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The demand for low-calorie products has increased. One of them is a natural product extracted from stevia (Stevia rebaudiana Bert.) whose diterpene glucosides generate outstanding sweetening power. The organic cultivation of this species is increasing in the southern region of Chiapas, México, but its asexual establishment through cuttings presents agronomic limitations. This can be improved with the incorporation of biofertilizers. It is the case of the efficiency in N fixation by Azospirillum brasilense and the endomycorrhizae Rhizophagus intraradices, which improve the transport of nutrients and water to the host plant. These microorganisms have been used in biofertilizer induction programs in annual and perennial crops in México. The objective of this study was to evaluate the effect of R. intraradices (Schenck & Sm.) Walker & Schüßler and A. brasilense Tarrand, Krieg & Döbereiner in interaction with brassinosteroids in S. rebaudiana Bert. plants through morphological, physiological and biochemical changes. The cv Morita 2 was established in a greenhouse, in plastic bags with 6 kg of fluvisol-euthric soil plus washed river sand (1:1) and 4 g of each inoculant at the moment of transplant. The concentration of A. brasilense was 9×106 bacteria g-1 and that of R. intraradices 40 spores g-1 of soil with 95 % colonization of the root system. The brassinosteroid (CIDEF-4, Natura del Desierto, S. A. de C.V., steroid content 80 % with 10 % i.a.) (2 mg L-1 of water) was sprayed on the foliage after 28 d of sowing, plus three later applications with frequency of 14 d. The experimental design was completely random with seven repetitions; the treatments were the microorganisms and the brassinosteroid applied individually and combined, and the control without application or with inoculum. The experimental unit was one pot. The variables recorded were yield, percentage of mycorrhizae root colonization, content of sweetener in leaves, and of N and P in shoots 90 d after sowing. The inoculated plants and treated with brassinosteroid in different modality increased the leaf area (1728 cm2 per plant), content of stevioside (35.8 mg g-1), rebaudioside (29.5 mg g-1) and steviol (5 mg g-1) compared to the control (p≤0.05). The content of N and P increased with the application of R. intraradices alone and combined with brassinoesteroid. [ABSTRACT FROM AUTHOR]

Unregulated logging in tropical forests promotes invasion of secondary vegetation that compete with species of medium and slow growth, limiting their natural regeneration. Many reforestation efforts fail for this situation endangering the ecosystem balance by differences in density and distribution of species. In order to promote a greater growth in shorter time of the timber species (Tabebuia donnell-smithii Rose) primavera under nursery conditions, the effect of inoculation with Rhizophagus intraradices (Schenck et Sm.) Walker et Schuessler, Glomus spp., and Azospirillum brasilense Tarrand, Krieg et Döbereiner was evaluated. Nursery was established with seeds collected in the Soconusco, Chiapas, Mexico, in plastic bags of 5 kg with a mixture of Andosolmollic soil and river sand washed (1:1) with 2 g of inoculant at the time of transplantation. Azospirillum brasilense had a concentration of 9×106 bacteria g-1 and R. intraradices 40 spores g-1 of soil with 95 % of colonization to the root system. The experimental design was completely randomized treatments were individual, combined microorganisms, and a control with four replications. Morphological and physiological variables of yield, percentage of mycorrhizal root colonization were recorded every 28 d from the 56 d to 168 d after sowing. Plants with R. intraradices, double symbiosis A. brasilense+R. intraradices and Glomus sp. (V) showed higher increase of biomass at 112 d (p⩽0.05), whereas the highest nitrogen content was found in roots of plants inoculated with R. intraradices and in the shoot with A. brasilense. The phosporus content was higher with A. brasilense and R. intraradices on the shoot. The final significant differences in plant height were 6 to 8 cm among inoculated treatments and of 16 cm of these with respect to the control. [ABSTRACT FROM AUTHOR]

La explotación forestal sin regulación en bosques tropicales promueve la invasión de vegetación secundaria que compite con especies de mediano y lento crecimiento, limitando su regeneración natural. Muchos esfuerzos de reforestación fracasan por esta situación poniendo en riesgo el equilibrio del ecosistema por diferencias en densidad y distribución de especies. Con el fin de promover un mayor crecimiento en menor tiempo de la especie maderable primavera (Tabebuia donnell-smithii Rose) en condiciones de vivero, se evaluó el efecto de la inoculación con Rhizophagus intraradices (Schenck et Sm.) Walker et Schuessler, Glomus spp., y Azospirillum brasilense Tarrand, Krieg et Döbereiner. El vivero se estableció con semillas recolectadas en el Soconusco, Chiapas, México, en bolsas de plástico de 5 kg con una mezcla de suelo andosol-mólico y arena de río lavada (1:1) con 2 g de inoculante al momento del trasplante. Azospirillum brasilense tuvo una concentración de 9×106 bacterias g-1 y R. intraradices 40 esporas g-1 de suelo con 95 % de colonización al sistema radical. El diseño experimental fue completamente al azar y los tratamientos fueron los microorganismos individuales, combinados y un testigo, con cuatro repeticiones. Las variables morfológicas y fisiológicas del rendimiento, porcentaje de colonización radical micorrízica se registraron cada 28 d desde los 56 d hasta 168 d después de siembra. Las plantas con R. intraradices, la simbiosis doble A. brasilense+R. intraradices y Glomus sp. (V) presentaron mayor aumento de biomasa a 112 d (p⩽0.05), mientras que el mayor contenido de nitrógeno se encontró en raíces de plantas inoculadas con R. intraradices y en el vástago con A. brasilense. El contenido de fósforo fue mayor con R. intraradices y A. brasilense en el vástago. Las diferencias significativas finales en altura de planta fueron de 6 a 8 cm entre tratamientos inoculados y de 16 cm de éstos respecto al testigo. [ABSTRACT FROM AUTHOR]