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Revista chilena de historia natural

Print version ISSN 0716-078X

Rev. chil. hist. nat. vol.81 no.3 Santiago Sept. 2008

http://dx.doi.org/10.4067/S0716-078X2008000300002 

Revista Chilena de Historia Natural 81: 321-333, 2008

ARTICLES

Effects of low temperature acclimation on photosynthesis in three Chilean Proteaceae

Efectos de la aclimatación a baja temperatura sobre la fotosíntesis de tres proteáceas chilenas

 

MAURICIO CASTRO-ARÉVALO1, MARJORIE REYES-DÍAZ1-2*,MIREN ALBERDI3-2, VALERIA JARA-RODRÍGUEZ1, CAROLINA SANHUEZA1, LUIS J. CORCUERA1 & LEÓN A. BRAVO1

1 Laboratorio de Fisiología Vegetal, Departamento de Botánica, Universidad de Concepción, Casilla 160-C, Concepción, Chile.
2 Dirección actual: Instituto de Agroindustria y Departamento de Ciencias Químicas, Facultad de Ingeniería, Ciencias y Administración, Universidad de La Frontera, Casilla 54-D, Temuco, Chile.
3 Instituto de Botánica, Facultad de Ciencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile.
*e-mail for correspondence: reyesm@ufro.cl


ABSTRACT

Embothrium coccineum, Gevuina avellana, and Lomaría ferruginea grow in habitats contrasting in temperature and light intensity. Embothrium coccineum is a pioneer species that establishes itself in open sites completely exposed to variable sunlight and temperature. Gevuina avellana is usually found in more protected sites. Lomaría ferruginea regenerates under the canopy in sites with low thermal oscillations and high humidity. In order to establish an association of their habitats with functional attributes of each species, we studied their photosynthetic responses to temperature and light intensity. We expected that E. coccineum, which grows at full sun, is able to acclimate much better its photosynthetic apparatus to different temperatures than the shade tolerant L. ferruginea and the semi-shade tolerant G. avellana. One group of plants of each species containing six individuals each was subjected to 4 °C (cold-acclimated plants). Another group with the same number of individuals was subjected to 20 °C (non-acclimated plants). In non-acclimated plants of E. coccineum, the photosynthetic rate as measured by 02 evolution presented its maximum at 30 °C (16.5 ^mol 02 m"2 s"1) with an optimum between 20 and 35 °C, while in G. avellana and L. ferruginea the highest photosynthetic rate (~13 ^mol 02 m"2 s") was obtained at 25 °C. Cold acclimation significantly reduced the photosynthetic rates of the investigated species. The Q10 for 02 evolution decreased significantly in cold-acclimated E. coccineum and G. avellana but not in L. ferruginea. The fluorescence parameters of PSII showed that E. coccineum presents a higher effective quantum yield (cPPSII) at both growth temperatures. Photochemical quenching (qP) was more affected by the photosynthetic photon flux density (PPFD) in L. ferruginea than in the other species. Lomaría ferruginea presented the highest non-photochemical quenching (NPQ) at lower PPFD. Thus, the photosynthetic apparatus of each species presents functional differences according to the characteristics of light availability and temperature changes of their habitats.

Key words: Proteaceae, cold acclimation, photosynthetic oxygen evolution, photochemical efficiency of PSII.


RESUMEN

Embothrium coccineum, Gevuina avellana y Lomaría ferruginea crecen en habitat contrastantes en temperatura e intensidad lumínica. Embothrium coccineum es una especie pionera que se establece en sitios abiertos, completamente expuestos a luminosidad y temperatura variables. Gevuina avellana se encuentra usualmente en sitios más protegidos. Lomaría ferruginea regenera bajo el dosel en sitios con bajas oscilaciones térmicas y alta humedad. Con el objetivo de establecer una asociación de los habitats que ocupan estas especies y los atributos funcionales de cada una de ellas, se estudió las respuestas fotosintéticas a la temperatura e intensidad lumínica. Un grupo de plantas de cada especie (seis individuos por especie) se aclimataron a 4 °C (plantas aclimatadas al frío). Otro grupo con el mismo número de individuos fue mantenido a 20 °C (plantas no aclimatadas al frío). Plantas no aclimatadas de E. coccineum, presentaron una tasa fotosintética máxima, medida como evolución de 02, a 30 °C (16,5 ^mol 02 m"2s_1) con un óptimo entre 20 y 35 °C, mientras que en G. avellana y L. ferruginea la mayor tasa fotosintética (~13 ^mol 02 m"2 s"1) se obtuvo a 25 °C. La aclimatación al frío redujo significativamente las tasas fotosintéticas de las especies. El Q10 para la evolución de 02 fue similar en las tres especies en el estado no aclimatado al frío. La aclimatación al frío disminuyó Q10 en E. coccineum y G. avellana, pero permaneció invariable en L. ferruginea. Los parámetros de fluorescencia del PSII mostraron que E. coccineum presentó un mayor rendimiento cuántico efectivo del PSII (<t>PSII) a ambas temperaturas crecimiento. El apagamiento fotoquímico (qP) fue más afectado por la densidad de flujo fotónico fotosintético (PPFD) en L. ferruginea que en las otras especies. Lomatia ferruginea presentó el mayor apagamiento no fotoquímico (NPQ) a una baja PPFD. Por lo tanto, el aparato fotosintético de cada especie presenta diferencias funcionales de acuerdo a las características de disponibilidad lumínica y cambios de temperatura de sus habitat.

Palabras clave: Proteaceae, aclimatación al frío, evolución fotosintética de oxígeno, eficiencia fotoquímica del PSII.


 

INTRODUCTION

One of the most important factors that limit development and distribution of plants is temperature, because different physiological processes have different temperature optima for their functioning (Larcher 1995). Optimal temperature for gas exchange is known to vary among species (Berry & Bjorkman 1980. Badger et al. 1982, Ferrar et al. 1989, Makino et al. 1994, Hikosaka et al. 1999, Yamasaki et al. 2002). Low temperature decreases all biochemical processes including photosynthesis, respiration and protein synthesis (Hopkins 1999). This is due to the sensibility of cellular membranes to low temperature, independently of the damage that may occur due to intracellular formation of ice nuclei, which is usually lethal (Alberdi & Corcuera 1991). Plants growing in different environments have different optima for photosynthesis, with plants in colder environments having lower temperature optima than those in warmer climates (Ngugi et al. 2003). Furthermore, photosynthesis plasticity in response to temperature is decisive for the success of a species in different environments and microclimates (Cabrera 2002).

Embothrium coccineum J. R. et G. Forster. Gevuina avellana Mol. and Lomatia ferruginea (Cav.) R. Br. are woody species of Proteaceae that prosper in different biotopes along the latitudinal and altitudinal gradient in the forests of the southern tip of South America. Embothrium coccineum and L. ferruginea present the widest and most southerly latitudinal distribution (35 to 56° S and 35°30' to 52° S), respectively, whereas G. avellana has a narrower distribution (33 to 45° S) (Rodriguez et al. 1983). In addition to the differences in distribution, other important characteristics of ecological and/or physiological nature exist among these species.

Embothrium coccineum is a pioneer sun plant that colonizes open areas after natural or anthropic disturbance events, where it grows quickly forming dense stands. Gevuina avellana grows in temperate habitats and although it can grow directly exposed to sun, it needs some protection under other trees in the coldest biotopes. The best biotope for L. ferruginea presents small thermal oscillations and high environmental humidity. These species differ in their pioneer or successional role because they occupy different habitats varying in their exposition to light and temperature.

In plants, high radiation levels may cause photoinhibition of photosynthesis and photooxidative destruction of the photosynthetic apparatus (Osmond 1994). The exposure of plants to low temperature produces adverse effects on metabolic functions and one of the sensitive processes is photosynthesis (Larcher 1995). Species that are sensitive to cool temperature have a tendency to exhibit photoinhibition of photosynthesis when they are exposed to low temperature, even at moderate light intensity. On the other hand, species that are able to acclimate to low temperature are much less sensitive to low temperature-induced photoinhibition (Alves et al. 2002). We studied the effects of temperature on the photosynthesis of three species of the Proteaceae that differ in their biotopes preferences. The study by Read & Hill (1985) about Nothofagus species and the present study are the first studies performed in species of the Chilean temperate rain forest on the relationship between temperature acclimation of photosynthesis and their habitats. It is postulated that E. coccineum, which grows at full sun, is able to acclimate its photosynthetic apparatus much better to different temperatures than the shade tolerant L. ferruginea and the semi-shade tolerant G. avellana.

MATERIAL AND METHODS

Experimental design

One year-old seedlings of the three species were collected from the field in Katalapi Park (Pichiquillaipe), Décima Región, Chile, in winter. Seedlings were transferred to pots filled with soil from their habitats and maintained in climatic chambers for six months. Then, one group of plants of each species was maintained for two months at 20 °C (non-acclimated), 14:10 h day:night photoperiod, with a photosynthetic photon flux density (PPFD) of 200-250 mmol photons nr2 sA at the top of canopy and a relative humidity of 85 %. Another group of plants of the investigated species was maintained for the same period at 4 °C (cold-acclimated) at the same photoperiod and PPFD. The light source consisted of cool-white fluorescent tubes F-40CW (General Electric, Charlotte, NC, USA). Seedlings were irrigated once a week and fertilized with Phostrogen (Solaris, Buckingghamshire, United Kingdom) using 0.2 g L_1 once every two weeks.

Photosynthesis (oxygen evolution)

Photosynthesis determined as oxygen evolution (Pn) was measured in leaf discs of each species with a gas phase oxygen electrode unit, using a Model LD2/3 oxygen electrode chamber and the PC operated oxygen electrode control unit, Oxylab (Hansatech Instruments Ltd. King's Lynn, Norfolk, England). Measurements were performed in non-acclimated (20 °C) and cold-acclimated (4 °C) plants at different temperatures between 5-40 °C controlled by a circulating water bath (Thermo Haake K15, Electron Corporation, Germany) and at a light irradiance of 250 [xmol photons m 2 s_1, obtained from the light saturation curves, measured as mentioned below. A Leaf disc of 10 cnr2 was put inside of LD2/3 measuring chamber and adapted for 10 min to each temperature. Measurements were performed at saturating CO2, obtained by adding a couple of drops of 1M carbonate/bicarbonate buffer at pH 9 into the measurement chamber. Light saturation levels for each species were obtained from photosynthesis light response curves made with nine irradiance values over the range of 0-800 [xmol photons nr2 sA given by an array of red light emitting diodes (Model LH36/2R, Hansatech Instruments Ltd., King's Lynn, Norfolk, England).

The temperature dependence of photosynthesis, measured as the rate of oxygen evolution (Pn), was estimated by calculating the temperature coefficient (Q10) for each species and treatments. Qi0 is the empirically measured change in the rate of a reaction for an alteration in temperature of 10 °C (Mohr & Schopfer 1995). The dependence of the reaction constant (k) upon temperature (T) could be described by the Arrhenius equation: In k = lnk0 - A/R*T, where k0 and A, are temperature dependent. Arrhenius constants were empirically determined: R is the universal gas constant. There is a linear relation between In k and T_1 following first equation. From the graphical presentation of the function In k = - (AR !) T_1 + In k0 (which correspond to y = -ax + b), A and k0 can be calculated. Substituting k for net photosynthesis rate (Pn) and considering that A/ R is the slope of the curve In Pn versus T_1, Q10 was obtained in the following equation: lnQi0 = A/R*(l/T - 1/T + 10) (Mohr & Schopfer 1995).

Fluorescence parameters

Fluorescence signals were measured by a pulse-amplitude modulated fluorometer of attached non shaded leaves (FMS 2, Hansatech Instruments Ltd., United Kingdom). The protocol of Reyes-Díaz et al. (2005) was followed. Non-acclimated and cold-acclimated leaves of the three species were dark adapted for 30 min (to obtain open reaction centers) with leaf-clips provided with a mobile shutter plate. Then the fiber-optic and its fiber-optic adapter were fixed to a controlled temperature chamber (20 or 4 °C), where the sample was placed and light pulses were applied. The minimal fluorescence (F„) was determined by applying a weak modulated light (0.4 \ivao\ photons nr2 s_1) and maximal fluorescence (Fm) was induced by a short pulse (0.8 sec) of saturating light (9,000 \ivao\ photons nr2 s_1). Fluorescence signals were followed until steady state level of fluorescence (Fs) was reached at different actinic light levels. To obtain Fm' (maximal fluorescence during illumination) a pulse of 9,000 \ivao\ photons nr2 sA was used. Fo', the minimal level of fluorescence, was determined after turning the actinic light off and immediately a 2 sec far red (FR) pulse was applied. In this paper, the data of Fv/Fm = variable fluorescence/maximal fluorescence, where Fv = (Fm-F0) (maximal fluorescence - minimal fluorescence) and 4>PSII = Fm' - (F/Fm') were used as indicators of the maximal and effective quantum yield of the PSII, respectively (Genty et al. 1989). Photochemical quenching (qP) was calculated as: qP = (Fm' - Fs) / (Fm' - F0'), and non photochemical quenching was calculated as: NPQ = (Fm - Fm') / Fm' (Maxwell & Johnson 2000).

The different actinic light were controlled by the light source of the FMSII apparatus and applied through an optic fiber. Light intensity at the leaf surface was calibrated using a LI-250 light meter (Li-Cor).

Determinations of total carotenoids and chlorophylls a and b in leaf extracts

Leaf extracts of non-acclimated (20 °C) and cold acclimated (4 °C) plants were made using 0.10 g FW with 5 ml of ethanol 96 % and 0.3 mg mL1 of CaCC>3 in cold and dim light. Then the extracts were centrifuged for 3 min at 12000 rpm (about 5,000 g) and 4 °C. Absorbance was measured in aliquots of the supernatant at 470, 649 and 665 nm (Shimadzu UV-1203) according to the method proposed by Lichtenthaler & Wellburn (1983).

Statistical analyses

Reported values correspond to the means of three replicates for pigment experiments, four replicates for fluorescence parameters and three replicates for photosynthesis. For determining the effect of temperature on Pmax of each species, we performed a one way ANOVA, followed by a Tukey test to identify those values with significant differences. From these differences the optimum temperature for each species was established. The data were also subjected to a two-way ANOVA (where the factors were species and temperature of acclimation) and a Tukey test for multiple comparisons. The analyses were performed with the software STATISTICA (version 6, Stat Soft Inc.). Differences between the values were considered significant at a =0.05.

RESULTS

Effect of the temperature acclimation on photosynthesis rate (Pmax)

Photosynthesis determinations obtained by oxygen evolution showed differences in the temperature for maximum photosynthesis between the non-acclimated and cold-acclimated species (Fig. 1A and IB). Non-acclimated plants of E. coccineum had their maximum photosynthesis (Pmax) at 30 °C (16.5 ^mol C^nr2 s_1), with an optimum between 20 ° and 35 °C while in G. avellana and L. ferruginea the highest photosynthetic rate (~13 ^mol O2 m"2 s_1) was obtained at 25 °C. Statistically significant differences among the Pmax °f non-acclimated E. coccineum and the other species were found (P < 0.05). Differences in oxygen evolution were found between non-acclimated and cold-acclimated plants of the three species being lower the photosynthesis in cold acclimated than in not acclimated ones (P < 0.05). Cold-acclimated E. coccineum showed two similar Pmax at 10 °C and 35 °C, whereas G. avellana Pmax was at 30 °C, and L. ferruginea at 25 °C (Fig. IB). At this temperature L. ferruginea species showed the highest Pmax (around 2 fold-higher) with respect to the other species at the same temperature and 1.3- and 1.7-fold higher than Pmax of G. avellana and E. coccineum respectively. Non-acclimated plants evidenced net oxygen consumption (respiration) at the lowest (5 °C) and at the highest (40 °C) temperature, whereas cold-acclimated plants showed net respiration only at 40 °C (Fig. 1A). In the non-acclimated state the highest rate of respiration at 5 °C was found in L. ferruginea (-2.6 \ivao\ O2 nr2 s_1) and the lowest in E. coccineum (-0.6 \ivao\ O2 nr2 s"1) (P < 0.05), while at 40 °C E. coccineum and G. avellana presented statistically significant higher respiration rates (around -3.3 ^mol O2 nr2 s_1) than L. ferruginea (-0.5 \ivao\ 02 nr2 s1) (P < 0.05) (Fig. 1A). Nevertheless, cold acclimated L. ferruginea evidenced higher respiration rate (-2.5 \ivao\ O2 m 2 s_1) than E. coccineum (around -0.67 \ivao\ O2 nr2 s_1) at 40 °C (P < 0.05) (Fig. IB). A statistically significant interaction between the acclimation treatment and temperature for oxygen evolution was found for each species (P < 0.001). Temperature coefficients (Q10) of non acclimated plants were very similar among them. In contrast, in cold-acclimated plants Qi0 decreased more than 23 % with respect to the non-acclimated plants (P < 0.05), with the exception of the shade-tolerant L. ferruginea (Table 1).



Effect of cold acclimation on light response of net photosynthesis and fluorescence parameters of PSII

Light response of net photosynthesis was distinctive for the non-acclimated studied species and consistent with light levels observed in their natural environments. Thus, in the non- acclimated state, the shade species L. ferruginea exhibited the highest dark respiration (-5.4 [xmol C^nr2 s_1 at 0 PPFD) and the lowest maximum rate of net photosynthesis (3.8 [xmol C^nr2 s_1) (Fig. 2A). G. avellana, the shade-semitolerant species, showed an intermediate value of maximum net photosynthesis (10.4 [xmol O2 nr2 s_1) and E. coccineum the highest (16 [xmol O2 nr2 s_1) (Fig. 2A). Cold acclimation reduced significantly (around in 80 %) the net photosynthesis of E. coccineum and G. avellana (P < 0.05), but did not significantly affect photosynthesis of L. ferruginea (Fig. 2B). Dark respiration rates remained similar in both cold-acclimated and non-acclimated plants (Fig. 2A, 2B at 0 PPFD). Maximal and effective quantum yield of PS II (Fv/Fm and 4>PSII, respectively), photochemical quenching (qP) and non-photochemical quenching (NPQ) in non-acclimated and cold-acclimated plants of the three studied species were measured (Fig. 3A-3H). Fv/Fm values of non-acclimated plants remained between 0.7 to 0.85, at increasing light intensities, indicating an optimal physiological state (Bjorkman & Demmig 1987, 1995), specially for E. coccineum and G. avellana, which had the highest values (Fig. 3A). Contrarily, in cold-acclimated plants the Fv/Fm values of G. avellana, decreased significantly above 500 ^mol photons m 2 s_1 reaching photoinhibitory values (around 0.6) (Bjorkman & Demmig 1987, 1995), whereas L. ferruginea and E. coccineum showed normal values during the light treatment (Fig. 3B). In the three species, 4>PSII of non-acclimated plants decreased significantly at increased irradiances (Fig. 3A) Light response of <E>PSII of non-acclimated plants of L. ferruginea had a clear difference with respect to the other two species, showing the lowest light saturation and exhibiting the lowest values at high light indicating that this species would have the lowest electron transport rate (Fig. 3A). No major differences were observed in light response of <E>PSII between G. avellana and E. coccineum. Cold acclimated plants showed a decrease in light saturation of OPSII of all species, being G. avellana which exhibited the most conspicuous decrease with light intensity, being around 0.15 even at 200 mmol photons nr2 sA (Fig. 3B). Nonetheless, over 1,000 mmol photons nr2 s_1 similar values were reached in all species. Non-acclimated plants of E. coccineum maintained the highest fraction of open reaction centers (higher qP) at irradiances from 200-1,000mmol photons m 2 s_1, exhibiting no differences with G. avellana at 1,600 mmol photons nr2 s_1 (Fig. 3C). Lomatia ferruginea was clearly the species that kept the lowest proportion of open reaction centers at the lowest irradiance (qP about 0.2 from 400 mmol photons nr2 s_1 ahead), which indicates that even a low irradiance caused a significant excitation pressure of PSII in this species. Cold acclimation sharply reduced the light intensity at which qP was saturated for all species (Fig. 3D), being more obvious in G. avellana where its qP decreased to one third of the value observed in the non-acclimated plants even at 200 mmol photons nr2 s_1. Cold acclimated plants of E. coccineum also decreased qP with respect to non-acclimated ones reaching about 50 % of qP at about 200 mmol photons nr2 s_1. Thermal dissipation of excess energy (NPQ) of non-acclimated plants was saturated at a lower irradiance (400 mmol photons nr2 s_1) in L. ferruginea than in the other species (Fig. 2E). In this treatment, NPQ of G. avellana was saturated at about 1,100 mmol photons nr2 s_1, while E. coccineum NPQ did not reach the saturation point. NPQ decreased in cold-acclimated plants of all species with respect to non-acclimated plants (Fig. 3E and 3F). Interestingly, cold acclimated L. ferruginea maintained the highest capacity for heat dissipation and a slightly higher light saturation of NPQ than in non-acclimated state (Fig. 3F). NPQ of cold treated G avellana and E. coccineum were readily saturated at 200 and 500 mmol photons nr2 sA respectively.



Variation in the content of pigments

Because pigments are strongly associated with the photochemical performance and photosynthesis of plants, pigments contents of non-acclimated and cold-acclimated plants of the investigated species were studied. Non-acclimated plants of E. coccineum and L. ferruginea showed similar content in total chlorophylls and carotenoids, both in higher amounts than in G. avellana (P < 0.05), (Table 2). These species showed a similar Chi alb ratio. A decrease in total chlorophyll contents (around 2-fold) was found in the cold-acclimated individuals with respect the non-acclimated ones of the three species (Table 2). Total chlorophyll contents of cold acclimated species presented slightly differences between them. Embothrium coccineum and G. avellana (P < 0.05) decreased noticeably their carotenoids contents with respect to non-acclimated plants, exhibiting a lower carotenoids/chlorophyll ratio and also a low Chi alb ratio than non-acclimated plants. Interestingly, cold-acclimated plants of L. ferruginea had a similar Chi alb ratio and carotenoids/chlorophyll ratio than non-acclimated ones (Table 2). Independently of the treatment, carotenoid contents of L. ferruginea were the highest among the species. Statistically significant interactions between treatments and species were obtained for chlorophyll a (P = 0.016), total chlorophyll (P = 0.028) and total carotenoids (P = 0.041).


DISCUSSION

Photosynthesis and respiration are strongly dependent on leaf temperature (Larcher 1995). These rates generally increase exponentially with temperature, although it has been demonstrated that after the photosynthetic parameters reach their maximum level, they decrease at higher temperature. The quantization of the photosynthetic response of individual species to temperature and light is important for comparing the plasticity or adaptability of different species to environmental conditions (Dungan et al. 2003). According to Krause (1994) photosynthetic rates diminished in cold-acclimated plants at 4 °C due to the highest proportion of inactive PSII centers caused by low temperature. This assumption is consistent with the lower proportion of open reaction centers (qP) and lower light saturation of qP and OPSII exhibited by cold acclimated plants of G. avellana and E. coccineum respect to non-acclimated ones (Fig. 3B and 3D). In addition, it has been reported that a higher dark respiration level in cold-acclimated plants may be associated with low levels of net photosynthesis (Graham & Patterson 1982). However, in our study, this appears not to be the case because a slightly decreased or similar dark respiration was observed in cold acclimated and non-acclimated plants of the studied species (Fig. 2A and 2B). It has been reported that low growth temperature may decrease the activation energy of enzymatic reactions (Pereira 1995, Hikosaka et al. 2006) leading to a lower temperature requirement to reach maximum net photosynthesis. This was the case only for L. ferruginea and E. coccineum. Furthermore cold acclimated plants exhibited a higher photosynthesis stability at low temperature, preserving higher positive net photosynthesis at the lowest temperature than non-acclimated plants (Fig.1 A and IB). Non-acclimated plants of E. coccineum presented a high photosynthesis rate accompanied by a wide optimum temperature range for photosynthesis. This is consistent with its typical character of sun plant (Weinberger 1974) and their widest latitudinal and altitudinal distribution. On the other hand, G. avellana and L. ferruginea present a more restricted optimum range of photosynthesis than E. coccineum, which is consistent with their more shade tolerance features and their lower latitudinal distribution for G. avellana but not for L. ferruginea which has a wider latitudinal distribution than G. avellana. Non-acclimated plants of E. coccineum showed significant differences with respect to the other species in photosynthesis level, but not in the cold-acclimated state, indicating that its photosynthetic apparatus is more sensitive to variations of temperature (Fig. 1A and IB). Nonetheless, E. coccineum seems to be more adapted to low temperature than the other two species because it presents the highest photosynthesis rate, together with a high OPSII even at high light intensity, which demonstrated that it possesses a more efficient photosynthetic apparatus than the other species. The high values of photosynthesis are consistent with the behavior of species that are often among the first to become established in mesic successional sequences (Dungan et al. 2003) as it occurs with E. coccineum. Moreover, this species can colonize highly unfavorable open habitats exposed to low temperature and high irradiance (Weinberger 1974, Alberdi & Donoso 2004). Sun plants possess efficient energy dissipation mechanisms, including the xanthophylls cycle (carotenoids) (Loggini et al. 1999) and other physiological processes such as photorespiration and Mehler reaction (Niyogi et al. 1998, Pérez-Torres et al. 2004). During sunny days, the most abundant pigment of the xanthophyll cycle is zeaxanthin. Most plants require a higher photoprotective potential during the summer (Niyogi et al. 1998, Loggini et al. 1999). Also during the summer some plants present the lowest chlorophyll concentration, being capable of reducing the overexcitement, together with maximizing the xanthophyll cycle function (Bjorkman & Deming-Adams 1995, Kyparissis et al. 2000). Shade species present frequently smaller total carotenoids and higher chlorophylls contents than sun plants (Adams et al. 2004). However, non-acclimated plants of L. ferruginea and E. coccineum showed higher chlorophyll content than the other investigated species. In cold-acclimated plants, however, their contents decreased with respect to non-acclimated plants. This is most likely due to an acclimation response before the decrease in the enzymatic activity of the photosynthesis process (Pérez-Torres et al. 2004). Embothrium coccineum, G. avellana, and L. ferruginea decreased their total carotenoids content in cold-acclimated plants. However, L. ferruginea maintained a higher content of total carotenoids than the other species under the same condition. This could indicate that the exposition of this species to low temperature induced a major photosensitivity of its photosynthetic apparatus, and thereby a higher demand for photoprotection, with a higher NPQ, contrary to the other species (Fig. 3F). The decrease in pigments contents occurs as a consequence of cold-acclimation (Alves et al. 2002). It is notorious that G. avellana, in non-acclimated state, presents an intermediate behavior in the measured pigments parameters, while under cold-acclimation conditions it was more affected. A higher acclimation capacity would be advantageous when radiation or temperature suddenly increase having important effects on forest composition and regeneration (Oguchi et al. 2003). NPQ in cold-acclimated E. coccineum and L. ferruginea showed a strong increase from 0 to 200 \ivao\ photons nr2 s_1. This agrees with a decrease in qP and OPSII, indicating a high saturation of the reaction centers at relatively low light intensities (Fig. 3). This was probably related to PSII photoprotective thermal energy dissipation of excess absorbed light (Colom et al. 2003). The saturation of OPSII, qP and NPQ at low irradiance in L. ferruginea explains why this plant is a shade-tolerant species. E. coccineum shows a light response in heat dissipation (NPQ) similar to the semi-shade tolerant species (Fig. 3E) and a higher level of photosynthesis (Fig. 2), thus confirming the higher capacity of E. coccineum to respond to environmental variations. Two different strategies for light utilization in relation to growth and cold acclimation have been described (Savitch et al. 2002). The first strategy consists in maintaining high levels of photosynthetic rates, no chlorophyll losses, and minimal NPQ. The second strategy shows general depression of photosynthesis, pigments losses, and high and sustained capacity for NPQ. In our case, the three species appear to belong to the second strategy, but with lower NPQ with respect to the non-acclimated species. It is also important to mention that G. avellana seems to be more affected by cold acclimation than the other species, which is corroborated by the low Fv/Fm values.

It is well known that temperature decreases with increasing latitude and altitude (Bannister 2007). Cabrera et al. (1998) reported lower photosynthesis in species of the tropical high Andes distributed at higher altitudes with respect those located at lower ones. A similar behavior was found by Pipper et al. (2006) in Kageneckia angustifolia growing in the Chilean Andes at different altitudes.

In general, it appears that the photosynthetic apparatus of L. ferruginea is less plastic than the other two species. This is consistent with their habitat preferences, in which L. ferruginea, a shade tolerant plant with more restricted latitudinal distribution than E. coccineum, is usually protected from drastic environmental variations. In contrast, E. coccineum shows a much higher plasticity in its photosynthetic performance, which would be required by the greater changes in radiation and temperature of its habitats, located at the most southerly easterly, altitudinal and latitudinal distribution (Escobar et al. 2006). G. avellana and L. ferruginea are distributed at lower altitudes in the occidental sites of the Andes, with more equilibrated thermal regimes (Donoso et al. 2006, Donoso & Utreras 2006). In general, the distribution of these species is concordant with their lower photosynthetic performance.

ACKNOWLEDGMENTS

The authors thank FONDECYT 1050640 for financial support and to Alexis Estay and Valeria Neira provided technical assistance. Sandra Gacitúa provided advice on statistical analyses of the data. Frida Piper helped with the literature review.

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Associate Editor: Thomas Kitzberger
Received October 25, 2007; accepted February 04, 2008.

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