The paper proposes the classification of seeds in relation to the forms of phytochrome instead of classical photoblastism. On the basis of published data all seeds have phytochrome and the term photoblastism should be replaced by forms of phytochrome that control germination: 1. Positive photoblastic seeds have phyB (and, to a lesser extent, phyD and phyE) controlling the germination process through Low Fluence Responses (LFR); 2. Negative photoblastic seeds have phyA controlling germination through High Irradiance Responses (HIR) and, when the pre-existing Pfr level is high enough to induce germination in darkness, through LFR by phyB; and 3. light insensitive seeds have phyA controlling germination through Very Low Fluence Responses (VLFR).

The objective of this work was to compare the contents of polyamines (putrescine, spermidine and spermine), and total soluble phenols and flavonoids in parts of plants grown under either organic or conventional cropping, commonly discarded during food preparation. The contents of free polyamines, total phenols and total soluble flavonoids in peels (zucchini squash, banana, potato, eggplant, orange, lime, mango, passion fruit and radish), leaves (zucchini squash, broccoli, carrot, collard, cassava, radish and grape), stalks (broccoli, collard and spinach) and zucchini seeds were analysed. Most analysed vegetables presented higher contents of polyamines and total phenols under organic cropping, contrary to the results obtained for total flavonoids, possibly because of the cultural practices adopted.

Good reviews have been published over the years regarding many aspects of plant response to light, such as important advances in understanding the molecular mechanisms of light perception, signaling and control of gene expression by the photoreceptors. Moreover, many efforts have been undertaken on the manipulation of these mechanisms to improve horticultural crop production. In this paper we present an overview about the photoreceptors, the relationship between their absorptive and reflective properties and their control of plant development as well perspectives focused on photomorphogenesis manipulation.

Estudos de alelopatia investigam os efeitos positivos e negativos que metabólitos secundários de plantas, microrganismos ou fungos exercem sobre o desenvolvimento de indivíduos vizinhos. Nesse trabalho foram investigados os efeitos de extratos aquosos de folhas, cascas de tronco e cascas de raízes de canela-sassafrás (Ocotea odorifera (Vell.) Rowher) sobre a germinabilidade das sementes, desenvolvimento do sistema radicular e da parte aérea, teor de clorofila da parte aérea e respiração das células radiculares de plântulas de sorgo (Sorghum bicolor L. Moench cv. Embrapa BR 303). Os extratos foram preparados adicionando o material vegetal seco e moído oriundo de cada órgão à água destilada e deionizada na proporção 1:10 (p/v). Após agitação constante por 24 h, as soluções foram decantadas e filtradas através de papel de filtro sob vácuo, constituindo os extratos testados. Os extratos aquosos de cascas de tronco e de raízes causaram inibição do desenvolvimento do sistema radicular das plântulas de sorgo e a sua parte aérea teve o crescimento estimulado pelo extrato de cascas de raízes. Os extratos de folhas e de cascas de tronco induziram aumento da biomassa fresca do sistema radicular e diminuição significativa do teor de clorofila. Todos os extratos causaram efeitos negativos sobre a respiração radicular das plântulas de sorgo. Concluiu-se que metabólitos secundários da canela-sassafrás causaram alterações metabólicas e morfológicas nas plantas de sorgo e que, portanto, a presença de canela-sassafrás no ambiente pode desencadear efeitos similares sobre outras espécies vegetais na sua vizinhança.

-(Seed germination of Dyckia tuberosa (Vell.) Beer (Bromeliaceae) under different temperature in light and darkness). The effects of temperature and light on the germination of seeds of Dyckia tuberosa (Vell.) Beer, a terrestrial bromeliad from "cerrado", were studied. Seeds collected at Estação Ecológica of Itirapina, São Paulo State, were incubated at constant temperatures in the range of 10 to 45 °C, under continuous white light and darkness. The range of temperature with highest germinability under light was from 15 to 40 °C and from 20 to 40 °C in darkness, with no germination at 10 and 45 ºC for light and darkness. For both, light and darkness, the optimum temperature was in the range of 30 to 35 °C. The high germinability at high temperatures shows that D. tuberosa is able to germinate in open areas and the light insensitivity of the seeds indicates that they can also germinate in conditions of completely darkness. Apparently, the germination process is not a limiting factor for this species.

ABSTRACT

Recebido em 18/06/1998. Aceito em 17111/1998 RESUMO -( Estudo da germinação de sementes de espécies arhóreas pioneiras: TibouellinQ pulellra Cogn. e Tibouellina granulosa Cogn. (MelaslOmalaceae)). A inlluência da luz no processo de germinação de sementes de Tibouellino l'ldellra e T. granulosa foi eSludada. Sementes de Tibouehina fJulellra e T. granulosa submetidas a diferentes qualidades e quantidades de luz demonstram forte dependência para o processo de germinação. Em condições de escuro e irradiação com vermelho-extremo, as sementes não germinaram, demonslrando o envolvimento do l'itocromo no processo, como evidenciado através de curvas de fluência-resposta e de curvas de escape da ação do l'itocromo. Após 12 meses de estocagem sob baixa temperatura as sementes apresentaram a mesma percentagem de sementes viáveis. A partir destes resultados conclui-se que T. I'ulellra and T. granulosa são espécies pioneiras.Palavras-chave -espécies pioneiras, germinação de sementes. fitocromo, TibouellinQ ABSTRACT -(Seed gel'1nination of pioneer spccies 01' Tiboucllina pulell/'{{ Cogn. and Tibouchil1a granulosa Cogn. (Melastomataceae)). Seeds 01' Tiboucllina IJ/dcllra and T. granulosa suhmitted to dilTerent light treatments showed light dependence 01' the gel'1nination processo In conditions 01' complete darkness and when suhmitted to far-red light. seeds did not germinate. demonstrating the involvement 01' phytochrome in the processo This conclusion was corroborated by curves 01' fluence response for seed germination and escape curves 01' phytochrome action. Seeds of both species presented the same percelltage 01' viahle seeds aI' ter twelve months storage under low temperature. Our resu1ts indicates Ihal T. pulellra and T. granulosa are pioneer species.Key \Vords -pioneer species, seed germination. phytm:hromc. TibouchinQ IntroduçãoA quantidade e a qualidade espectral da luz, disponíveis para a germinação de sementes, são distintas nas diferentes situações da floresta tropical natural (Lopes-Quiles & Vazquez-Yanes 1976; Vazquez-Yanes ef aI. 1990). A luz pode tanto promover quanto inibir a germinação, até mesmo em sementes da mesma espécie. Algumas sementes I

Seeds (nutlets) of Rumex obtusifolius L. fail to germinate in darkness at 25° C, but are stimulated by short exposure to red light (R) the effectiveness of which can be negated by a subsequent short exposure to far red light (F) indicating phytochrome control. Short periods of elevated temperature treatment (e.g. 5 min at 35° C) can induce complete germination in darkness. Although short F cannot revert the effect of 35° C treatment, cycling the phytochrome pool by exposure to short R before short F results in reversion of at least 50% of the population. Prolonged or intermittent F can also revert the germination induced by 35° C treatment. The effect of elevated temperature treatment is interpreted on the basis of two possible models; (i) that it increases the sensitivity of the seeds to a low level of pre-existing active form of phytochrome (Pfr) (ii) that it induces the appearance of Pfr in the dark. In both cases it is envisaged that elevated temperature treatment and Pfr control germination at a common point in the series of reactions that lead to germination.