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Helianthus Que Amenazan La Produccion De Semilla De Girasol En El Valle Bonaerense Del Rio Colorado

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Helianthus Que Amenazan La Produccion De Semilla De Girasol En El Valle Bonaerense Del Rio Colorado Boletín Técnico Nº 16 ISSNA 0328-3321 HELIANTHUS QUE AMENAZAN LA PRODUCCION DE SEMILLA DE GIRASOL EN EL VALLE BONAERENSE DEL RIO COLORADO Miguel Cantamutto, Alejandro Presotto, Mónica Poverene, Julio Rivas, Raúl Matarazzo y Juan Renzi EEA Hilario Ascasubi Boletín Técnico Nº 16 ISSNA 0328-3321 HELIANTHUS QUE AMENAZAN LA PRODUCCION DE SEMILLA DE GIRASOL EN EL VALLE BONAERENSE DEL RIO COLORADO Miguel Cantamutto(a), Alejandro Presotto(a,b), Mónica Poverene(a,b), Julio Rivas(c), Raúl Matarazzo(c), Juan Renzi(a,c) (a) Departamento de Agronomía, Universidad Nacional del Sur. (b) Centro de Recursos Renovables de Zonas Semiáridas, Universidad Nacional del Sur-Consejo Nacional de Investigaciones Científicas y Técnicas. (c) Estación Experimental Agropecuaria Hilario Ascasubi. Instituto Nacional de Tecnología Agropecuaria EEA HILARIO ASCASUBI Ruta Nacional Nº 3 - km. 794 8142 – HILARIO ASCASUBI – Tele-Fax 02928 – 491141/491011 Internet:http://www.inta.gov.ar [email protected] INTA República Argentina Secretaría de Agricultura, Ganadería, pesca y Alimentos Centro Regional Buenos Aires Sur Estación Experimental Agropecuaria Hilario Ascasubi Diciembre 2007 RESUMEN En Argentina se han naturalizado cuatro especies de Helianthus originarias del centro de los EEUU. Entre ellas, H. annuus annuus y H. petiolaris son dos anuales que han invadido la región central de Argentina donde muestran un intenso flujo génico con el cultivo de girasol H. annuus var. macrocarpus. El patrón de migración, descripto en varias escalas de análisis, sugeriría que el valle bonaerense del Río Colorado (VBRC) es vulnerable a la invasión por ambas especies. Si ello ocurriera, la producción de semilla híbrida de girasol se vería perturbada, dado que fecundan al cultivo y generan plantas fuera de tipo que podrían originar poblaciones ferales a partir del empleo de semilla contaminada. Para evitar este escenario se alerta sobre esta situación a las personas implicadas en la cadena de producción de semilla. Se realiza una breve descripción de la capacidad invasora de las especies, el análisis de riesgo de invasión y una propuesta de medidas preventivas para evitar la colonización del agroecosistema del VBRC, incluyendo la descripción de los taxones de interés. HELIANTHUS QUE AMENAZAN... SECCION I El porqué de la amenaza, las consecuencias de una invasión. INTRODUCCION Una semilla comercialmente competitiva de un cultivar híbrido de girasol (Helianthus annuus L.) debe producir un cultivo uniforme de plantas de alto rendimiento económico y cumplir normas de calidad muy exigentes. Debe estar libre de contaminantes (Res. SAGyP 2270/93) y originar más del 95% de plantas del híbrido comercializado (OECD, 2006). Si durante la producción de semilla existe polen diferente al del genotipo paterno, como es de polinización cruzada entomófila, pueden producirse fecundaciones no controladas que originarán plantas fuera de tipo. Para evitarlo, los lotes de producción de semilla deben aislarse de otras fuentes de polen, separándolos hasta 5 km en el caso de la semilla categoría fundación (Anfinrud 1997). En el valle bonaerense del Río Colorado (VBRC) el girasol comenzó a desarrollarse en la década del ´80, como un cultivo alternativo a la cebolla, que predominaba entre los sistemas productivos. Si bien se conocía su alto potencial de rendimiento (Hernández, Oroli 1982, Rivas et al. 1987) el girasol no se expandió hasta que se superaron limitantes del cultivo, incluyendo las pérdidas por loro barranquero (Cyanoliseus patagonus). Ello se logró a mediados de los ´90 con el aumento del área de cultivo y siembras tempranas, impulsadas por el INTA EEA Hilario Ascasubi y la empresa Oleaginosa Moreno Hermanos SA. En los últimos años ha aumentado la superficie destinada a producción de semilla fiscalizada, que es de mayor valor. Según información del INASE (2007) en el VBRC se cultiva más del 85% de los lotes destinados a este fin, con la presencia de la mayor parte de las empresas del sector. Para mantener esta interesante actividad económica, la zona debe continuar destacándose por sus altos estándares de calidad de semilla. LA CAPACIDAD INVASIVA DE LOS HELIANTHUS El género Helianthus (Asterales: Asteraceae) comprende unas 50 especies, originarias del centro de América del Norte, donde aún existen en estado salvaje (Heiser et al. 1978). Entre ellas se encuentra el H. annuus annuus silvestre, que fue la fuente para la domesticación de H. annuus var. macrocarpus L., variedad botánica utilizada por los genetistas rusos para obtener las primeras poblaciones mejoradas (Vranceanu et al. 1977), que fueron introducidas a nuestro país por inmigrantes hebreos en el siglo XIX (Kugler y Godoy 1964). Argentina, donde el girasol es la segunda oleaginosa cultivada, ocupa los primeros lugares en el comercio internacional de su aceite (www.sagpya.gov.ar). Unas pocas especies del género se encuentran difundidas en Argentina (www.darwin.edu.ar). Entre las anuales, H. petiolaris Nutt. registrada desde la década del `50 (Covas, 1984), ocupa sectores de la zona semiárida de las provincias de Buenos Aires, La Pampa, San Luis y Córdoba. Helianthus annuus L. comprende la forma cultivada, sus descendientes denominados "guachos" (Marzocca 1994), y la subespecie salvaje H. annuus annuus, naturalizada en la región central Argentina (Poverene et al. 2002). Helianthus argophyllus T.&G., es otra especie anual, que con escasa frecuencia, puede hallarse en jardines, cultivada por la atractiva coloración plateada de sus tallos y hojas, pero no se ha naturalizado. Entre las perennes en Argentina se encuentran Helianthus tuberosus L. o topinambur y la especie híbrida H. x laetiflorus Pers. La primera fue introducida como forrajera en la segunda mitad del siglo pasado, mediante clones mejorados. Si bien actualmente presenta escasa difusión, algunas poblaciones se mantienen fuera de cultivo por sus tubérculos engrosados, que son persistentes. El híbrido botánico H. x laetiflorus Pers. ornamental cultivado, se ha naturalizado en el sudeste de la provincia de Buenos Aires (Sala et al. 1990). Si bien no produce semilla, posee tubérculos alargados, que le confieren mayor capacidad invasora que el topinambur. Resulta muy complejo conocer con exactitud el origen de las poblaciones ferales de Helianthus que se registran fuera del centro de origen. En Europa se estima que las poblaciones invasoras de H. tuberosus habrían escapado de cultivo por cursos de agua que arrastraron sus rizomas, mientras que H. annuus se habrían iniciado mediante semilla híbrida de líneas maternas polinizadas por plantas silvestres, posiblemente introducida desde EEUU (Bervillé et al. 2005). Las poblaciones argentinas de H. annuus silvestre descenderían de introducciones realizadas con fines forrajeros antes de la década del ´50 (Bauer 1991), mientras que H. petiolaris habría ingresado como contaminante de semillas de sorgo importadas de Texas (A. Luciano, comunicación personal). El área invadida por las dos especies anuales de Helianthus en Argentina ha seguido un patrón agroecológico que puede ser descripto a escalas múltiples (Cantamutto et al. 2007). Dentro de la banda de transición entre el régimen ústico y el údico, Helianthus petiolaris invadió suelos arenosos, generalmente degradados por erosión eólica, mientras que H. annuus silvestre se difundió en suelos de textura más fina, a veces afectados por procesos de degradación ligados al agua (anegamiento, inundación, salinidad). Ambas especies han migrado por las vías de comunicación terrestre, posiblemente llevadas en forma no intencional por el transporte de granos y las maquinarias agrícolas. Sus pequeñas semillas han podido viajar retenidas en las partes externas de los implementos agrícolas y transportes. Helianthus annuus annuus también se ha difundido en las regiones de regadío de San Juan y Mendoza, donde se puede inferir que fue movilizada por el sistema de riego, y puede hallársela en los bordes de canales de riego y desagües. El VBRC no presentaría diferencias sustantivas con las regiones ya invadidas, por lo que podría ser un ambiente adecuado para su naturalización. EL IMPACTO DE UNA INVASION El flujo génico entre los Helianthus es muy intenso y se lo reconoce como una vía por la cual ha aumentado la capacidad invasora del género. Al menos tres nuevas especies, adaptadas a ambientes distintos a sus progenitores se han originado naturalmente por hibridación transgresiva (Rieseberg et al. 2006). El frecuente flujo génico entre los taxa anuales en Argentina (Cantamutto et al. 2003, Poverene et al. 2004) genera un escenario en el que podría aumentar la exo-feralidad del género, facilitados por la introgresión con el cultivo (Ellstrand 2003). Las especies anuales de Helianthus naturalizadas en Argentina pueden fecundar al cultivo y generar descendientes fértiles (Tabla 1). Helianthus annuus (Deines et al. 2004, Rosales-Robles et al. 2005) y H. petiolaris (Grichar et al. 2004) también se reconocen como malezas de los cultivos. En los EEUU, la presencia de estas especies en banquinas, taludes y contrafuegos limita la producción de semilla pues constituyen una fuente de polen contaminante (Anfinrud 1997), al igual que en Europa, donde las regiones invadidas por silvestres o sus cruzas se rechazan para la producción de semilla (Muller et al. 2007). Una vez instaladas, dado que ambas especies silvestres poseen dormición, sus poblaciones resultarían de difícil erradicación, pues podrían sobrevivir varios años en el banco de semillas del suelo. La polinización con abejas, que se utiliza en los
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