The role of Brazilian pine (Araucaria angustifolia) in the process of forest succession is a topic of increasing controversy. While some authors consider the species to be pioneer, others consider it to be a climax species in relict temperate forests. We designed a field experiment to assess the role of light availability on the regeneration of A. angustifolia. The following three forests, with contrasting patterns of the species regeneration, were selected at a National Forest in southern Brazil: a Pinus plantation, an Araucaria plantation and a native araucarian forest. We analysed the population structure of Brazilian pine, the vegetation architecture, the light regime experienced by seedlings and the height growth of seedlings and sprouts. Brazilian pine colonisation and regeneration were observed in the Pinus and Araucaria plantations, respectively. No seedlings were found in the native forest. The greatest foliage area index and canopy cover were found in the native forest and the smallest in the Pinus plantation. In spite of the architectural differences, the native forest and the Araucaria plantation had similar light conditions and they both had lower levels of irradiances than the Pinus plantation. Seedlings and sprouts of Brazilian pine were found to occupy spots in the Araucaria plantation with canopy cover greater than the average forest conditions. Considering the similarity of understorey light conditions between the Araucaria plantation and the native forest (where no regeneration was taking place), we concluded that light availability was not limiting the regeneration of the species. Tolerance to shading and other aspects of the species indicate that Brazilian pine is not strictly heliophyllous and pioneer and is able to be established in the forest understorey.

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Este trabalho visou quantificar o nitrogênio (N) disponível ao milho e avaliar seu efeito sobre o índice de área foliar (IAF), a produção de massa seca da parte aérea (MSPA), o N absorvido pela parte aérea e o rendimento de grãos. As avaliações foram realizadas aos 52, 65, 74, 82, 99 e 160 dias após a emergência (DAE) em um experimento de manejo de solo conduzido em plantio direto desde 1985, com os sistemas de culturas de aveia/milho (A/M), aveia + ervilhaca/milho + caupi (A+E/M+C) e ervilhaca/milho (E/M) e duas doses de N (0 N e 180 N - kg ha-1) aplicadas no milho, na forma de uréia. Na safra 2000/01 foram aplicadas doses de 0, 60, 120 e 180 kg ha-1 de N. O N estimado disponível ao milho e calculado a partir da contribuição do solo, plantas de cobertura e do adubo nitrogenado, variou de 27 a 192 kg ha-1, sendo os menores valores associados aos tratamentos apenas com gramíneas (A/M) e os maiores aos com N e presença de leguminosas no sistema (E/M e A+E/M+C). Todas as variáveis avaliadas apresentaram alta dependência do N disponível e tiveram seus valores maximizados próximos da maior disponibilidade de N (192 kg ha-1). Com esta disponibilidade foram observados, no estádio de polinização (65 DAE), o maior IAF (4,41) e a mais alta concentração de N na folha índice (2,49%), enquanto o maior acúmulo de N na parte aérea foi constatado no estádio de grão leitoso (74 DAE) e o acúmulo de MSPA no estádio de grão dentado (99 DAE). O rendimento máximo de grãos atingido foi de 8.920 kg ha-1, correspondendo a uma exportação de N pelo grão, de 112 kg ha-1.

RESUMO -(Crescimento e limitações à assimilação de carbono em Ricinus communis (Euphorbiaceae) sob condições de estresse hídrico do solo). A disponibilidade hídrica pode infl uenciar o ganho de carbono e o crescimento, com grande impacto na produtividade das plantas. Ricinus communis (L.), uma espécie resistente à seca, é uma cultura de grande importância econômica no Brasil, devido ao seu uso na indústria química e para a produção de biocombustíveis. Alguns dos mecanismos envolvidos na resistência à seca desta espécie foram analisados através de um progressivo estresse hídrico em plantas cultivadas em vasos sob condições de casa de vegetação. A suspensão da irrigação por 53 dias decresceu o conteúdo gravimétrico de água no solo e o potencial hídrico das folhas. O crescimento das plantas foi negativamente e signifi cativamente reduzido pelo aumento do défi cit hídrico do solo. Com a suspensão da irrigação, a assimilação de carbono e a transpiração foram reduzidas e permaneceram praticamente constantes ao longo do dia. A análise das curvas A/C i demonstrou o aumento da limitação estomática, indicando que a limitação imposta pelo fechamento dos estômatos é o principal responsável pela redução da fotossíntese. A efi ciência de carboxilação e a taxa de transporte de elétrons não foram afetadas pelo estresse hídrico até 15 dias de suspensão da irrigação. A resistência à seca da mamona parece estar relacionada a uma resposta inicial e pronunciada do crescimento, a um efi ciente controle estomático e a capacidade de manter uma alta taxa de fi xação de CO 2 sob condições de estresse hídrico. Palavras-chave: curvas A/Ci, estresse hídrico, fotossíntese, limitação estomática, padrão diário ABSTRACT -(Growth and limitations to carbon assimilation in Ricinus communis (Euphorbiaceae) under soil water stress conditions). Water availability may infl uence plant carbon gain and growth, with large impacts on plant yield. Ricinus communis (L.), a drought resistant species, is a crop with increasing economic importance in Brazil, due to its use in chemical industry and for the production of biofuels. Some of the mechanisms involved in this drought resistance were analyzed in this study by imposing progressive water stress to pot-grown plants under glasshouse conditions. Water withholding for 53 days decreased soil water gravimetric content and the leaf water potential. Plant growth was negatively and signifi cantly reduced by increasing soil water defi cits. With irrigation suspension, carbon assimilation and transpiration were reduced and remained mostly constant throughout the day. Analysis of A/ C i curves showed increased stomatal limitation, indicating that limitation imposed by stomatal closure is the main factor responsible for photosynthesis reduction. Carboxylation effi ciency and electron transport rate were not affected by water stress up to 15 days after withholding water. Drought resistance of castor bean seems to be related to a pronounced, early growth response, an effi cient stomatal control and the capacity to keep...

Iron (Fe) is an essential element to plants, but can be harmful if accumulated to toxic concentrations. Fe toxicity can be a major nutritional disorder in rice ( Oryza sativa ) when cultivated under waterlogged conditions, as a result of excessive Fe solubilization of in the soil. However, little is known about the basis of Fe toxicity and tolerance at both physiological and molecular level. To identify mechanisms and potential candidate genes for Fe tolerance in rice, we comparatively analyzed the effects of excess Fe on two cultivars with distinct tolerance to Fe toxicity, EPAGRI 108 (tolerant) and BR-IRGA 409 (susceptible). After excess Fe treatment, BR-IRGA 409 plants showed reduced biomass and photosynthetic parameters, compared to EPAGRI 108. EPAGRI 108 plants accumulated lower amounts of Fe in both shoots and roots compared to BR-IRGA 409. We conducted transcriptomic analyses of roots from susceptible and tolerant plants under control and excess Fe conditions. We found 423 up-regulated and 92 down-regulated genes in the susceptible cultivar, and 42 up-regulated and 305 down-regulated genes in the tolerant one. We observed striking differences in root gene expression profiles following exposure to excess Fe: the two cultivars showed no genes regulated in the same way (up or down in both), and 264 genes were oppositely regulated in both cultivars. Plants from the susceptible cultivar showed down-regulation of known Fe uptake-related genes, indicating that plants are actively decreasing Fe acquisition. On the other hand, plants from the tolerant cultivar showed up-regulation of genes involved in root cell wall biosynthesis and lignification. We confirmed that the tolerant cultivar has increased lignification in the outer layers of the cortex and in the vascular bundle compared to the susceptible cultivar, suggesting that the capacity to avoid excessive Fe uptake could rely in root cell wall remodeling. Moreover, we showed that increased lignin concentrations in roots might be linked to Fe tolerance in other rice cultivars, suggesting that a similar mechanism might operate in multiple genotypes. Our results indicate that changes in root cell wall and Fe permeability might be related to Fe toxicity tolerance in rice natural variation.