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C B Osmond - One of the best experts on this subject based on the ideXlab platform.
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rapid changes in xanthophyll cycle dependent energy dissipation and photosystem ii efficiency in two vines stephania japonica and smilax australis growing in the understory of an open eucalyptus forest
Plant Cell and Environment, 1999Co-Authors: William W. Adams, Barry A Logan, D H Barker, Barbara Demmigadams, C B OsmondAbstract:Leaves of Stephania japonica and Smilax australis were characterized in situ on the coast of north-eastern New South Wales, Australia, where they were growing naturally in three different light environments: deep shade, in the understory of an open Eucalyptus forest where they received frequent Sunflecks of high intensity, and in an exposed site receiving full sunlight. In deep shade the xanthophyll cycle remained epoxidized during the day and the vast majority of absorbed light was utilized for photosynthesis. In the exposed site both deepoxidation and epoxidation of the xanthophyll cycle and changes in the level of xanthophyll-dependent thermal energy dissipation largely tracked the diurnal changes in photon flux density (PFD). In the understory the xanthophyll cycle became largely deepoxidized to zeaxanthin and antheraxanthin upon exposure of the leaves to the first high intensity Sunfleck and this high level of deepoxidation was maintained throughout the day both during and between subsequent Sunflecks. In contrast, thermal energy dissipation activity, and the efficiency of photosystem II, fluctuated rapidly in response to the changes in incident PFD. These findings suggest a fine level of control over the engagement of zeaxanthin and antheraxanthin in energy dissipation activity, presumably through rapid changes in thylakoid acidification, such that they became rapidly engaged for photoprotection during the Sunflecks and rapidly disengaged upon return to low light when continued engagement might limit carbon gain.
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Rapid changes in xanthophyll cycle‐dependent energy dissipation and photosystem II efficiency in two vines, Stephania japonica and Smilax australis, growing in the understory of an open Eucalyptus forest
Plant Cell and Environment, 1999Co-Authors: William W. Adams, Barry A Logan, Barbara Demmig-adams, D H Barker, C B OsmondAbstract:Leaves of Stephania japonica and Smilax australis were characterized in situ on the coast of north-eastern New South Wales, Australia, where they were growing naturally in three different light environments: deep shade, in the understory of an open Eucalyptus forest where they received frequent Sunflecks of high intensity, and in an exposed site receiving full sunlight. In deep shade the xanthophyll cycle remained epoxidized during the day and the vast majority of absorbed light was utilized for photosynthesis. In the exposed site both deepoxidation and epoxidation of the xanthophyll cycle and changes in the level of xanthophyll-dependent thermal energy dissipation largely tracked the diurnal changes in photon flux density (PFD). In the understory the xanthophyll cycle became largely deepoxidized to zeaxanthin and antheraxanthin upon exposure of the leaves to the first high intensity Sunfleck and this high level of deepoxidation was maintained throughout the day both during and between subsequent Sunflecks. In contrast, thermal energy dissipation activity, and the efficiency of photosystem II, fluctuated rapidly in response to the changes in incident PFD. These findings suggest a fine level of control over the engagement of zeaxanthin and antheraxanthin in energy dissipation activity, presumably through rapid changes in thylakoid acidification, such that they became rapidly engaged for photoprotection during the Sunflecks and rapidly disengaged upon return to low light when continued engagement might limit carbon gain.
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Responses of Rainforest Understorey Plants to Excess Light during Sunflecks
Functional Plant Biology, 1997Co-Authors: Jennifer R. Watling, Sharon A. Robinson, Ian E. Woodrow, C B OsmondAbstract:Responses of Alocasia macrorrhiza (L.) G. Don, Castanospora alphandii (F. Muell.) F. Muell. and Alpinia hylandii R. Smith, growing in a tropical rainforest understorey, to excess light during Sunflecks were investigated using chlorophyll fluorescence techniques and by analysing xanthophyll cycle activity. A fourth species, the pioneerOmalanthus novo-guineensis (Warb.) Schum., growing in a small gap, was also studied. In all three understorey species there were large and rapid decreases in the proportion of open Photosystem II (PSII) centres, as indicated by qP, on illumination with saturating light and a concurrent increase in non-photochemical quenching. qP remained low (< 0.4) throughout the period of illumination (~15 min), although it did increase gradually, probably reflecting photosynthetic induction. Sustained declines (up to 120 min) in quantum yield, indicated by Fv/Fm, occurred in all three understorey species following exposure to saturating Photon flux density (PFD) during Sunflecks. When pPSII was monitored during Sunflecks it was found to be very sensitive to changes in PFD, declining rapidly with even modest rises in the latter. There was rapid and continuing net conversion of violaxanthin (V) to antheraxanthin plus zeaxanthin (A+Z) on exposure of A. macrorrhiza and C. alphandii to saturating Sunflecks. On returning to low light A. macrorrhiza retained its high levels of A+Z for up to 60 min, while C. alphandii rapidly converted back to V. O. novo- guineensis responded to high light by changing its leaf angle to reduce interception and showed no indication of photoinhibition during or after exposure.
Robert W. Pearcy - One of the best experts on this subject based on the ideXlab platform.
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Two decades of Sunfleck research: looking back to move forward.
Tree physiology, 2012Co-Authors: Robert W. Pearcy, Danielle A. WayAbstract:The study of how plants make use of Sunflecks, relatively short duration but high-intensity patches of light in the understory and shaded tree canopy, has been of interest for decades. The potential ecological significance of Sunflecks was recognized early in the 20th century (Allee 1926, Evans 1956), and the first few studies attempting to quantify the contribution of Sunflecks to carbon gain in understories were undertaken at
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Sunflecks in trees and forests: from photosynthetic physiology to global change biology
Tree physiology, 2012Co-Authors: Danielle A. Way, Robert W. PearcyAbstract:Sunflecks are brief, intermittent periods of high photon flux density (PFD) that can significantly improve carbon gain in shaded forest understories and lower canopies of trees. In this review, we discuss the physiological basis of leaf-level responses to Sunflecks and the mechanisms plants use to tolerate sudden changes in PFD and leaf temperature induced by Sunflecks. We also examine the potential effects of climate change stresses (including elevated temperatures, rising CO 2 concentrations and drought) on the ability of tree species to use Sunflecks, and advocate more research to improve our predictions of seedling and tree carbon gain in future climates. Lastly, while we have the ability to model realistic responses of photosynthesis to fluctuating PFD, dynamic responses of photosynthesis to Sunflecks are not accounted for in current models of canopy carbon uptake, which can lead to substantial overestimates of forest carbon fixation. Since Sunflecks are a critical component of seasonal carbon gain for shaded leaves, Sunfleck regimes and physiological responses to Sunflecks should be incorporated into models to more accurately capture forest carbon dynamics.
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Photosynthetic responses to dynamic light under field conditions in six tropical rainforest shrubs occuring along a light gradient
Oecologia, 1997Co-Authors: Fernando Valladares, Mitchell T. Allen, Robert W. PearcyAbstract:We examined in the field the photosynthetic utilization of fluctuating light by six neotropical rainforest shrubs of the family Rubiaceae. They were growing in three different light environments: forest understory, small gaps, and clearings. Gas exchange techniques were used to analyse photosynthetic induction response, induction maintenance during low-light periods, and lightfleck (simulated Sunfleck) use efficiency (LUE). Total daily photon flux density (PFD) reaching the plants during the wet season was 37 times higher in clearings than in the understory, with small gaps exhibiting intermediate values. Sunflecks were more frequent, but shorter and of lower intensity in the understory than in clearings. However, Sunflecks contributed one-third of the daily PFD in the understory. Maximum rates of net photosynthesis, carboxylation capacity, electron transport, and maximum stomatal conductance were lower in understory species than in species growing in small gaps or clearings, while the reverse was true for the curvature factor of the light response of photosynthesis. No significant differences were found in the apparent quantum yield. The rise of net photosynthesis during induction after transfer from low to high light varied from a hyperbolic shape to a sigmoidal increase. Rates of photosynthetic induction exhibited a negative exponential relationship with stomatal conductance in the shade prior to the increase in PFD. Leaves of understory species showed the most rapid induction and remained induced longer once transferred to the shade than did leaves of medium- or high-light species. LUE decreased rapidly with increasing lightfleck duration and was affected by the induction state of the leaf. Fully induced leaves exhibited LUEs up to 300% for 1-s lightflecks, while LUE was below 100% for 1-80 s lightflecks in uninduced leaves. Both induced and uninduced leaves of understory species exhibited higher LUE than those of species growing in small gaps or clearings. However, most differences disappeared for lightflecks 10 s long or longer. Thus, understory species, which grew in a highly dynamic light environment, had better capacities for utilization of rapidly fluctuating light than species from habitats with higher light availability.
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The Consequences of Sunflecks for Photosynthesis and Growth of Forest Understory Plants
Ecophysiology of Photosynthesis, 1995Co-Authors: Robert W. Pearcy, William A. PfitschAbstract:Plants in forest understories are subjected to light environments consisting of a very low background of diffuse light that is punctuated by often much brighter Sunflecks lasting from a few seconds to several minutes. These Sunflecks, although usually present for less than 10% of the time, typically contribute 10 to 80% of the photosynthetically active photon flux density (PFD) (Chazdon 1988). Therefore much of the photosynthesis of understory plants may occur under transiently changing light conditions that characterize Sunflecks. The environmental and physiological controls on photosynthesis during transient light changes are not necessarily the same as those that determine photosynthetic performance under steady-state conditions. The shade-plant syndrome of understory plants has been widely studied but mostly in terms of the controls on steady-state photosynthetic characteristics. Until recently, relatively little attention has been given to the mechanisms regulating the use of Sunflecks.
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Growth and Reproductive Allocation of Adenocaulon Bicolor Following Experimental Removal of Sunflecks
Ecology, 1992Co-Authors: William A. Pfitsch, Robert W. PearcyAbstract:We used experimental and multiple-regression approaches to test the hy- pothesis that the availability of direct light (Sunflecks) is important for the growth and reproduction of Adenocaulon bicolor, a perennial herb occurring in the understory of a coastal redwood forest. Plants were shaded from direct-beam light (PFD = photosynthetic photon flux density) with shadow bands that still allowed receipt of most of the diffuse light. After two growing seasons the shaded plants receiving only diffuse light were signif- icantly smaller than control plants that received both direct and diffuse light. In general, shaded plants decreased in size whereas those receiving Sunflecks remained at the same size. Reproduction was reduced more than vegetative size in the shaded plants. Despite the dramatic effect of removing Sunflecks from the light energy available to individual plants, we found no significant relationship between plant performance and the amount of direct PFD available in a microsite, as predicted from hemispherical canopy photographs. Instead, the size and reproductive output of plants growing under natural light regimes was correlated with the average daily diffuse light availability. Although additional direct PFD could be expected to increase carbon gain, the lack of a correlation between the amount of direct PFD and plant performance may be due to other factors such as water or nutrient stress that increase in microsites with increased direct light.
William W. Adams - One of the best experts on this subject based on the ideXlab platform.
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rapid changes in xanthophyll cycle dependent energy dissipation and photosystem ii efficiency in two vines stephania japonica and smilax australis growing in the understory of an open eucalyptus forest
Plant Cell and Environment, 1999Co-Authors: William W. Adams, Barry A Logan, D H Barker, Barbara Demmigadams, C B OsmondAbstract:Leaves of Stephania japonica and Smilax australis were characterized in situ on the coast of north-eastern New South Wales, Australia, where they were growing naturally in three different light environments: deep shade, in the understory of an open Eucalyptus forest where they received frequent Sunflecks of high intensity, and in an exposed site receiving full sunlight. In deep shade the xanthophyll cycle remained epoxidized during the day and the vast majority of absorbed light was utilized for photosynthesis. In the exposed site both deepoxidation and epoxidation of the xanthophyll cycle and changes in the level of xanthophyll-dependent thermal energy dissipation largely tracked the diurnal changes in photon flux density (PFD). In the understory the xanthophyll cycle became largely deepoxidized to zeaxanthin and antheraxanthin upon exposure of the leaves to the first high intensity Sunfleck and this high level of deepoxidation was maintained throughout the day both during and between subsequent Sunflecks. In contrast, thermal energy dissipation activity, and the efficiency of photosystem II, fluctuated rapidly in response to the changes in incident PFD. These findings suggest a fine level of control over the engagement of zeaxanthin and antheraxanthin in energy dissipation activity, presumably through rapid changes in thylakoid acidification, such that they became rapidly engaged for photoprotection during the Sunflecks and rapidly disengaged upon return to low light when continued engagement might limit carbon gain.
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Rapid changes in xanthophyll cycle‐dependent energy dissipation and photosystem II efficiency in two vines, Stephania japonica and Smilax australis, growing in the understory of an open Eucalyptus forest
Plant Cell and Environment, 1999Co-Authors: William W. Adams, Barry A Logan, Barbara Demmig-adams, D H Barker, C B OsmondAbstract:Leaves of Stephania japonica and Smilax australis were characterized in situ on the coast of north-eastern New South Wales, Australia, where they were growing naturally in three different light environments: deep shade, in the understory of an open Eucalyptus forest where they received frequent Sunflecks of high intensity, and in an exposed site receiving full sunlight. In deep shade the xanthophyll cycle remained epoxidized during the day and the vast majority of absorbed light was utilized for photosynthesis. In the exposed site both deepoxidation and epoxidation of the xanthophyll cycle and changes in the level of xanthophyll-dependent thermal energy dissipation largely tracked the diurnal changes in photon flux density (PFD). In the understory the xanthophyll cycle became largely deepoxidized to zeaxanthin and antheraxanthin upon exposure of the leaves to the first high intensity Sunfleck and this high level of deepoxidation was maintained throughout the day both during and between subsequent Sunflecks. In contrast, thermal energy dissipation activity, and the efficiency of photosystem II, fluctuated rapidly in response to the changes in incident PFD. These findings suggest a fine level of control over the engagement of zeaxanthin and antheraxanthin in energy dissipation activity, presumably through rapid changes in thylakoid acidification, such that they became rapidly engaged for photoprotection during the Sunflecks and rapidly disengaged upon return to low light when continued engagement might limit carbon gain.
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The response of xanthophyll cycle-dependent energy dissipation in Alocasia brisbanensis to Sunflecks in a subtropical rainforest
Australian Journal of Plant Physiology, 1997Co-Authors: D H Barker, Barry A Logan, William W. Adams, Barbara Demmig-adamsAbstract:The photosynthetic responses of leaves of Alocasia brisbanensis (F.M. Bailey) Domin (Araceae) to Sunflecks were monitored via chlorophyll fluorescence beneath the canopy of a subtropical rainforest in Australia. Additionally, the size and conversion state of the xanthophyll cycle were determined. Acclimation to understory environments that regularly experienced Sunflecks involved small increases in the size of the xanthophyll cycle pool in comparison to understory plants that never received Sunflecks. In understory plants that regularly experienced Sunflecks the rate of photochemistry and the level of xanthophyll cycle-dependent energy dissipation closely tracked changes in incident PFD. Subsequent to the first Sunfleck plants tended to retain their pool of xanthophyll cycle carotenoids as the deepoxidised forms (antheraxanthin and zeaxanthin) throughout the day. Retention of these deepoxidised forms apparently allows the trans-thylakoid membrane proton gradient to engage and disengage dissipation rapidly in response to a Sunfleck, thereby mitigating photooxidative damage and ensuring a rapid return to efficient light utilisation via photosynthesis in limiting light. Our results were also in agreement with previous studies that demonstrated a requirement for light activation of photosynthesis.
Matti Mõttus - One of the best experts on this subject based on the ideXlab platform.
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Measurement and modelling of the vertical distribution of Sunflecks, penumbra and umbra in willow coppice
Agricultural and Forest Meteorology, 2004Co-Authors: Matti MõttusAbstract:A 3D model of Salix viminalis canopy describing the coppice as consisting of randomly located shoots is used to simulate the fractional areas of Sunflecks, penumbra and umbra within the coppice canopy. The model is based on phytometrical measurements of willow coppice; results are compared with theoretical formulae, a simple model describing the canopy as consisting of randomly located plates, and measured canopy transmittance. The model predicts different Sunfleck, penumbra, and umbra fractional areas compared with the simple plate model, differences in the distribution of direct solar irradiance in penumbra are less significant. The 3D model also indicates that the congregation of foliage into shoots induces the dependence of the clumping index on measurement height and solar zenith angle.
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Statistical treatment of umbra length inside willow coppice.
Agricultural and Forest Meteorology, 2000Co-Authors: J. Ross, Matti MõttusAbstract:Abstract Measurements of the statistical characteristics of umbra and Sunflecks at different depths inside a willow coppice — Salix viminalis and Salix dasyclados — were carried out at Tartu Observatory, Toravere, Estonia, in 1997. A new instrument, the Sunfleck indicator, constructed by M. Sulev, was used. This instrument, moving perpendicularly to rows in the horizontal direction , counts the number and length of Sunflecks and umbrae at a level where downward cumulative leaf area index is L . During statistical data processing, several umbra characteristics — umbra length distribution function, mean number of umbrae, mean umbra length, fractional area of umbra, etc. — were calculated at different measurement heights as the functions of the optical path length τ = L/ sin h , where h is the solar elevation. The number of umbrae N U increases rapidly at small τ , has a maximum at τ ≈ 3–4 and decreases slowly with further increase in τ . This interrelationship was fitted by an exponential function. Umbra length distribution function can be divided into three regions: small umbrae (0–10 cm in length), medium-length umbrae (10–20 cm) and long umbrae (up to 100 cm). At all depths the number of small umbrae exceeds the number of medium-length and long umbrae by 3–10 times. The fractional area of umbra k U ( τ ) increases with τ and was approximated by a rectangular hyperbola. In lower layers ( τ = 8–12) k U ( τ ) reaches 0.85–0.90 and these layers are dominated by umbra.
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Statistical treatment of Sunfleck length inside willow coppice
Agricultural and Forest Meteorology, 2000Co-Authors: J. Ross, Matti MõttusAbstract:Abstract Different Sunfleck characteristics, length and number of Sunflecks per metre and Sunfleck fractional area, were measured at different depths inside willow ( Salix viminalis ) canopy. All these characteristics depend on the pathlength of the direct solar radiation beam into the canopy, τ = L /sin h , where L is the downward cumulative leaf area index and h is the solar elevation. The mean Sunfleck length l S decreases exponentially with τ . The maximum length of short Sunflecks (length τ , but not exponentially. Like the number of short Sunflecks, the total number of Sunflecks increases with τ , reaches a maximum value of 10–12 Sunflecks m −1 at τ =2.5 and then decreases slowly to zero at τ =6. The vertical profiles of the Sunfleck fractional area are different for long (length >6 cm) and short Sunflecks. The fractional area of long Sunflecks decreases with τ exponentially; the fractional area of short Sunflecks, k SS , can be approximated by the formula k SS =0.19 τ exp(−0.15 τ 2 ), and the fractional area of all Sunflecks k S , by the formula k S =exp(−0.18 τ 2 ). Within the canopy, direct solar radiation is present in two areas — in Sunflecks and in penumbra. The total flux of direct solar radiation in penumbra is smaller than it is in Sunflecks and reaches its maximum value of 50% at the pathlength τ =2. There exists a good correlation with R 2 =0.94 between the fractional area of umbra, k U , and the fractional area of Sunflecks, k S , which can be fitted by the exponential formula k U =0.63 exp(−2.30 k S ).
Malcolm C. Press - One of the best experts on this subject based on the ideXlab platform.
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Physiological and ecological significance of Sunflecks for dipterocarp seedlings
Journal of experimental botany, 2004Co-Authors: Andrew D. B. Leakey, Julie D. Scholes, Malcolm C. PressAbstract:Irradiance is highly dynamic in many plant canopies. Photosynthesis during Sunflecks provides 10-90% of daily carbon gain. The survivorship of tree seedlings in the deeply shaded understorey of tropical rain forests is limited by their ability to maintain a positive carbon balance. Dipterocarp seedlings from the SE Asian rain forest were used as a model system to test novel aspects of the physiological and ecological significance of Sunflecks. First, understorey seedlings experienced leaf temperatures up to 38 degrees C in association with Sunflecks. Under controlled environment conditions, the inhibition of carbon gain at 38 degrees C, compared with 28 degrees C, was significantly greater during a sequence of Sunflecks (-59%), than under uniform irradiance (-40%), providing the same total photosynthetic photon flux density (PPFD). Second, the relative enhancement effects of elevated [CO2] were greater under Sunflecks (growth +60%, carbon gain +89%), compared with uniform irradiance (growth +25%, carbon gain +59%), supplying the same daily PPFD. Third, seedling growth rates in the forest understorey were 4-fold greater under a dynamic irradiance treatment characterized by long flecks, compared with a regime of short flecks. Therefore, stresses associated with dynamic irradiance may constrain photosynthetic carbon gain. Additionally, seedling photosynthesis and growth may be more responsive to interactions with abiotic factors, including future changes in climate, than previously estimated. The sensitivity of seedling growth to varying patterns of dynamic irradiance, and the increased likelihood of species-specific responses through interactions with environmental factors, indicates the potential for Sunflecks to influence regeneration processes, and hence forest structure and composition.
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High-temperature inhibition of photosynthesis is greater under Sunflecks than uniform irradiance in a tropical rain forest tree seedling
Plant Cell & Environment, 2003Co-Authors: Andrew D. B. Leakey, Malcolm C. Press, Julie D. ScholesAbstract:The survival of dipterocarp seedlings in the understorey of south-east Asian rain forests is limited by their ability to maintain a positive carbon balance. Photosynthesis during Sunflecks is an important component of carbon gain. Field measurements demonstrated that Shorea leprosula seedlings in a rain forest understorey received a high proportion of daily photon flux density at temperatures supra-optimal for photosynthesis (72% at ≥ 30 ∞ C, 14% at ≥ 35 ∞ C). To investigate the effect of high temperatures on photosynthesis during Sunflecks, gas exchange and chlorophyll fluorescence measurements were made on seedlings grown in controlled environment conditions either, under uniform, saturating irradiance (approximately 539 m mol m - 2 s - 1 ) or, shade/fleck sequences (approximately 30 m m m mol m - - 2 s - 1
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Relative enhancement of photosynthesis and growth at elevated CO2 is greater under Sunflecks than uniform irradiance in a tropical rain forest tree seedling
Plant Cell & Environment, 2002Co-Authors: Andrew D. B. Leakey, Malcolm C. Press, Julie D. Scholes, Jennifer R. WatlingAbstract:The survivorship of dipterocarp seedlings in the deeply shaded understorey of South-east Asian rain forests is limited by their ability to maintain a positive carbon balance. Photosynthesis during Sunflecks is an important component of carbon gain. To investigate the effect of elevated CO 2 upon photosynthesis and growth under Sunflecks, seedlings of Shorea leprosula were grown in controlled environment conditions at ambient or elevated CO 2 . Equal total daily photon flux density (PFD) ( ~ ~ ~ 7·7 mol m - - 2 d - 1 ) was supplied as either uniform irradiance ( ~ ~ ~ 170 m mol m - 2 s - 1 ) or shade/ fleck sequences ( ~ 30 m m m mol m - 2 s - 1 / ~ ~ ~ 525 m mol m - 2 s - 1 ). Photosynthesis and growth were enhanced by elevated CO 2 treatments but lower under flecked irradiance treatments. Acclimation of photosynthetic capacity occurred in response to elevated CO 2 but not flecked irradiance. Importantly, the relative enhancement effects of elevated CO 2 were greater under Sunflecks (growth 60%, carbon gain 89%) compared with uniform irradiance (growth 25%, carbon gain 59%). This was driven by two factors: (1) greater efficiency of dynamic photosynthesis (photosynthetic induction gain and loss, post-irradiance gas exchange); and (2) photosynthetic enhancement being greatest at very low PFD. This allowed improved carbon gain during both clusters of lightflecks (73%) and intervening periods of deep shade (99%). The relatively greater enhancement of growth and photosynthesis at elevated CO 2 under Sunflecks has important potential consequences for seedling regeneration processes and hence forest structure and composition.
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Elevated CO2 and Sunflecks interact to increase photosynthesis and growth in the tropical rainforest tree seedling Shorea leprosula
Science Access, 2001Co-Authors: Adb Leakey, Malcolm C. Press, Julie D. Scholes, Jennifer R. WatlingAbstract:To maintain a positive carbon balance and survive in the deeply shaded understorey of S.E. Asian rain forests, dipterocarp seedlings must utilize Sunflecks for photosynthetic carbon gain. Despite the potentially important ecological consequences, very little is known about the response of dynamic photosynthesis to elevated CO2. How, therefore, do photosynthesis and growth in Shorea leprosula respond to elevated CO2 under Sunflecks, compared with uniform irradiance? Steady state photosynthesis acclimated to elevated CO2, with no difference between plants grown under flecked and continuous irradiance. There was a reduction in the ratio of Vcmax/Jmax and greater light-saturated photosynthetic rates, under the elevated CO2 treatment. Light response curves indicated greater apparent quantum yield and a lower light compensation point, as well as greater photosynthetic rates at all PPFDs, in leaves grown and measured at elevated CO2. The responses to elevated CO2 of the dynamic components of photosynthesis during Sunflecks included: faster photosynthetic induction gain, greater post-irradiance CO2 fixation and lower post-irradiance CO2 burst. When these effects are added to changes in photosynthetic capacity, seen during steady state photosynthesis, there is an 87% increase in carbon gain.
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Photosynthetic Induction and Stomatal Oscillations in Relation to the Light Environment of Two Dipterocarp Rain Forest Tree Species
The Journal of Ecology, 1997Co-Authors: Simon W. Zipperlen, Malcolm C. PressAbstract:1 This paper reports on changes in photosynthetic induction and stomatal conductance (g,) in response to (i) continuous saturating light and (ii) darkness, for seedlings of two climax tree species (Shorea leprosula and Dryobalanops lanceolata, Dipterocarpaceae) growing in three differing rain forest light environments (ranging from 2 4 to 21 8 mol photon m-2 day-') in Malaysian Borneo. 2 For some species in understorey environments, a high proportion of diurnal carbon gain is attained during Sunflecks (transient periods of high light), and D. lanceolata is slower growing than S. leprosula but its seedlings can survive under deeper shade conditions. Our aim was to determine whether interspecific differences in dynamic photosynthetic responses to light could further explain observed differences in seedling ecology. 3 In contrast to the expected trend for shade-tolerant species, D. lanceolata showed faster induction in higher light environments and S. leprosula showed no relationship between light environment and induction rate. However, both species showed greater potential Sunfleck utilization efficiency in low light environments through slower rates of both induction loss and stomatal closure. 4 Shorea leprosula attained greater rates of maximal photosynthesis and gs, had faster rates of induction and retained a higher level of induction in prolonged darkness (> 30 min) than D. lanceolata in all light environments. However, S. keprosula showed faster induction loss and stomatal closure in the short term (10 min) than D. lanceolata, which, together with a potentially negative carbon balance between Sunflecks, may limit its distribution to microsites of higher Sunfleck frequency. 5 The balance between photosynthesis (A) and gs during induction resulted in a constant intercellular CO2 concentration of c. 270 p.p.m. after c. 11 min, which may represent a physiological optimum for both species. 6 In some circumstances transient peaks in A and ga were observed during induction, rather than a rise to a stable maximum, which we attribute to overcompensation of the stomatal response to light. In some cases for S. leprosula this initiated synchronized damped oscillations in gs and A that continued for c. 1 h in both continuous and discontinuous (80-s light/80-s dark) light. 7 Shorea leprosula plants with the ability to 'peak' or oscillate had the potential to increase both the rate and magnitude of response to Sunflecks in comparison with simple sigmoidal induction.
