Padrões de atividade fotoquímica em graminóides de ecossistemas campestres

A complexidade da vegetação campestre pode ser explicada a partir de uma abordagem de tipologia funcional, juntando espécies diferentes em grupos relacionados a estratégias de captura ou conservação de recursos. Os grupos assim formados podem relacionar os aspectos adaptativos das plantas aos processos dominantes no ecossistema. Os campos de dupla estrutura contemporânea no Sul do Brasil, mostram-se como resultante da interação de múltiplos fatores ambientais expressados nos fenótipos de uma composição vegetal pouco estudada. Assim, o objetivo deste estudo foi investigar a atividade fotoquímica de graminóides dominantes, buscando testar a hipótese da existência de um padrão fisiológico compatível com a tipologia funcional de captura ou conservação de recursos. Posteriormente, foi feita a descrição dos mecanismos da atividade fotoquímica de Axonopus affinis (Poaceae) que sustentam a hipótese de que a rápida ciclagem de nutrientes intrínseca de plantas de captura de recursos é precedida por plasticidade fisiológica no uso da energia luminosa O estudo foi conduzido em vegetação campestre de áreas experimentais da Embrapa Pecuária Sul em Bagé/RS e Hulha Negra/RS (primavera-verão 2014-2016). A amostragem preferencial foi precedida por reconhecimento visual da composição e estrutura da vegetação, utilizando unidades amostrais de espécies dos tipos funcionais de captura ou conservação de recursos (folhas jovens, não danificadas e totalmente expandidas). Os resultados encontrados corrobarama abordagem de tipologica funcional construída a partir do teor de matéria seca e área foliar específica em estudos de ecossistemas campestres de contemporânea dupla estrutura. A similaridade para respostas na atividade fotoquímica de espécies graminóides de mesmo tipo funcional encontrada mostrou que emissões de fluorescência basal e maior pool de transportadores de elétrons fotossintetizantes, dependência de mecanismos do quenching não fotoquímico e fotoinibição dinâmica foram características do tipo funcional de captura de recursos. Entretanto, o tipo funcional de conservação de recursos mostrou maior estabilidade em emissão de fluorescência basal e menor pool de transportadores de elétrons associado a mecanismos de quenching fotoquímico e tamanho efetivo da antena do complexo coletor de luz. O padrão estável de atividade fotoquimica observado em A. affinis foi direcionado por mecanismos de quenching da emissão de fluorescência de clorofila a e pelo comportamento do transiente OJIP, os quais revelaram plasticidade fisiológica a partir de mudanças reversiveis na estrutura e função de cloroplastos.The complexity of grassland vegetation can be explained based on functional typology, grouping different species regarding strategies for capture or conservation of resources. The groups thus formed can be related to adaptive aspects of plants to the dominant processes in the ecosystem. The grassland having double structure in southern Brazil are the result of interaction of multiple envrionmental factors, expressed in the phenotypes of a vegetative composition that has not been widely studied. Thus, the objective of this study was to investigate the photochemical activity of the dominant graminoids, by testing the hypothesis of the existence of a physiological pattern compatible with the functional typology of capture or conservation of resources. This analysis was followed by examination to describe the mechanisms of the photochemical activity of Axonopus affinis (Poaceae), supporting the hypothesis that the rapid cycling of nutrients that is intrinsic to plants for capture of resources is preceded by physiological plasticity in the use of light energy. The study was conducted in grassland vegetation of experimental grassland of Embrapa Pecuária Sul (Embrapa Southern Livestock research unit), located in the municipalities of Bagé and Hulha Negra, both in the state of Rio Grande do Sul, in the spring-summer of 2014-2016. The preferential sampling was preceded by visual recognition of the composition and structure of the vegetation, using sampling units consisting of species of the functional types for capture or conservation of resources (fully expanded and undamaged young leaves). The results found corroborate the typological-functional approach formulated based on the dry matter content and specific leaf area in studies of grassland ecosystems with double contemporary structure The similarity of responses in the photochemical activity of species of graminoids with the same functional type found showed that emission of basal fluorescence, a greater pool of photosynthetic electron transporters, dependence on non-photochemical quenching, and dynamic photoinhibition were characteristics of the functional type for resource capture. However, the functional type for conservation of resources showed greater stability in emission of basal fluorescence and a smaller pool of electron transporters associated with photochemical quenching mechanisms and effective size of the antenna of the light collection complex. The stable pattern of photochemical activity observed in A. affinis was guided by quenching mechanisms in the emission of chlorophyll a fluorescence and by the behavior of the OJIP transient, revealing the physiological plasticity based on reversible changes in the structure and function of chloroplasts

Analyses of OJIP transients in leaves of two epiphytic orchids under drought stress

The tolerance to low water availability is a decisive factor for growth and survival of orchids in their natural environment. The objective of this study was to characterize the photochemical traits of two epiphytic orchids (Cattleya warneri and Miltonia spectabilis) under water deficit (WD). Chlorophyll a fluorescence signals were recorded from young and fully expanded leaves of 5 plants/ species after dark-adaption for 60 minutes, between 6-9 a.m. after 0, 30, 60, and 90 days of WD, using a Handy-PEA fluorometer (Hansatech, UK). Increases of O-J and J-I phases and L and K-bands and decreases of I-P phase were observed after 30 days of WD, especially in C. warneri. Decreases in the capacity to photochemically reduce quinone A (QA) and the kinetic properties required for redox reactions of the plastoquinone pool, the loss of energetic connectivity between units of PSII, inactivation of the oxygen evolution complex, and decrease of the overall rate of reducing the electron acceptor pool of photosystem I were observed in M. spectabilis, a more tolerant species. The greater ability of this species to maintain higher relative water content (RWC) in photosynthetic tissues allows greater photochemical activity

Vegetation composition and forage mass in grassland with a double structure under two winter grazing regimes

ABSTRACT: Remnant areas of Pampas grassland have a distinct double structure. Efficiency of livestock farming on these grasslands depends on practices that are synchronized with natural variation. This study examined the changes in vegetation composition and forage mass during winter to understand the effects of grazing methods in a natural pasture with a double structure that was grazed by heifers. An experimental area in the municipality of Bagé, Rio Grande do Sul, was subjected to continuous or rotational grazing treatments, with two replicates each. Frequency of the structural composition and forage mass of the lower stratum and animal weight was measured at the beginning and end of the experimental period (June 7 to October 7, 2016, respectively). Data were analysed with PCA ordination, regression and variance analysis. Both structural composition and forage mass changed during the experimental period. Axonopus affinis and Paspalum notatum were characteristic of continuous grazing, while Mnesithea selloana and Axonopus argentinus characterized rotational grazing (the latter had the highest levels of forage mass). Although, grazing methods changed the composition of forage mass in winter, they did not affect average daily gain of the animals

Exposition of three Cattleya species (Orchidaceae) to full sunlight: effect on their physiological plasticity and response to changes in light conditions

In order to establish a link between the evolutionary history and the photochemical attributes, measurements of chlorophyll (Chl) a fluorescence were made in Cattleya warneri, C. shofieldiana and C. harrisoniana exposed to high irradiance for 5, 35, and 120 min (hereafter referred to as treatments T5, T35, and T120, respectively). The following questions are addressed: (1) Is the increased energy dissipation enough to counterbalance the excess energy that drives photosynthesis at different times of high irradiance exposure? (2) Is there an influence of the incidence and duration of light radiation on Cattleya species in full sunlight, compared to Cattleya species submitted to low irradiance? Higher relative variable fluorescence at the J-step (Vj) values followed by the lower quantum yield of electron transport (ψEo) indicate the accumulation of reduced Quinone A (QA) proportionally of sunflecks exposure time in C. warneri. The higher performance index (PIABS) and plasticity index values in C. schofieldiana indicate higher efficiency in modulating the photosynthetic apparatus under sunflecks. C. harrisoniana shows the lowest plasticity index, suppression of maximum fluorescence (Fm), and no recovery of PIABS after sunflecks. This study evidences the importance of physiological plasticity in the current geographic distribution of Cattleya in response to light pulses in species derived from fragmented habitats and the maintenance of shade to species of more primitive clades