Plants have a large number of bioactive compounds with high antioxidant activity. Studies for the determination of the antioxidant activity of different plant species could contribute to revealing the value of these species as a source of new antioxidant compounds. There is a large variety of in vitro methods to quantify antioxidant activity, and it is important to select the proper method to determine which species have the highest antioxidant activity. The aim of this work was to verify whether different methods show the same sensitivity and/or capacity to discriminate the antioxidant activity of the extract of different plant species. To that end, we selected 12 species with different content of phenolic compounds. Their extracts were analyzed using the following methods: 2,2-di-phenyl-1-picrylhydrazyl (DPPH) radical scavenging capacity assay, ferric reducing (FRAP) assay, Trolox equivalent antioxidant capacity (ABTS) assay, and reducing power (RP) assay. The four methods selected could quantify the antioxidant capacity of the 12 study species, although there were differences between them. The antioxidant activity values quantified through DPPH and RP were higher than the ones obtained by ABTS and FRAP, and these values varied among species. Thus, the hierarchization or categorization of these species was different depending on the method used. Another difference established between these methods was the sensitivity obtained with each of them. A cluster revealed that RP established the largest number of groups at the shortest distance from the root. Therefore, as it showed the best discrimination of differences and/or similarities between species, RP is considered in this study as the one with the highest sensitivity among the four studied methods. On the other hand, ABTS showed the lowest sensitivity. These results show the importance of selecting the proper antioxidant activity quantification method for establishing a ranking of species based on this parameter.

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The production of labdanum exudate byCistus ladanifer L. is highly seasonal, reaching a maximum concentration during summer and a minimum concentration in winter. Because this exudate strongly absorbs in the wavelength range of 260-400 nm (the near-UV-visible range), it may be important biologically as an UV-visible filter. Separation of exudate components has been achieved by reverse-phase high-performance liquid chromatography (HPLC).The retention times of HPLC chromatograms and the spectral characteristics (absorption and fluorescence) of the exudate identify flavonoids as the most relevant chromophores regarding the potency of the exudate as a UV-visible filter. HPLC studies show that kaempferol-3-(O)methyl, kaempferol-3,7-di(O)methyl, and apigenin-4'-(O)methyl are the most enriched flavonoids in the exudate.Other flavonoids [apigenin, apigenin-7-(O)methyl, apigenin-7,4'-di(O)methyl, kaempferol-3,4'-di(O)methyl and kaempferol-3,7,4'-tri(O)methyl] are present in the exudate as minor components, e.g., each contributes by less than 10% to total flavonoids.The ratio of kaempferols to apigenins of the exudate also shows seasonal variation (maximum value in summer and minimum in spring). However, due to the similar absorption spectra of both groups of flavonoids, this has a minor influence on the exudate's ability to filter near-UV-visible radiation.

Summary1. The allelopathic effect of the exudate secreted by Cistus ladanifer leaves was tested on different plant species. Cynodon dactylon and Rumex crispus, species absent from C. ladanifer scrub (jarales), were clearly inhibited by the exudate. 2. Species present in the jarales, Medicago polymorpha and Lolium rigidum, showed no direct inhibition of germination, but the process this was delayed, and cotyledon and root size was diminished. This implies inhibition of seedling growth, and may explain the low abundance of these species inside the jarales. 3. The active constituents in the inhibition of germination are low molecular weight phenolic compounds. The flavonoids appear to exert no direct effect on germination of the species tested, but do provoke a reduction in cotyledon and root size, and could thereby inhibit subsequent seedling development.

Autotoxic species are those which adversely affect their own seeds' germination and/or seedling development. Cistus ladanifer L (labdanum or jara) has been shown to have a pattern of allelopathic behaviour against the herbs that share its habitat. The present work studied whether an autotoxic effect also exists. The aqueous solution obtained from washing jara leaves was found by itself to inhibit germination and cotyledon emergence of the species' seeds. When these same trials were carried out in soils, autotoxicity was observed only from leaves and soils collected in winter. This was so both in soils collected away from the influence of the jaral to which was added the greatest concentration of aqueous extract prepared from the leaves, and in soils collected within the jaral, except that in the latter group of soils germination was inhibited with or without the addition of C. ladanifer extracts. This autotoxic behaviour could be involved in the species' own population control, and would explain the scant self-regeneration within established C. ladanifer stands.