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Tropical cloud forest canopy and subcanopy adapt to different light environments by regulating photosynthetic pigments

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Title:
Tropical cloud forest canopy and subcanopy adapt to different light environments by regulating photosynthetic pigments
Translated Title:
El dosel y el subdosel del bosque nuboso tropical se adaptan a diferentes ambientes de luz mediante el control de pigmentos fotosintéticos ( )
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Book
Language:
English
Creator:
Wallentine, Bradley D
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Subjects / Keywords:
Forest canopies   ( lcsh )
Chlorophyll--Measurement   ( lcsh )
Sunshine   ( lcsh )
Doseles forestales
Clorofila--Medida
Luz del sol
Tropical Ecology 2006
Forest subcanopies
Ecología Tropical 2006
Subdoseles forestales
Genre:
Reports   ( lcsh )
Reports

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Abstract:
The canopy and subcanopy of a Tropical Cloud Forest provide distinctly different light environments. Here, the amounts and ratios of photosynthetic pigments in leaves from a Cloud Forest canopy and subcanopy plants are compared. The pigments of forty canopy and subcanopy leaf samples are extracted using acetone and analyzed using a spectrophotometer. It is found that canopy and subcanopy plants possess equivalent means of concentrations of photosynthetic pigments per mass of leaf tissue (x = 0.21± 0.09 mg/g and 0.22 ± 0.11 mg/g, respectively). Therefore, plants from these two microhabitats invest the same quantity in major pigments for photosynthesis. However, the availability of light cause canopy plants to produce a higher concentration of photosynthetic pigments per area (x = 0.0079 ± 0.0026 mg/cm²) than subcanopy plants (x = 0.0059 ± 0.0019 mg/cm²). Based on the ratio of chlorophyll a to chlorophyll b, it appears that canopy plants (x =1.63 ± 0.57) use their photosynthetic pigments to maximize their rate of light processing. Subcanopy plants (x = 0.98 ± 0.26), in contrast, appear to maximize light absorption. Using the ratio of carotenoids to chlorophyll b, canopy plants (x = 1.24 ± 0.27) may be using carotenoids to prevent photoinhibition. Subcanopy plants, having a much lower carotenoids to chlorophyll b ratio (x = 0.97 ± 0.27), are possibly using carotenoids for further light absorption.
Abstract:
El dosel y subdosel de un bosque nuboso tropical proporcionan ambientes de luz muy diferentes. En este caso, se comparan las cantidades y proporciones de los pigmentos fotosintéticos en hojas de un dosel del bosque nuboso y las plantas del subdosel.
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Text in English.
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Born Digital

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The canopy and subcanopy of a Tropical Cloud Forest provide distinctly different light environments. Here, the amounts and ratios of photosynthetic pigments in leaves from a Cloud Forest canopy and subcanopy plants are compared. The pigments of forty canopy and subcanopy leaf samples are extracted using acetone and analyzed using a spectrophotometer. It is found that canopy and subcanopy plants possess equivalent means of concentrations of photosynthetic pigments per mass of leaf tissue (x = 0.21 0.09 mg/g and 0.22 0.11 mg/g, respectively). Therefore, plants from these two microhabitats invest the same quantity in major pigments for photosynthesis. However, the availability of light cause canopy plants to produce a higher concentration of photosynthetic pigments per area (x = 0.0079 0.0026 mg/cm) than subcanopy plants (x = 0.0059 0.0019 mg/cm). Based on the ratio of chlorophyll a to chlorophyll b, it appears that canopy plants (x =1.63 0.57) use their photosynthetic pigments to maximize their rate of light processing. Subcanopy plants (x = 0.98 0.26), in contrast, appear to maximize light absorption. Using the ratio of carotenoids to chlorophyll b, canopy plants (x = 1.24 0.27) may be using carotenoids to prevent photoinhibition. Subcanopy plants, having a much lower carotenoids to chlorophyll b ratio (x = 0.97 0.27), are possibly using carotenoids for further light absorption.
El dosel y subdosel de un bosque nuboso tropical proporcionan ambientes de luz muy diferentes. En este caso, se comparan las cantidades y proporciones de los pigmentos fotosintticos en hojas de un dosel del bosque nuboso y las plantas del subdosel.
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Forest canopies
Chlorophyll--Measurement
Sunshine
4
Doseles forestales
Clorofila--Medida
Luz del sol
653
Tropical Ecology 2006
Forest subcanopies
Ecologa Tropical 2006
Subdoseles forestales
655
Reports
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CIEE
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t Monteverde Institute : Tropical Ecology
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Tropical Cloud Forest canopy and subcanopy adapt to different light environments by regulating photosynthetic pigments Bradley D. Wallentine Department of Biological Sciences, University of Texas at Austin Abstract The canopy and subcanopy of a Tropical Cloud Fo rest provide distinct ly different light environments. Here, the amounts and ratios of photosynthetic pigments in leaves from a Cloud Forest canopy and subcanopy plants are compared. The pigments of forty canopy and subcanopy leaf samples are extracte d using acetone and analyzed using a spectrophotometer. It is found that ca nopy and subcanopy plants possess equivalent means of concentrations of photosynthetic pigments per mass of leaf tissue (x = 0.21 0.09 mg/g and 0.22 0.11 mg/g, respectively). Therefore, plants from these two microhabitats invest the same quantity in ma jor pigments for photosynthesis. However, the availability of light cause canopy plan ts to produce a higher concentration of photosynthetic pigments per area (x = 0.0079 0.0026 mg/cm) than subcanopy plants (x = 0.0059 0.0019 mg/cm). Based on the ratio of chlorophyll a to chlorophyll b, it appears that canopy plants (x =1.63 0.57) use their photosynthetic pigments to maximize their rate of light processing. S ubcanopy plants (x = 0.98 0.26), in contrast, appear to maximize light absorption. Using the ratio of carotenoids to chlorophyll b, canopy plants (x = 1.24 0.27) may be using carotenoids to prevent photoinhibition. Subcanopy plants, having a much lower carotenoids to chlorophyll b ratio (x = 0.97 0.27), are possibly using carotenoids for further light absorption. Resumen El pabelln y el estrato infe rior de un bosque nuboso tropical proporcionan distintamente diversos ambientes ligeros Aqu, las cantidades y los cocientes de pigmentos fotosintticos en hojas de las plantas del pabelln y estrato inferior de un bosque nuboso se comparan. Los pigmentos de las hojas de cuarenta pabelln y estrato inferior se extraen usando la acetona y se analizan usando un espectrofotmetro. Se encuentra que las plantas del pabelln y el estrato poseen medios equivale ntes de concentraciones de pigmentos fotosintticos por la masa del te jido fino de la hoja (x = 0.21 0.09 mg/g y 0.22 0.11 mg/g, respectivamente). Por lo ta nto, las plantas de estos dos ambientes ligeros invierten la misma cuantidad en los pi gmentos importantes para la fotosntesis. Sin embargo, la disponibilidad de las planta s ligeras del pabelln de la causa para producir una concentracin ms alta de pigm entos fotosintticos por rea (x = 0.0079 0.0026 g/cm) que las plantas estrato infe rior (x = 0.0059 0.0019 g/cm). De acuerdo con el cociente de la clorofila a a la clorofila b, aparece que las plantas del pabelln ( 0.57 de x =1.63) utilizan sus pigmentos foto sintticos para maximizar su ndice del proceso ligero. Plantas del estrato in ferior (x = 0.98 0.26), en cambio, aparezca 1

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maximizar la absorcin ligera. Usando el cociente de carotenos a la clorofila b, las plantas del pabelln (x = 1.24 0.27) utilizan los carotenos pa ra prevenir la destruccin de las molculas de la clorofila. Plantas del estrato inferior, te niendo un cociente de carotenos a clorofila b (x = 0.97 0.27) ms bajo, estn utilizando posiblemente los carotenos para la abso rcin ligera adicional. Introduction A Tropical Forest has two do minant strata, characterized by a tall, closed canopy of climax trees with a subcanopy of small shade-tolerant trees and shrubs (Richards, 1996). The canopy is characterized by high leaf densities and leat hery leaf thickness. Many canopy leaves are angled diagonall y to allow penetration of li ght to lower l eaves (Rundel and Gibson, 1996). The leaves of the subca nopy are characterized as horizontal, longer, and thinner to maximize exposure to availabl e light. Subcanopy plants have spatially arranged leaves in order to minimize self-shading. Since sunlight can be the most important and often the most limiting, resource in the Tropical Forest, it is not surprising that close connect ions have been found between leaf physiology and the environmental conditi on of these forest layers (Chazdon and Pearcy, 1991; Kabakoff and Chazdon, 1996; Mulkey, Chazdon, and Smith, 1996; Richards, 1996). Our understanding of light environments in the Tropical Forests has improved remarkably in the last three decades due to numerous comprehensive field studies (Mulkey, Chazdon, and Smith, 1996). Th ese field studies have discovered the dramatic variation, in quantity and quality, of solar that penetrates from the canopy down to the subcanopy. Since solar radiation is unobstructed in reaching the top of the Tropical Forest, irradiance levels in the canopy on a clear day often exceed 2000 mol m-2 s-1 (Hopkins, 1995; Rundel and Gibson, 1996). Al though light is a valuable resource for plants, high intensities of light can be destructive, lowe ring the efficiency of photosynthesis, a process known as photoinhibition. Even though photoinhibiti on can occur at all light intensities, it is most pronounced and destructive at high intensities (Tyystjrvi and Aro, 1996). However, cloud formation greatly effects lig ht diffusion in a cloud forest, reducing the sunlight that reaches the canopy up to 75% (Mulkey, Chazdon, and Smith, 1996). In contrast to the canopy, onl y about 0.5% of solar light effectively penetrates to the subcanopy level of the Tropical Forest (Chazdon and Fetcher, 1984). This diffuse background radiation below the canopy us ually only produces between 5 and 10 mol m2 s-1 (Chazdon and Fetcher, 1984; Mulkey, Ch azdon, and Smith, 1996). Therefore, most important for light accessibility in the subcanopy, the low li ght environment, however, is occasionally interrupted by brief intervals of almost direct sunlight called sunflecks. Although most of these periods of high sunlight last mere seconds and rarely reach full sunlight intensity, sunflecks account for up to 60% of light that the plants of the subcanopy receive (Anderson, Chow, and Goodch ild, 1988). The light penetrating to the subcanopy is also spectrally filtered by the stratum above. Much of the wavelengths of light reaching the subcanopy are in the fa r-red region, which is not photosynthetically functional (Chazdon and Pearcy 1996). The di ffusion of light by prenumbral effects also improves the light accessibility of the subcanopy especially in a cloud forest (Richards, 1996). The penumbral effect is a result of cloud cover scattering s unlight at different angles, allowing more light to penetrate the canopy cover (Chazdon and Fetcher, 1984). 2

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This scattered light from cloud cover, however, has only minor functionality in photosynthesis because it is largely enriched with far-red light (Lee and Downum, 1991). The pigments primarily responsible for ha rvesting light energy to be used in photosynthesis are chlorophylls (Hopkins, 1995). Specifically, chlorophyll athe most abundant pigment is the pigment solely re sponsible for the tran sformation of light energy into usable chemical energy. Chlorophyll b and carotenoids, both known as accessory pigments because they are not requ ired or actually directly involved in the transformation of light energy, are less abundant but help to amplify the amount of light absorption (Raven, Evert, and Eichhorn, 1999; Taiz and Zeiger, 1991). These three principal pigmentschlorophyll a, chlorophyll b, and carotenoids have absorption maxima at distinctively unique wave lengths of light. Since chlorophyll b is valuable in absorbing different wavelengths of light, the ratio of chlorophyll a to chlorophyll b is used as an indicator the range of light abso rbed by a plant (Watts and Eley, 1981; Melis, 1989; Smith et al. 1990; Hendrey and Price, 1993). In addition to absorbing light used in photosynthesis, carotenoids have recently been determined to aid in the prevention of photoinhibition. Because of the additional f unction of carotenoids, the ratio of total chlorophylls to carotenoids has been used as an indicator of plant response to high light intensities (Hendrey and Price, 1993; G oncalves, Marenco, and Vieira, 2001). The validity of this indicator will be discussed later. The location and function of the photosynthetic pigments must be briefly explained to fully understand the impact of variation in photosynthetic pigments. The photosynthetic pigments of vascular plants are found in two separate sites within the chloroplasts: photosystem I and photosystem II (Fig. 1). A plant must use both photosystems in order for photosynthesis to be efficient. Each of these photosystems is comprised of both a reaction cen ter and an associated light-h arvesting complex which are 3

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directly linked. The reaction centers, containing only a pair of chlorophyll a molecules, are the primary site where light energy initiall y begins to be convert ed to chemical energy (Smith et al., 1990). In intense light e nvironments, the rate of photosynthesis is determined by the abundance of reaction ce nters. The light-harvesting complexes, consisting of chlorophyll a, chlorophyll b, and carotenoids, function to absorb light and channel it to the reaction center (Hopkins, 1995). In low lig ht environments, the amount of light absorbed is dependent on these light -harvesting complexes. The light-harvesting complex of photosystem I contains mostly chlorophyll a molecules (Melis, 1989). Photosystem II, which comprises a majority of the chlorophyll b in a leaf, has two distinctive forms: photosystem II and photosystem II (Hopkins, 1995). Photosystem II possesses the largest light-harve sting complex of the photosystems as well as the highest concentration of chlorophyll b; photosystem II, having the smallest light-harvesting complex associated with it, still contains a higher concentration of chlorophyll b than photosystem I (Melis, 1989). Although photosystem II and photosystem II are structurally different, these two systems ha ve the same function in photosynthesis. Leaves have the ability to regulate the concen trations of the reaction centers and sizes of the light-harvesting systems in order to maximize photosynthetic rates and light absorption in different light conditions (Anderson et al. 1988, Smith et al. 1990). The purpose of this study is to analyze the presence of photosynthetic pigments in leaves and connect the significance of th eir relative abundanc e in the canopy and subcanopy of a Tropical Cloud Fore st. Many past studies of photosynthesis in the natural environment do not account for effects that individual pigments have on the transformation of light to chemical energy. On the other hand, laboratory research examining the significance of individual pigmen ts seldom relates the importance of these pigments to the photosynthetic capabilities of plants in a natural ecosystem. Relative comparisons of pigment concentrations between the two forest layers will be made on both a per mass and a per area basis. Comparison of photosynthetic pigment types within the leaves of a single plant, using ra tios, will be used to reveal the methods in which canopy and subcanopy plants have beco me adapted to their particular light environment. Based on the greater precedence for ma ximizing light absorption in a shaded environment, higher concentration of photosynthetic pigments per mass are expected in subcanopy plants. No significant differen ce in pigment concen tration per area is anticipated. This expectation is due to fact that more photos ynthetic pigments are expected per mass in the subcanopy, but the th icker leaves of the canopy will allow them to have equal amounts of photosynthetic pigmen ts. Furthermore, the ratio of chlorophyll a to chlorophyll b is expected to be n earest to one for plants inhabiting the subcanopy since this would balance of types of ch lorophyll and enhances the range of light absorption. The ratio of carotenoids to tota l chlorophyll is expected to be higher for canopy plants since the necessity for the prevention of photoinhibition is greater in intense light environments. Materials and Methods This study was conducted in the Tropical Clou d Forests of Montev erde, Costa Rica. Twenty canopy samples were collected from an aerial tram in the secondary forest of Natural Wonders with pole-clippers. Twenty subcanopy samples were collected from the 4

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understory of a closed canopy mixed primary and secondary forest near the Estacion Biologica using a pocketknife. All leaf samples were obtai ned from the crown of the plants. To reduce damage to the light absorb ing pigments, all samples were placed in an ice bath with minimal light. To further control pigment damage, the leaf samples evaluated in the laboratory the day of extrac tion. Leaves from each individual plant were measured and cut to an area 50 cm using sten cils and a single edge d razor blade. The 50 cm of leaf were then subseque ntly cut and reduced to very fi ne leaf fragments. The mass of the 50 cm, now in fine leaf fragments, was recorded in grams using a FischerScientific T top loading balance. P hotosynthetic pigments were extracted from the leaf fragments using 7 ml of 85% acetone solution. This solution was sustained at a pH of 6.5 with 2 ml of phosphate buffer (pH 6.5), which most accurately resembles the pH level of chloroplasts (Arnon, 1949). Th e photosynthetic pigments were allowed to precipitate in acetone for 15 minutes. During this 15 minute period, the solutions were shaken for 30 seconds in order to increase the interactions with acetone every 5 minutes. The mixture was then centrifuged at 4000 rp m with a Premiere XC-1000 centrifuge to separate the leaf cel ls and fragments from the acetone -pigment solution. The purified acetone-pigment solution was then decanted a nd the volume measured in milliliters using a graduated cylinder. Two ml of the acetone-pigment solution were subsequently added to 8 ml of 85% acetone in a cuve t. Using a Sequoia-Turner Model 340 spectrophotometer, absorption readings of the diluted aceton e-pigment solution were then taken at light wavelengths of 663, 646, and 470 nm The concentrations of pigments, per area and per mass, were determined using the following empirically derived equations (Lichtenthaler and Welbur, 1983): Chlorophyll a (mg/g) = [12.21 (Abs 663 ) 2.81 (Abs 646 )] x [Purified Volume (ml)] [200] x [Mass of Leaf Used (g)] Chlorophyll a (mg/cm) = [12.21 (Abs 663 ) 2.81 (Abs 646 )] x [Purified Volume (ml)] [200] x [Area of Leaf Used (cm)] Chlorophyll b (mg/g) = [20.13 (Abs 646 ) 5.03 (Abs 663 )] x [Purified Volume (ml)] [200] x [Mass of Leaf Used (g)] Chlorophyll b (mg/cm) = [20.13 (Abs 646 ) 5.03 (Abs 663 )] x [Purified Volume (ml)] [200] x [Area of Leaf Used (cm)] Carotenoids (mg/g) = {1000 (Abs 470 ) 3.27[chl a] 104 [chl b]} x {Purified Volume (ml)} {45400} x {Mass of Leaf Used (g)} Carotenoids (mg/cm) = {1000 (Abs 470 ) 3.27[chl a] 104 [chl b]} x {Purified Volume (ml)} {45400} x {Area of Leaf Used (cm )} Using the concentrations, ratios of chlorophyll a to chlorophyll b, carotenoids to total chlorophyll, and caroten oids to chlorophyll b were determined. 5

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Results DESCRIPTIONS. Overall, chlorophylls constituted the majority (68.3 0.9%) of the photosynthetic pigments extracted from th e plants. Specifically, chlorophyll a was the most abundant pigment, representing 37.7 0.5% of the measured pigments. Chlorophyll b, having the lowest amount, comprised only 30.1 0.4% while carotenoids generated 31.6 0.4% of the measured pigments. CONCENTRATION ANALYSIS. The concentrations of photosynthetic pigments per mass of leaf tissue (Fig. 2) were not signif icantly different between canopy (x = 0.30 0.13 mg/g) and subcanopy plants (x = 0.32 0.17 mg/g); (t-test, t = 0.35, df = 38.44, P < 0.05). Based on area, concentra tions of pigments (Fig. 3) were significantly higher in canopy plants (x = 0.0079 0.0026 mg/cm) th an subcanopy plants (x = 0.0059 0.0019 mg/cm); (t-test, t = 2.79, df = 36.89, P < 0.05). RATIOS The ratio of chlorophyll a to chlorophyll b was significantly lower (Fig. 4) in subcanopy plants (x = 0.98 0.26) in relation to canopy plants (x = 1.63 0.57); (t-test, t = 4.63, df = 38.00, P < 0.05). The canopy (x = 0.48 0.10) and subcanopy (x = 0.49 0.13) demonstrated no significant difference in the ratio of carotenoids to total chlorophyll (Fig. 5); (t-test, t = 0.31, df = 40.14, P < 0.05). The ratio of carotenoids to chlorophyll b was significantly higher (Fig. 6) in canopy plants (x = 1.24 0.27) compared to subcanopy plants (x = 0.97 0.27); (t-test, t = 3.14, df = 40.98, P < 0.05). 6

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Discussion Differences in both photosynthetic pigment con centrations and ratios suggest that canopy and subcanopy plants employ different techniques for light ab sorption in their respective light environments. The measure of concen tration of photosynthetic pigments per mass of leaf tissue can be used as a relative desi gnation for which plants make the most effort to absorb light. Since both canopy and subcanopy plants ha ve similar pigment content per mass of leaf, these two growth fo rms are investing similar amounts for photosynthesis. The concentration of photosynthetic pigmen ts per area can be used to determine the amount of light per leaf area is absorbed. Since most leaves already absorb 80 to 85% of available light, it would take more pigmen t per area for a canopy leaf to absorb this high percentage of light (Bjrkman, 1981). Fo r this reason, canopy plants in this study and studies of other plants in high levels of sunlight have shown a higher concentration of photosynthetic pigment per area of leaf compar ed to shaded plants (Goncalves, Marenco, and Vieira, 2001). Although subcanopy plants, w ith much less light per leaf area, would prefer to absorb a highe r percentage of light, doub ling the concentration of photosynthetic pigments per area would onl y increase light absorption by 3 to 6% (Bjrkman, 1981). Therefore, increasing their pigment concentrati on per area would be ineffective and thus metabolically uneconomi cal. Therefore in accordance with this study and others, subcanopy leaves are expend ing much fewer photosynthetic pigments per area of leaf to absorb the same high percentage of available light (Goncalves, Marenco, and Vieira, 2001). Ratios of individual pigments within a leaf are the best indicator to determine the photosynthetic adaptation of a plant to its light environment. Based on their chlorophyll a to chlorophyll b ratio, canopy plants are using mainly chlorophyll a for photosynthesis. Based on the chlorophyll a to chlorophyll b ratio for the subcanopy, these plants are using equal amounts of the chlorophylls for photosynthesis. Because each photosystem has a unique size and concentration of each ch lorophyll, the manipulation of these photosystems produces the observed changes in the relative amounts of each chlorophyll type. 7

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(Fig. 7A) Plants grown in high light exhi bit the reduction in all light-harvesting complex sizes as well as the use of photosystem II, which is associated with a smaller light-harvesting complex (Smith et al., 1990). Canopy plants in this study, having a smaller proportion of chlorophyll b, reflect this trend si nce employing photosystem II and smaller light-harvesting complexes w ould show this decrease in chlorophyll b. Since light-harvesting complexes are primarily used to absorb additiona l light, canopy plants do not heavily in these structures. Since phot osynthetic pigment amounts are equal per mass in canopy and subcanopy plants, the chlorophyl l content in the canopy leaves reflect an abundance of reaction centers. More reac tion centers would help expedite light processing in high irradiance conditions and th erefore would be advantageous to canopy plants. The acceleration of the photochemical light process would not only allow light to be converted to chemical energy quicker, but it would also preven t photoinhibition since photosynthetic chemicals are most commonly damaged when they absorb light that cannot immediately be transf erred (Smith et al., 1990). (Fig. 7B) Plants located in shaded en vironments are characterized with an increase in light-harvesting complex sizes in addition to an increase in the larger photosystem II, which leads to a higher concentration of chlorophyll b (Anderson, 1988). Subcanopy plants in this research de monstrate this photosystem manipulation since they display a larg er amount of chlorophyll b. The enhancement of photosystem II, which is associated with a larger light-harv esting complex, as well as the amplification of all light-harvesting systems allow a higher percentage of light to be absorbed by subcanopy plants. Although an increase in light harvesting systems would result in a decrease in reaction centers, a low concentration of reaction ce nters is not detrimental to the plant since rapid light pr ocessing is not required in the low light environment. Subcanopy plants are thus reduc ing light conversion rate in exchange for an increase in light absorption (Smith et al., 1990). The ratio of chlorophyll a to chlorophyll b observed in the canopy and subcanopy plants of the Tropical Cloud Forest also demonstrate a response to penumbral effects. The concentration of chlorophyll b in both the canopy and sub canopy is much higher than has 8

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been shown for most plants (Watts and El ey, 1981; Anderson et al., 1988; Melis, 1989; Hopkins 1995). Since the cloud c over reduces the light availability in the canopy, the high amount of chlorophyll b, relative to other su n leaf means, reveals that the canopy is acting as if it were moderately shaded. As common in shaded environments, the canopy plants in the Cloud Forest are probabl y still maintaining more photosystem II and reasonably sized light-harvesting complexes in or der to increase their percentage of light absorbed. The quality of the light available to the subcanopy due to penumbral effects must also be considered. Light filtration due to light scattering from clouds, as well as canopy leaves, provides the subcanopy with a large am ount of photosynthetically inactive light. However, far-red light is still absorbed by subcanopy plants, and its presence may be significant. Past research has demonstrated that the absorption of light near this wavelength causes the chlorophyll content in the leaf to increase in addition to an increase in the concentration of photos ystem II (Chow, Melis, and Anderson, 1990). I propose that this subsequent chlorophyll a nd photosystem II accumulation in reaction to far-red light absorption is an adaptive respons e by the plant in anticipation of possible photosynthetically functional sunlight in s unflecks. The occurrence of far-red light reveals to a plant that daylight is present and that usable light could instantly become accessible. The accumulation of photosynthetic material in the leaves allows the subcanopy plant to be prepared for and effici ently utilize a sunfleck if one develops. In the past, the ratio of carotenoids to chlorophyll has been used to indicate the response of plants to high light intensities However, the ambiguity of carotenoid function may invalidate the use of this indicat or. Given that carote noids can function in the prevention of photoinhibiti on in addition to light absorption for photosynthesis, a direct comparison of carotenoids to total chlorophyll may not successfully reveal the actual manner in which the plan t is using carotenoids. This study disproves the use of this indicator since the carotenoid to total chlorophyll ratio was not significantly different between the canopy and subcanopy. Since a high concentration of chlorophyll b reveals that a plant is attempting to maximize light absorption, I propose the ratio of carotenoids to chlorophyll b would be a more reliable indicator the carotenoid use by the plant. If a plant is in a low light environment and it is exploiting carotenoids for photosynthetic light absorption, the equally high concentration of chlorophyll b in response to shading will lead to a lower ratio of carotenoids to chlorophyll b. On the other hand, if a plant is in a high light environment and is using its carotenoids in high concentrations for the prevention of photoinhibition, the low concentration of chlorophyll b will result in a larger carotenoid to chlorophyll b ratio and, thus, demonstrate the response of the plant to elevated sunlight intensity. Therefore, this proposed ratio of carotenoids to chlorophyll b is a more adequate indicator in of the functionality of carotenoids within a plant. This indicator succeeds in this report since canopy plants had a higher carotenoids to chlorophyll b ratio, suggesting the use of carotenoids fo r photoinhibition, compared to the subcanopy which is most likely using carotenoids for phot osynthetic light absorp tion. The empirical data of Goncalves, Marenco, and Vieira (2001) on mahogany (t-t est, t = 11.32, df = 20.73, P < 0.05) and tonka beans (t-test, t = 20.24, df = 20.14, P < 0.05) in sun and shade environments also support this new indicator. 9

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Although there has recently been great progress in understanding the process and ability of plants in photosynthesis, ther e is still more to be discovered. Although chlorophyll a, chlorophyll b, and carotenoids have been shown to be the predominant photosynthetic pigments, the identification a nd significance of other accessory pigments in plants could be important in understanding this complicated process. Other pigments that absorb light not used in photosynthesis have been discovered in plants. Many of these pigments are used by plants in order to identify their surroundings. The understanding of these pigments and their func tions within the plant may provide insight into the adaptability of the worlds flora. Acknowledgements I thoroughly appreciate those, mentioned and unmentioned, who assisted in this study. I am very grateful to Alan Masters for all of his help, especially for obtaining much of the laboratory equipment needed for this research. I would also like to thank Karen Masters for acquiring the chemicals needed in this study as well as for her help in locating the usable canopy site at the Natural Wonders Tr am. I appreciate the Natural Wonders Tram company for allowing me to use their aerial tram to collect samples for this experiment. Finally, I would like to thank Tom McFarland and Cam Pennington for all of their help and patience during my research. Literature Cited Arnon, D. 1949. Copper enzymes in isolated chloroplasts. Polyphenoloxidase in Beta vulgaris Plant Physiology. 24: pp. 1-14. Anderson, J., W. Chow, and D. Goodchild. 1988. Thylakoid membrane organization in sun/shade acclimation. Aust. J. Plant Physiology 15: pp 11-26. Bjrkman, O. 1981. Responses to di fferent quantum flux densities. Physiological Plant Ecology I. Encyclopedia of Plant Physiology Springer-Verlag. New York. Mulkey, S., R. Chazdon, and A. Smith. 1996. Tropical Forest Plant Ecophysiology Chapman & Hall, New York, NY, pp. 1-44. Chazdon, R. and N. Fetcher. 1984. Photosynthetic light environments in lowland Tropical Rainforest in Costa Rica. Journal of Ecology 72: pp 222-230. Chazdon R. and R. Pearcy. 1991. The importan ce of sunflecks for forest understory plants. BioScience 41: pp. 760-766. Chow, W., A. Melis, and J. Anderson. Adjustments of photosystem stoichiometry in chloroplasts improve the quantum efficiency of photosynthesis. Proceedings of the National Academy of Sciences of the United States of America 87: 7502-7506. 10

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