A bacia amazônica tem mais de seis milhões de quilômetros quadrados e abriga a maior floresta tropical do mundo, sendo particularmente importante pela sua biodiversidade e pelo seu papel na ciclagem de água e carbono. Fotossíntese, condutância estomática e fluxo de seiva de espécies florestais da Amazônia apresentam variação ao longo do dia seguindo a variação diurna observada na irradiância, temperatura e o déficit de pressão de vapor. Em decorrência da fotorrespiração, cerca de 25% do carbono fixado é retornado para a atmosfera. Os aumentos na concentração de CO2 na atmosfera previstos para as próximas décadas poderão apresentar efeito positivo na assimilação de carbono deste ecossistema florestal. Em comparação à época chuvosa, redução da umidade do solo e aumento no déficit de pressão de vapor (associado à baixa umidade do ar e alta temperatura) favorecem o fechamento dos estômatos em detrimento da fotossíntese. Desse modo, em comparação com a estação chuvosa, a fotossíntese líquida é menor no período seco. No geral, na Amazônia as árvores que atingem o dossel da floresta crescem a taxas maiores na época chuvosa. Exceto em anos de menor precipitação, o ecossistema florestal atua como sumidouro de carbono na época chuvosa. Mais estudos são necessários para determinar como e de que forma fatores específicos do ambiente físico influenciam a assimilação de carbono e o crescimento de árvores nos diversos grupos funcionais na Amazônia.

Stomata are turgor-operated valves that control water loss and CO 2 uptake during photosynthesis, and thereby water relation and plant biomass accumulation is closely related to stomatal functioning. The aims of this work were to document how stomata are distributed on the leaf surface and to determine if there is any significant variation in stomatal characteristics among Amazonian tree species, and finally to study the relationship between stomatal density (S D ) and tree height. Thirty five trees (>17 m tall) of different species were selected. Stomatal type, density (S D ), size (S S ) and stomatal distribution on the leaf surface were determined using nail polish imprints taken from both leaf surfaces. Irrespective of tree species, stomata were located only on the abaxial surface (hypostomaty), with large variation in both S D and S S among species. S D ranged from 110 mm -2 in Neea altissima to 846 mm -2 in Qualea acuminata. However, in most species S D ranges between 271 and 543 mm -2 , with a negative relationship between S D and S S . We also found a positive relationship between S D and tree height (r 2 = 0.14, p < 0.01), but no correlation was found between S D and leaf thickness. The most common stomatal type was anomocytic (37%), followed by paracytic (26%) and anisocytic (11%). We conclude that in Amazonian tree species, stomatal distribution on the leaf surface is a response most likely dependent on the genetic background of every species, rather than a reaction to environmental changes, and that somehow S D is influenced by environmental factors dependent on tree height. Keywords: Amazonia, hypostomatous leaves, stomatal density, stomatal length, tropical rainforest. Densidade, tamanho e distribuição estomática em 35 espécies de árvores na Amazônia Central resumoEstômatos são válvulas operadas a turgor que controlam a perda de água e a captura de CO 2 durante a fotossíntese. Assim, as relações hídricas e o acumulo de biomassa vegetal são fortemente influenciadas pelo funcionamento estomático. Os objetivos deste trabalho foram: documentar como os estômatos estão distribuídos na superfície foliar e determinar se existe variação das características estomáticas entre espécies da Amazônia, estudar a relação entre densidade estomática (S D ) e altura arbórea. Trinta e cinco árvores (>17 m de altura) de diferentes espécies foram selecionadas. Tipo de complexo estomático, S D , tamanho (S S ) e distribuição na superfície foliar foram determinados utilizando impressões de ambas as superfícies foliares com esmalte incolor. Independente da espécie, os estômatos foram encontrados apenas na superfície abaxial (hipoestomatia) com ampla variação na S D e no S S entre espécies. A densidade estomática variou de 110 mm -2 em Neea altissima a 846 mm -2 em Qualea acuminata. Entretanto, a maioria das espécies apresentou S D entre 271 e 543 mm -2 , com uma relação negativa entre S D e S S. Observou-se uma relação positiva entre S D e altura arbórea (r 2 = 0.14, p < 0.01), não havendo relação entre S D e espess...

Most Amazonian soils are highly weathered and poor in nutrients. Therefore, photosynthesis and plant growth should positively respond to the addition of mineral nutrients. Surprisingly, no study has been carried out in situ in the central Amazon to address this issue for juvenile trees. The objective of this study was to determine how photosynthetic rates and growth of tree saplings respond to the addition of mineral nutrients, to the variation in leaf area index of the forest canopy, and to changes in soil water content associated with rainfall seasonality.We assessed the effect of adding a slow-release fertilizer. We determined plant growth from 2010 to 2012 and gas exchange in the wet and dry season of 2012. Rainfall seasonality led to variations in soil water content, but it did not affect sapling growth or leaf gas exchange parameters. Although soil amendment increased phosphorus content by 60 %, neither plant growth nor the photosynthetic parameters were infl uenced by the addition of mineral nutrients. However, photosynthetic rates and growth of saplings decreased as the forest canopy became denser.Even when Amazonian soils are poor in nutrients, photosynthesis and sapling growth are more responsive to slight variations in light availability in the forest understory than to the availability of nutrients. Therefore, the response of saplings to future increases in atmospheric [CO 2 ] will not be limited by the availability of mineral nutrients in the soil.

-Crabwood (Carapa guianensis Aubl.) is a fast growing tree species with many uses among Amazonian local communities. The main objective of this study was to assess the effect of seasonal rainfall pattern on growth rates, and seasonal and diurnal changes in leaf gas exchange and leaf water potential (Ψ L ) in crabwood. To assess the effect of rainfall seasonality on growth and physiological leaf traits an experiment was conducted in Manaus, AM (03° 05´ 30" S, 59° 59´ 35" S). In this experiment, six 6-m tall plants were used to assess photosynthetic traits and Ψ L . In a second experiment the effect of growth irradiance on stomatal density (S D ), size (S S ) and leaf thickness was assessed in 0.8-m tall saplings. Stomatal conductance (g s ) and light-saturated photosynthesis (A max ) were higher in the wet season, and between 09:00 and 15:00 h. However, no effect of rainfall seasonality was found on Ψ L and potential photosynthesis (CO 2 -saturated). Ψ L declined from -0.3 MPa early in the morning to -0.75 MPa after midday. It increased in the afternoon but did not reach full recovery at sunset. Growth rates of crabwood were high, and similar in both seasons (2 mm month -1 ). Leaf thickness and S D were 19% and 47% higher in sun than in shade plants, whereas the opposite was true for S S . We conclude that Ψ L greatly affects carbon assimilation of crabwood by reducing g s at noon, although this effect is not reflected on growth rates indicating that other factors offset the effect of g s on A max .Keywords: Photosynthesis, Stomatal density, Crabwood and Amazonia. CRESCIMENTO, CARACTERÍSTICAS FOLIARES E ESTOMÁTICAS

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