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Background: Secondary metabolites in stevia leaves offer important therapeutic benefits. The germination problems of stevia seeds and the high heterogeneity of the resulting populations make micropropagation the preferred tool for obtaining a large number of plants in a short time. Until now, studies have focused on optimizing the action of growth regulators for the improvement of stevia micropropagated plantlets. In this project, we study alternative mineral nutrients in the substrate and two culture systems on micropropagated stevia plantlets to obtain proper amounts of enhanced plantlets. Methods: Criolla and Morita varieties; MS, MSM, and G substrates, and temporary immersion (TIS) and agar-medium systems were used. Biometric variables and damage to the cell membrane of the resulting plantlets were tested. Results: More productivity in the Criolla plantlets growing in MS solid medium and TIS. The Morita plantlets presented higher productivity in the solid MS, MSM, and G substrates and TIS. The explants growing in MS and MSM presented the highest productivity. The solid substrate was generally better than TIS. No damage to the cell membrane was found. Conclusions: The micropropagation efficiency of stevia plantlets of the two varieties has been enhanced without negative effects on their physiological condition. Consequently, any of the culture systems used, including bioreactors, can be of industrial application on a large scale. [ABSTRACT FROM AUTHOR]

Allelopathic potential of buckwheat roots on the radicle growth of the broomrape weed species Orobanche cumana and Phelipanche ramosa was studied. Buckwheat root exudates induced a significant growth inhibition in P. ramosa radicles but radicles of O. cumana were not affected. Among the metabolites present in the root organic extract we identified the flavonol quercetin and the stilbene p-coumaric acid methyl ester with only quercetin showing inhibitory effect on P. ramosa. The activity of quercetin was compared with other two similar flavanoids, the flavone apigenin and the dihydroflavanol 3-O-acetylpadmatin extracted respectively from Lavandula stoechas and Dittrichia viscosa plants. In this comparative assay only 3-O-acetylpadmatin besides quercetin, showed inhibition activity of radicle growth while apigenin was inactive. These results indicated that the presence of two ortho-free hydroxy groups of C ring, like catechol, could be an important feature to impart activity while the carbon skeleton of B ring and substituents of both A and B rings are not essential. Besides reduction of radicle growth, haustorium induction was observed at the tip of P. ramosa radicles treated with quercetin which swelled and a layer of papillae was formed. Activity of quercetin on haustorium induction in P. ramosa was assayed in comparison with the known haustorium-inducing factor 2,6-dimethoxy-p-benzoquinone (DMBQ) and a three partial methyl ether derivatives semisynthetized from quercetin. Results indicated that P. ramosa haustorium was induced by DMBQ at concentrations of 1–0.5 mM and quercetin and its derivatives at concentration range 0.1–0.05 mM. [ABSTRACT FROM AUTHOR]