Crescimento e atividade fotossintetica em duas especies de lianas de uma floresta estacional semidecidual

AUTOR(ES)
DATA DE PUBLICAÇÃO

2004

RESUMO

The lianas are copious in natural and man-made clearings and it appears that these forms of life require high irradiance for development and growth. Lianas are rooted in the soil and use other plants, especially trees as support. Since these plants begin their growth on the forest floor, then ascend to the canopy, they can experience extreme differences in radiation quantity and spectral quality. In the present study, the influence of photosynthetically active radiation (PAR) on some traits of growth and photosynthetic activities were evaluated in two liana species, Canavalia parviflora Benth. (Fabaceae) and Gouania virgata Reissk (Rhamnaceae). The species grew naturally in a fragment of a tropical semi-deciduous forest, located in Campinas, SP. Three aspects were taken into account. First, the effects of light quality and quantity on growth responses and morphology were examined in both controlled conditions and natural deep shade imposed by a closed canopy. In controlled conditions, the species were maintained under contrasting red/far-red ratios, characteristic of the open and shade forest understorey, but under the same PAR. After that, the role of cotyledons and primary leaves was verified. Experiments of removal of parts (cotyledons or primary leaves) were conducted under greenhouse conditions and under reduced irradiance. Finally, gas exchange, stomatal conductance and transpiration were measured in these species to investigate the constraints on these responses under high (full sunlight), medium (44% PAR), intermediate (10% PAR) and low irradiance (1,5% PAR). Alterations in chlorophyll fluorescence, changes in specific leaf area and pigment composition were also investigated. In spite of their occurrence, both species were capable of exhibiting adaptations to a low irradiance and altered spectral quality. Canavalia was sensitive to shade with low red ar red, with responses that included reductions in leaf area and leaf number, increases in petiole length, reductions in specific leaf area and leaf area ratio when compared with plants growing in neutral shade (i.e. shade without altered quality). Also, a shift in allocation to stem mass over leaf mass was influenced by low red ar-red in Canavalia. Gouania showed significantly greater growth in height that was paralleled with increases in internode length under shade with low red ar-red at 1,8% of PAR, but patterns of allocation were not affected by altered spectral quality. Under natural conditions (closed canopy and clearings), Canavalia responded to deep shade with characteristics that promoted increases in photosynthetic tissues in detriment to support tissues, for instance, increasing the specific leaf area, the leaf area ratio and the leaf mass ratio. On the other hand, Gouania showed decreases in the leaf mass ratio and no significant effect was observed in their root mass ratio for deep shade plants compared to clearing plants. In both species, relative growth ratio (RGR) under high light conditions was significantly correlated with their physiological component, the net assimilation rates. Under low PAR, Canavalia growth was achieved by morphological adjustments such as increases in leaf area ratio, specific leaf area and leaf mass ratio. In Gouania no relationship was observed between RGR and leaf area ratio as well as between RGR and specific leaf area. A compensatory growth was evident in Canavalia when only one leaf was removed from this plant, in both greenhouse and 14% of full sunlight conditions, reflecting their capacity to cope with possible destructive events. The growth was adversely affected by total removal of cotyledons in these species, ascribing importance of the cotyledons for initial establishment. Gouania was notably tolerant to the removal of the cotyledons or primary leaves, as indicated by their high survival rate in 40, 14 e 1.6% PAR. In addition, the lost tissue did not affect the RGR and patterns of carbon allocation in Gouania, irrespective of light growth conditions. Features such as slow initial growth and high root/shoot area in this species probably provides carbon source for supporting growth. Although the species are found in gaps and forest margins, the carbon uptake in Canavalia and Gouania was slightly limited by high irradiances and high temperatures. In both species the highest photosynthetic rates were attained during morning, when the PAR had not reached its maximum values. After that, decreases in photosynthetic rates were detected, due to lower stomatal conductance. This response prevented large loss of water, as evidenced by reductions in transpiration rates. Canavalia exhibited a high ratio of leaf carbon gain to a leaf water loss, water use efficiency, reflecting patterns of water conservation rather than only carbon gain. In Gouania, higher water use efficiency was verified in 1.5% of PAR. Both species showed physiological acc1imation to different light levels. Canavalia displayed pronounced responses, with decreases in maximum rates of photosynthesis, transpiration and stomatal conductance that followed reductions in imposed irradiance. Gouania exhibited narrower responses, with high photosynthetic rates from full sunlight to 10% PAR. Plants growing under full sunlight showed light saturation of photosynthesis at ca 600 /?mol m-2 S-l, while under 1.5% of PAR the plants achieve their maximum rates of photosynthesis at lower irradiance. Canavalia exhibited a lower light compensation point in full sunlight and a higher one at 1.5% of PAR, while Gouania showed a reverse pattern. Remarkable reductions in effective quantum yield (?F/Fm ) of photosystem II was observed at midday in both species growing under either full sunlight or 44% PAR. After sunset, recovery of ?F/Fm took place for these plants. Large decreases in ?F/Fm were compensated by increases in non-photochemical quenching (NPQ) at midday. Decreases in ?F/Fm were less pronounced in plants growing under shade (1.5% PAR) than in plants growing under higher irradiances. Photosynthetic performance was reduced when shade plants were submitted to brief flashes of saturating light, as indicated by a sharper drop of ?F/Fm and lower values of photosynthetic electron transport rates (ETR). Growth light conditions resulted in alterations of the spectral proprieties of leaves. Plants of Canavalia growing at 1.5% PAR presented greater absorbance when compared with plants growing under higher irradiances that was accompanied with decreases in transmittance and reflectance, whereas for Gouania reductions in reflectance under 1.5% PAR were the onlymodification detected. Despite Gouania did not exhibit marked responses in their spectral properties, this species was superior in the efficiency of absorption when compared with Canavalia. The increases in the efficiency of absorption occurred as a consequence of a higher specific leaf are a, in both species. Increases in leaf chlorophyll a, b content and total chlorophyll content were significant for both species growing in the shade, but the chlorophyll a ratio was not altered. Greater content of total carotenoid per chlorophyll content was detected in plants under full sunlight, probably indicating its function for protection from excess light. These two lianas species are found in places of high irradiance e.g. Forest edge, but this specific distribution does not imply typical physiological response to be similar to the high-light-demanding species. In fact, as shown in the present work, lianas share attributes of both high-light-demanding and shade tolerant species. superior in the efficiency of absorption when compared with Canavalia. The increases in the efficiency of absorption occurred as a consequence of a higher specific leaf are a, in both species. Increases in leaf chlorophyll a, b content and total chlorophyll content were significant for both species growing in the shade, but the chlorophyll a ratio was not altered. Greater content of total carotenoid per chlorophyll content was detected in plants under full sunlight, probably indicating its function for protection from excess light. These two lianas species are found in places of high irradiance e.g. Forest edge, but this specific distribution does not imply typical physiological response to be similar to the high-light-demanding species. In fact, as shown in the present work, lianas share attributes of both high-light-demanding and shade tolerant species

ASSUNTO(S)

florestas fotossintese biologia vegetal

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