-Known since ancient times, the phenomenon of allelopathy has recently received greater attention from researchers and farmers worldwide. Crop failures and low yields caused by the reseeding, overseeding, crop rotation and replanting of fruit trees in orchards are believed to be caused by allelopathic activity. Moreover, the expensive and environmentally impacting herbicides for weed control have motivated studies directed at developing cheaper, environmentally-friendly alternatives. Despite the difficulties in separating chemical interference from other mechanisms, more modern approaches have produced consistent and conclusive data on this topic. The release and dynamics of allelochemicals in the soil are discussed herein. Examples of allelopathic crops, the allelochemicals produced and their uses in cropping systems are also presented along with current research trends regarding allelopathy. allelopathy / chemical interference / agriculture / potential use

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Unitermos: Florestas secas, Folin-Ciocalteau, plantas medicinais, semi-árido. ABSTRACT:The infl uence of seasonal climatic changes on tannin concentrations was examined in two caatinga (semi-arid) plant species (Myracrodruon urundeuva (Engl.) Fr. All. and Anadenanthera colubrina (Vell.) Brenan). The Folin-Ciocalteau method was used to quantify total phenols, and the casein precipitation method was used for tannins. In general, there is a close relationship between tannin levels and rainfall, although species seem to adopt different strategies of tannin compound production in response to periods of drought and rainfall in the caatinga.Keywords: Dry forest, Folin-Ciocalteau, medicinal plant, semi-arid. INTRODUCTIONIt is well documented that some ecological variables can quantitatively and qualitatively affect tannin production in plants (Hatano et al., 1986;Teixeira et al., 1990;Furlan et al., 1999;Simon et al., 1999;Salminen et al., 2001;Paiva et al., 2002). Seasonal changes, for example, alter plant metabolism and, consequently, biosynthetic pathways (Hatano et al., 1986;Salminen et al., 2001). Periods of high temperatures are known to increase the concentration of phenolic compounds in many plants. Species of Lotus, for example, demonstrate seasonal variations in tannin concentration (Gebrehiwot et al., 2002), which have been shown to be signifi cantly higher during the spring and summer. The same response was seen with Alnus rubra Bong (González-Hernández et al., 2000). Comparisons between plants growing in tropical sub-humid and tropical semi-arid regions of Africa have demonstrated that total phenols levels increase in regions with more severe climates and with less available humidity (Makkar; Becker, 1998).In Brazil, only a limited number of studies have examined the infl uence of environmental variables on tannin production, and most of these have focused on plant defenses in areas of cerrado (savannah) and Atlantic Coastal Forest vegetation (Pais;Varanda, 2003;Pansera et al., 2003;Yariwake et al., 2005). Fewer studies have been undertaken in the caatinga yet and it certainly remains the most incompletely studied Brazilian biome. The caatinga is also the least protected region in Brazil, with less than 2 % of its lands included within any type of conservation area. Research programs in this semiarid region are therefore necessary, especially in light of the rapid alteration of this biome, and substitution of the native vegetation even before it can be effectively studied (Fernandes, 2002; Giullieti et al., 2002;Castelletti et al., 2003).In the present work, the tannin levels of two caatinga medicinal plants, Myracrodruon urundeuva (Engl.) Fr. All. and Anadenanthera colubrina (Vell.) Brenan, were quantifi ed and compared with local seasonal rainfall data. Studies which increase our understanding of the behavior of caatinga plants are extremely important at this moment for conservation and sustainable. Management strategies are needed to protect and preserve many semi-arid species threatened with extinction...

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