Spatial patterns of photosynthesis in thin- and thick-leaved epiphytic orchids: unravelling C3-CAM plasticity in an organ-compartmented way.

BACKGROUND AND AIMS: A positive correlation between tissue thickness and crassulacean acid metabolism (CAM) expression has been frequently suggested. Therefore, this study addressed the question of whether water availability modulates photosynthetic plasticity in different organs of two epiphytic orchids with distinct leaf thickness. METHODS: Tissue morphology and photosynthetic mode (C3 and/or CAM) were examined in leaves, pseudobulbs and roots of a thick-leaved (Cattleya walkeriana) and a thin-leaved (Oncidium 'Aloha') epiphytic orchid. Morphological features were studied comparing the drought-induced physiological responses observed in each organ after 30 d of either drought or well-watered treatments. KEY RESULTS: Cattleya walkeriana, which is considered a constitutive CAM orchid, displayed a clear drought-induced up-regulation of CAM in its thick leaves but not in its non-leaf organs (pseudobulbs and roots). The set of morphological traits of Cattleya leaves suggested the drought-inducible CAM up-regulation as a possible mechanism of increasing water-use efficiency and carbon economy. Conversely, although belonging to an orchid genus classically considered as performing C3 photosynthesis, Oncidium 'Aloha' under drought seemed to express facultative CAM in its roots and pseudobulbs but not in its leaves, indicating that such photosynthetic responses might compensate for the lack of capacity to perform CAM in its thin leaves. Morphological features of Oncidium leaves also indicated lower efficiency in preventing water and CO2 losses, while aerenchyma ducts connecting pseudobulbs and leaves suggested a compartmentalized mechanism of nighttime carboxylation via phosphoenolpyruvate carboxylase (PEPC) (pseudobulbs) and daytime carboxylation via Rubisco (leaves) in drought-exposed Oncidium plants. CONCLUSIONS: Water availability modulated CAM expression in an organ-compartmented manner in both orchids studied. As distinct regions of the same orchid could perform different photosynthetic pathways and variable degrees of CAM expression depending on the water availability, more attention should be addressed to this in future studies concerning the abundance of CAM plants.

Fitossociologia de um Cerrado denso em área de influência de torre de fluxo de carbono, Pé-de-Gigante, Parque Estadual de Vassununga, SP / Phytossociology of a dense Cerrado on the footprint of a carbon flux tower, Pé-de-Gigante, Vassununga State Park, SP

O domínio do Cerrado compreende uma área contínua nos estados centrais do Brasil e áreas disjuntas em outros estados, incluindo São Paulo. Essa vegetação ocupava originalmente 21 por cento do território brasileiro, restando atualmente apenas 21,6 por cento de sua extensão original. A área recoberta por essa vegetação em São Paulo cobria 14 por cento de sua área total e seus remanescentes recobrem menos de 1 por cento da ocorrência original dessa vegetação. Estudos recentes indicam que o valor da produtividade líquida no Cerrado Pé-de-Gigante (SP) constitui um pequeno dreno de carbono e indicou que a sazonalidade foi o fator determinante do valor observado. Os estudos dos fluxos de carbono em ecossistemas terrestres são raramente acompanhados de abordagens ecofisiológicas de modo a explorar a relação funcional das espécies que compõem o ecossistema e os valores líquidos obtidos para o mesmo. Assim, o objetivo deste trabalho foi caracterizar estruturalmente a vegetação presente na área de maior influência da torre de fluxo instalada no Cerrado Pé-de-Gigante, visando possibilitar estudos relacionados à quantificação em longo prazo da dinâmica dos fluxos de água, energia e CO2 na vegetação de Cerrado. Para isso foram levantadas 20 parcelas (10 x 10 m) em 0,2 ha de Cerrado, e amostraram-se todas as plantas com perímetro ao nível do solo >6 cm (exceto lianas e árvores mortas). A distribuição das classes de diâmetro e estrutura vertical, assim como os parâmetros fitossociológicos foram analisados. Encontramos 1451 indivíduos, distribuídos em 85 espécies, 52 gêneros e 31 famílias. A densidade absoluta e área basal foram de 7255 ind. ha-1 e de 7,9 m².ha-1, respectivamente. A família Leguminosae apresentou o maior número de espécies (13). O Índice de diversidade de Shannon (H') foi 3,27 nats.ind-1. A distribuição em classes de diâmetro mostrou uma curva de "J" invertido, estando a maioria dos indivíduos na primeira classe. Concluímos que a área deve ser classificada como Cerrado denso, devido principalmente à dominância pela espécie arbórea Anadenanthera falcata, cuja ocorrência no estado foi relatada apenas em locais com solos ricos em saturação de bases na região das Cuestas Basálticas, devido também à maior área basal dos indivíduos, comparando com outros fragmentos de Cerrado. Além da espécie citada, Myrcia lingua e Xylopia aromatica, apresentaram os maiores IVI (Valor de importância).

The Cerrado vegetation covers the Brazilian central areas (core area) which include extensive and continuous areas in the central States and some marginal areas in other States, including São Paulo, accounting originally for 21 percent of Brazilian territory. Nowadays, the remnants are 21,6 percent of its original coverage. About of 14 percent of the São Paulo State territory was originally covered with Cerrado vegetation, and today remnants account for less than 1 percent of the original distribution. Recent studies indicate that the net productivity in the Pé-de-Gigante Cerrado constitutes a small carbon sink, with seasonality playing a major role. The carbon flux studies in terrestrial ecosystems have rarely been accompanied by ecophysiological studies in order to explore the functional relationships among species, ecosystem and the values achieved. Therefore, our objectives were to characterize the vegetation structure in the meteorological and flux tower footprint. The flux tower allows studies related to long term measurements and dynamics of water, solar energy and CO2 fluxes in Cerrado vegetation. Plots (10 x 10 m) were established in 0.2 ha area of Cerrado and all vascular plants with stem perimeter at ground level >6 cm (except vines and dead individuals) were sampled. The distribution of diameter classes and vertical structure, as well as the phytosociological parameters were analyzed. We recorded 1,451 individuals, distributed in 85 species belonging to 31 different families. The absolute density and the basal area were 7,255 ind.ha-1 and 7.9 m².ha-1, respectively. Leguminosae showed the highest number of species (13). The Shannon's Index was 3.27. The distribution of diameter classes showed a curve in inverted "J", most of the individuals being in the initial classes. We concluded that the area must be classified as dense Cerrado, mainly due to Anadenanthera falcata dominance, which occurrence in São Paulo State was reported only in rich soil sites in Cuestas Basaltic regions and also due to major basal area of these individuals, comparing with other Cerrado vegetation sites. A. falcata, Myrcia lingua and Xylopia aromatica showed the highest IVI (Importance value index).

Specific leaf areas of the tank bromeliad Guzmania monostachia perform distinct functions in response to water shortage.

Leaves comprise most of the vegetative body of tank bromeliads and are usually subjected to strong longitudinal gradients. For instance, while the leaf base is in contact with the water accumulated in the tank, the more light-exposed middle and upper leaf sections have no direct access to this water reservoir. Therefore, the present study attempted to investigate whether different leaf portions of Guzmania monostachia, a tank-forming C(3)-CAM bromeliad, play distinct physiological roles in response to water shortage, which is a major abiotic constraint in the epiphytic habitat. Internal and external morphological features, relative water content, pigment composition and the degree of CAM expression were evaluated in basal, middle and apical leaf portions in order to allow the establishment of correlations between the structure and the functional importance of each leaf region. Results indicated that besides marked structural differences, a high level of functional specialization is also present along the leaves of this bromeliad. When the tank water was depleted, the abundant hydrenchyma of basal leaf portions was the main reservoir for maintaining a stable water status in the photosynthetic tissues of the apical region. In contrast, the CAM pathway was intensified specifically in the upper leaf section, which is in agreement with the presence of features more suitable for the occurrence of photosynthesis at this portion. Gas exchange data indicated that internal recycling of respiratory CO(2) accounted for virtually all nighttime acid accumulation, characterizing a typical CAM-idling pathway in the drought-exposed plants. Altogether, these data reveal a remarkable physiological complexity along the leaves of G. monostachia, which might be a key adaptation to the intermittent water supply of the epiphytic niche.