Ontogenetic changes in leaf traits of tropical rainforest trees differing in juvenile light requirement

Relationships between leaf traits and the gap dependence for regeneration, and ontogenetic changes therein, were investigated in juvenile and adult tropical rainforest tree species. The juveniles of the 17 species included in the study were grown in high light, similar to the exposed crowns of the adult trees. The traits were structural, biomechanical, chemical and photosynthetic. With increasing species gap dependence, leaf mass per area (LMA) decreased only slightly in juveniles and remained constant in adults, whereas punch strength together with tissue density decreased, and photosynthetic capacity and chlorophyll increased. Contrary to what has been mostly found in evergreen tropical rainforest, the trade-off between investment in longevity and in productivity was evident at an essentially constant LMA. Of the traits pertaining to the chloroplast level, photosynthetic capacity per unit chlorophyll increased with gap dependence, but the chlorophyll a/b ratio showed no relationship. Adults had a twofold higher LMA, but leaf strength was on average only about 50% larger. Leaf tissue density, and chlorophyll and leaf N per area were also higher, whereas chlorophyll and leaf N per unit dry mass were lower. Ranking of the species, relationships between traits and with the gap dependence of the species were similar for juveniles and adults. However, the magnitudes of most ontogenetic changes were not clearly related to a species’ gap dependence. The adaptive value of the leaf traits for juveniles and adults is discussed

Aclimatação de mudas de acariquara à alta irradiância Acclimation of manwood seedlings to full sunlight

O objetivo deste trabalho foi determinar as mudanças fisiológicas que ocorrem em acariquara (Minquartia guianensis) durante o processo de aclimatação à alta irradiância, bem como a estratégia de aclimatação dessa espécie. Plantas mantidas em baixa irradiância foram transferidas para alta irradiância por 290 dias. Durante esse período, foi medida a relação entre fluorescência variável e máxima (Fv/Fm), em folhas desenvolvidas à sombra e, após a senescência prematura por foto-oxidação, em folhas aclimatadas ao sol. Ao final do experimento, foram determinadas as características fotossintéticas e anatômicas da folha. A exposição à alta irradiância causou, logo após a transferência, forte fotoinibição e foto-oxidação parcial da folhagem, mas não provocou a morte da planta. Folhas produzidas no ambiente ensolarado apresentaram valores de Fv/Fm similares aos do controle. A fotossíntese saturada por luz e a fotossíntese saturada por CO2 foram 90 e 50% maiores em plantas aclimatadas à alta irradiância. A velocidade máxima de carboxilação da rubisco e a taxa máxima de regeneração da ribulose bisfosfato seguiram a mesma tendência. Folhas produzidas ao sol apresentaram maior densidade estomática e maior espessura foliar. A produção de folhas novas é a principal estratégia de aclimatação da acariquara à alta irradiância.<br>The aim of this work was to determine the physiological changes that occur in manwood (Minquartia guianensis) during the acclimation process to high irradiance, and also to determine the acclimation strategy of this species. Plants kept in low light were transferred to high irradiance for 290 days. During this period the ratio between variable fluorescence and maximum fluorescence (Fv/Fm) was measured in leaves developed in the shade, and after premature senescence due to photooxidation, in sun-acclimated leaves. At the end of the experimental period the anatomic and photosynthetic characteristics of leaves were determined. Exposure to high irradiance caused, just after transferring, strong photoinhibition and partial photooxidation, but did not cause plant death. Leaves produced in sunny environment showed Fv/Fm similar to that of control plants. Light-saturated photosynthesis and CO2 saturated-photosynthesis were 90 and 50% higher in plants acclimated to high irradiance. The maximum carboxylation velocity of rubisco and the maximum rate of ribulose bisphosphate regeneration followed the same trend. Leaves produced under sunlight had higher stomatal density and increased leaf thickness. The production of new leaves is the main acclimation strategy to high irradiance in manwood

Fotossíntese e fotoinibição em mogno e acariquara em função da luminosidade e temperatura foliar Photosynthesis and photoinhibition in mahogany and acariquara as a function of irradiance and leaf temperature

A fotoinibição pode ser intensificada por fatores de estresse como alta temperatura. O objetivo deste trabalho foi determinar os efeitos da luminosidade e temperatura na fotossíntese e na recuperação da fotoinibição sob baixa irradiância em mogno (Swietenia macrophylla King) e acariquara (Minquartia guianensis Aubl.). As plantas cresceram sob baixa irradiância: 0,20 (acariquara) e 1,43 mol m-2 dia-1 (mogno). A fotossíntese líquida e as características da fluorescência foram avaliadas em duas irradiâncias, 1.000 e 1.700 mmol m-2 s-1 , e duas temperaturas foliares, 32 e 38ºC. A 32ºC, a fotossíntese máxima (Amáx) foi maior em mogno do que em acariquara. Em mogno, a exposição a 38ºC causou redução gradativa na fotossíntese, sendo a taxa fotossintética após 50 min similar à encontrada em acariquara. A 32ºC, o aumento na irradiância, de 1.000 para 1.700 mmol m-2 s-1 , intensificou a fotoinibição, mas Amáx não foi alterada. Em 1.700 mmol m-2 s-1, o aumento da temperatura, de 32 para 38ºC, não afetou a fotoinibição. Após 72 horas em baixa irradiância, a recuperação da fotoinibição foi de 80% em acariquara e 89% em mogno. Não houve efeito da temperatura da folha na fotoinibição.<br>Photoinhibition may be exacerbated by stress factors such as high temperature. The aim of the work was to determine the effects of temperature and irradiance on photosynthesis and photoinhibition in mahogany (Swietenia macrophylla King) and acariquara (Minquartia guianensis Aubl.). Also to assess the effect of irradiance and temperature on the recovery from photoinhibition at low irradiance. Sapling were grown at low irradiance: 0.20 mol m-2 day-1 (mahogany) and 1.43 mol m-2 day-1 (acariquara). Photosynthesis and fluorescence characteristics were assessed in plants subjected to two irradiances 1,000 and 1,700 mmol m-2 s-1 , and two temperatures, 32 e 38ºC. At 32ºC, light saturated photosynthesis (Amax) was greater in mahogany than in acariquara. Mahogany plants exposed to 38ºC showed a consistent decline in carbon uptake, and after 50 min both species showed the same photosynthetic rates. In saplings exposed to 32ºC, increase in irradiance, from 1,000 to 1,700 mmol m-2 s-1 , exacerbated photoinhibition, but without reducing Amax values. At 1,700 mmol m-2 s-1 , the rise in temperature, from 32 to 38ºC, did not increase photoinhibition. After 72 hours at low irradiance, recovery from photoinhibition was 80% in acariquara and 89% in mahogany. There was no effect of leaf temperature on photoinhibition

Effects of photoacclimation on the light niche of corals: a process-based approach

The ecology of photosynthetic organisms is influenced by the need to adjust the photosynthetic apparatus to variable light environments (photoacclimation). In this study, we quantified different components of the photoacclimation process for a reef-building coral (Turbinaria mesenterina, Lamarck, 1816): including, variation in absorption cross-section, size of photosynthetic units, turnover time, chlorophyll content, and colony respiration. We used these calibrations to characterize this species’ light niche, and to determine the sensitivity of the niche boundaries to different processes of photoacclimation. Results showed that the breadth of the light niche was most sensitive to the size of the photosynthetic unit, absorption cross-section, and rates of respiration. Habitats with the highest light availability did not lead to maximal energy acquisition. This was because, although corals acclimated to high light have high rates of photosynthesis per unit chlorophyll, their chlorophyll content was strongly reduced. This suggests that potential energetic benefits that could be achieved through increased light harvesting (i.e., increased chlorophyll content) in high-light habitats are outweighed by costs associated with photoprotection. Such costs appear to place an upper bound on the habitat distributions of coral species. Our approach reveals how the photophysiological processes involved in photoacclimation interact to determine the light niche