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RESUMO O tegumento e conteúdo de água das sementes podem interferir na sua tolerância a choques térmicos, assim como o seu ambiente de origem. Para testar essas possibilidades, sementes de Plathymenia reticulata e Stryphnodendron adstringens foram coletadas no norte do Estado de Minas Gerais (representando savana seca) e no nordeste do Estado do Mato Grosso (representando savana úmida) do bioma Cerrado, e suas respostas a choques térmicos de 110, 140 e 170 ºC por 2,5 minutos foram avaliadas sob diferentes condições; intactas, escarificadas, e embebidas após escarificação. Sementes escarificadas e embebidas germinaram mais rapidamente para ambas as espécies. Sementes de savana seca apresentaram menor teor de água inicial e germinação mais lenta para ambas as espécies, porém maior tolerância a choques térmicos que sementes de savana úmida. Sementes de P. reticulata não sobreviveram após tratamento de 140 ºC, mas sementes de S. adstringens de savana seca suportaram tratamento de 170 ºC, independentemente de estarem escarificadas e/ou embebidas. Conclui-se que sementes de P. reticulata são menos tolerantes a choques térmicos que sementes de S. adstringens, que sementes secas são mais tolerantes a choques térmicos que sementes embebidas, e que sementes de savanas secas são mais tolerantes a choques térmicos que sementes de ambientes úmidos. ABSTRACT Seed coat and water content may be direct determinants of seed tolerance to heat shocks, which may also be determined by the origin of the seeds. To check this, seeds of Plathymenia reticulata and Stryphnodendron adstringens were collected in the north of Minas Gerais State (representing a dry savanna) and in the northeast of Mato Grosso State (wet savanna) of the Cerrado biome and subjected to heat shocks of 110, 140 and 170 ºC for 2.5 minutes under three conditions; intact, scarified, and scarified and soaked. Soaked and scarified seeds of both species germinated more quickly. For both species, seeds from dry savanna showed lower initial water content and slower germination, but greater tolerance to heat shocks than seeds from wet savannas. No survival was detected among seeds of P. reticulata after treatment of 140 ºC, while seeds of S. adstringens from dry savannas tolerated treatment of 170 ºC, regardless of being scarified and imbibed or not. We conclude that seeds of P. reticulata have a lower tolerance to heat shock than seeds of S. adstringens, that imbibed seeds are less tolerant to heat shock than non-imbibed seeds and that seeds from dry savannas are more tolerant to heat shock than those from wet savannas

The search for and identifi cation of new molecules for the development of herbicides has grown in recent decades. With the objective of developing effi cient, environmentally friendly herbicides, we evaluated the phytotoxic activity of 13 synthetic chalcones on the development of sesame Sesamum indicum L. and brachiaria Urochloa decumbens (Stapf) R.D. Webster. Growth experiments were conducted at 30°C within a photoperiod of 12 hours. The positive control was distilled water whereas the negative control was the herbicide Glyphosate®. Two chalcones (3,13) signifi cantly inhibited the growth of the target species, being this inhibition comparable to that produced by Glyphosate®. The inhibitory eff ects of these chalcones on the growth of sesame and brachiaria deserves deeper investigation, since the methodology for their synthesis is simple and the reagents employed are of low cost, what make their large-scale production economically viable. Also, these molecules do not produce toxic residues such as those generated by synthetic herbicides.

Estudos têm identificado potencial fitotóxico de tecidos vegetais de diversas espécies do Cerrado. Entretanto, a maior parte destes estudos foi conduzida em substrato inerte, ou fazendo uso de quantidades exageradas de material vegetal. Neste estudo investigamos o potencial fitotóxico de folhas de pequi adicionadas a substrato solo em quantidades compatíveis ao volume de liteira produzida por esta espécie no campo. Folhas de pequi foram trituradas e incorporadas a latossolo vermelho em proporções entre 0,75 e 3% (p/p), sendo o controle constituído por solo sem folhas. Estas misturas foram adicionadas a vasos plásticos irrigados diariamente. Sementes germinadas das espécies cultivadas sorgo e gergelim, da invasora braquiária, e da nativa ipê-roxo foram dispostas nos vasos e crescidas por cinco a sete dias a 30°C, com fotoperíodo de 12 h. Plântulas de todas as espécies apresentaram redução dose-dependente no crescimento, sendo o crescimento radicular mais afetado que o aéreo. Sinais de necrose foram observados nas raízes de sorgo, gergelim e braquária. Os efeitos fitotóxicos gerados por uma quantidade relativamente baixa de tecido vegetal, em substrato solo, e em razoável número de espécies de diferentes grupos funcionais sugerem que a liteira desta espécie possa exercer real potencial alelopático em condições de campo. Studies have identified the phytotoxicity of many native species of the Cerrado; however, most of them were conducted either in inert substrates, or using exaggerate proportions of plant material. We investigated the phytotoxicity of pequi leaves added to substrate soil in quantities compatible with the litter produced by this species. Pequi leaves were triturated and added to red latosol in concentrations of 0.75%, 1.5% and 3%; the control was constituted of leafless soil. These mixtures were added to pots and irrigated daily to keep them moist. Germinated seeds of the cultivated sorghum and sesame, of the invasive brachiaria and of the native purple ipê, were disposed in the pots to grow for five to seven days at 30°C within a photoperiod of 12 h. Seedlings of all the species presented a reduction in their initial growth in a dose-dependent way. In general, the root growth was more affected by the treatments than the shoot growth; moreover, signs of necrosis were observed in the roots of the sorghum, sesame and brachiaria. The phytotoxic effects generated by relatively small quantities of leaves, in a reasonable range of species within a soil substrate, suggest potential allelopathy of pequi leaves under natural conditions.

In this article, we review allelopathy studies conducted in Brazil or involving plant species that occur in the country. Conceptions and misconceptions associated with allelopathy, as well as some international criteria to be applied in allelopathic research, are presented and discussed. We observed a sharp increase in the number of papers on this subject conducted in Brazil between 1991 and 2010. However, most studies are conducted under laboratory conditions, lack a clear hypothesis or a solid justification, and typically make use of target species that do not co-exist with the donor species under natural conditions. We also found that most studies do not take the additional steps in order to purify and identify the bioactive molecules. We recommend that further studies be conducted in order to explore the potential of plant biodiversity in Brazil. Such studies could lead to the development of new molecular structures (allelochemicals) that could be used in the control of pests and weeds, thereby reducing the use of the harmful synthetic herbicides that are currently being widely employed.