants performed similarly in the open light environment, but that shade shoots substantially outperformed sun shoots in the shade light environment. The shoot architecture observed in sun plants appears to achieve an efficient compromise between maximizing carbon gain while minimizing the time that the leaf surfaces are exposed to PFDs in excess of those required for light saturation of photosynthesis and therefore potentially photoinhibitory.

Photosynthetic characteristics of a tropical forest understory herb, Alocasia macrorrhiza, and a related crop species, Colocasia esculenta grown in contrasting light environments.

Photosynthetic acclimation to 5 light environments ranging from 2 to 60% full sun was determined in Alocasia macrorrhiza, a shade tolerant species from tropical forest understories, and Colocasia esculenta, a cultivated species which occurs naturally in open marshy areas. Photosynthetic capacities of both species increased nearly 3 fold with increased photon flux density (PFD). In a given environment, however, photosynthetic capacities of C. esculenta were double those of A. macrorrhiza. Stomatal limitations explained only a small part of this difference. Respiration rates and estimated biochemical capacities increased in parallel to photosynthetic capacity. No differences were observed either between species or environments in the ratio of RuBP regeneration capacity to carboxylation capacity as determined from the CO2 dependence response of photosynthesis. Quantum yields of both species decreased only slightly with increasing growth PFD, providing little evidence for photoinhibition at high PFD. The results are discussed in terms of the mechanisms of and limitations on acclimation in these two species.

Daily carbon gain by Adenocaulon bicolor (Asteraceae), a redwood forest understory herb, in relation to its light environment.

The relationships between carbon gain and availability of sunfleck- and diffuse-light were determined for Adenocaulon bicolor by following the daily courses of assimilation and incident PFD on different days and locations in a redwood forest understory. Total PFD for the days sampled ranged from 1 to 4% of full sun values. Sunflecks accounted for 50 to 90% of the total PFD and were responsible for the majority of variation among days and locations. Each day had several clusters of sunfleck activity separated by relatively long intervals of diffuse light. Most sunflecks had maximum PFDs below the photosynthetic light-saturation point, and they had a median length and diffuse light interval separating them of 2 s. Daily carbon gain varied from 14 to 40 mmol m-2d-1 and was more strongly correlated with differences among days in total sunfleck PFD (r 2=0.81) than with variation in diffuse PFD (r 2=0.54). The assimilation that was attributable to sunflecks ranged from essentially zero on one day to 30 to 65% of the total on the other days. Carbon gain on most days was 70 to 80% of that predicted by a model based on the measured light dependences of assimilation. This model assumed an instantaneous response to changes in PFD, whereas incomplete photosynthetic induction probably limited the capacity to respond to sunflecks and therefore limited carbon gain on most days.