The role of a microbial consortium on gas exchange and water relations in Allium cepa L. under water and nutritional deficit conditions.

Synergistic studies of microorganisms in the last decade have been mostly directed towards their biofertilizing effects on growth and crop yield. Our research examines the role of a microbial consortium (MC) on physiological responses of Allium cepa hybrid F1 2000 under water and nutritional deficit in a semi-arid environment. An onion crop was established with normal irrigation (NIr) (100% ETc) and water deficit (WD) (67% ETc) and different fertilization treatments (MC with 0%, 50% and 100% NPK). Gas exchange (Stomatal conductance (Gs), transpiration (E) and CO2 assimilation rates (A)) and leaf water status were evaluated throughout its growth cycle. The MC + 50% NPK treatment with NIr maintained similar A rates to the production control. A. cepa decreased Gs by approximately 50% in the WD treatment. The highest water use efficiency (WUE) and an increase in the modulus of elasticity in response to water stress were obtained for the 100% NPK treatment under non-inoculated WD. The onion hybrid F1 2000 was tolerant to water stress and under non-limiting nutrient conditions, irrigation may be reduced. The MC facilitated the availability of nutrients under NIr allowing a 50% reduction in the application of high doses of fertilization without affecting yield, resulting in a suitable agroecological strategy for this crop.

Functional characteristics of the arborescent genus Polylepis along a latitudinal gradient in the high Andes

Polylepis es un género restringido a la Cordillera de los Andes, encontrándose de forma natural por encima del límite superior de bosque continuo. El propósito de este trabajo fue integrar y comparar las características funcionales, en términos de relaciones hídricas y de carbono y mecanismos de resistencia a bajas temperaturas, en diferentes especies de Polylepis a lo largo de un gradiente latitudinal. Las especies estudiadas fueron P. sericea en Venezuela, P. tarapacana en Bolivia y P. australis en Argentina. Se compararon medidas estacionales de potencial hídrico y osmótico foliares, conductancia estomática, asimilación de CO2 y respiración, y temperatura de congelamiento y daño. Se evidencia un gradiente de atributos funcionales a lo largo del rango ambiental. P. tarapacana es la especie más resistente al estrés hídrico, mientras que P. sericea evade condiciones menos severas de su habitat a través de ajuste osmótico y cambios en la elasticidad de las paredes celulares. Las tasas promedios de asimilación de CO2 fueron mayores en P. australis (9mmol·m-2·s-1) que en P. sericea (5mmol·m-2·s-1) y P. tarapacana (3mmol·m-2·s-1). La tasa promedio de respiración foliar nocturna fue similar para todas las especies (1-2mmol·m-2·s-1). En términos de resistencia a bajas temperaturas, P. sericea muestra ajuste osmótico diario y capacidad moderada de sobreenfriamiento (-9ºC). Las otras dos especies dependen de la tolerancia al congelamiento para soportar las temperaturas bajas más extremas. Los atributos funcionales descritos para las diferentes especies en un amplio rango ambiental pueden explicar algunos aspectos de su éxito en los gradientes latitudinales y altitudinales.

Estudio morfo-anatómico de dos orquídeas de una selva nublada tropical / Morpho-anatomical study of two tropical cloud forest orchilds

Se describe por primera vez la estructura morfo-anatómica y estrategias hídricas de dos orquídeas de las selvas nubladas andinas de Venezuela, Maxillaria miniata y Pleurothallis cardiantha. El estudio se efectuó en una selva nublada poco intervenida y en el bosque secundario del Jardín Botánico de la Universidad de los Andes, Venezuela. Las muestras recolectadas fueron fijadas en FAA y sometidas a métodos tradicionales de anatomía vegetal y de MEB. Se determinaron sus potenciales hídricos foliares durante el año 2004-2005, para determinar si presentan un comportamiento CAM típico, CAM facultativo o C3. Ambas especies exhiben caracteres xeromórficos en sus órganos vegetativos. M. miniata presenta seudobulbos como principales reservorios hídricos, hojas delgadas cartáceas, células epidérmicas de paredes engrosadas, hipodermis adaxial biestratificada y abaxial uniestratificada, clorénquima homogéneo con haces bien desarrollados, abundante esclerénquima, y cristales silíceos asociados a tejidos epidérmicos y esclerenquimáticos. En contraste, P. cardiantha presenta hojas suculentas a coriáceas, epidermis adaxial papilosa, mesófilo heterogéneo, haces pequeños, poco desarrollados e hipodermis acuífera pluriestratificada en ambas caras, que junto con los idioblastos acuíferos del clorénquima constituyen los tejidos de reserva de agua. Los potenciales hídricos registrados en estas especies se mantuvieron elevados y casi invariables durante todo el año, lo cual sugiere un metabolismo CAM en ambos casos.

Freezing tolerance in grasses along an altitudinal gradient in the Venezuelan Andes.

The tropical high Andes experience greater daily temperature oscillations compared to seasonal ones as well as a high frequency of night frost occurrence year round. Survival of organisms, under such environmental conditions, has been determined by selective forces which have evolved into adaptations including avoidance or tolerance to freezing. These adaptations have been studied in different species of trees, shrubs and perennial herbs in páramo ecosystems, while they have not been considered in grasses, an important family of the páramo. In order to understand survival of Poaceae, resistance mechanisms were determined. The study was performed along an altitudinal gradient (2,500-4,200 m a.s.l.) in the páramo. Supercooling capacity and frost injury temperature were determined in nine species in order to establish cold resistance mechanisms. Grasses registered a very low supercooling capacity along the altitudinal gradient, with ice formation between -6 and -3 degrees C. On the other hand, frost injury temperature oscillated between -18 and -7 degrees C. Our results suggest that grasses exhibit freezing tolerance as their main cold resistance mechanism. Since grasses grow at ground level, where greatest heat loss takes place, tolerance may be related to this life form as reported for other small life forms.

Cold resistance mechanisms in high desert Andean plants.

Freezing tolerance and freezing avoidance were studied, during the growing season, in plant species from two different elevations (3200 m and 3700 m) in a desert region of the high Andes (29° 45'S, 69° 59'W) in order to determine whether there was a relationship between plant height and cold resistance mechanisms. Freezing injury and supercooling capacity were determined in plants of different height, from ground-level (<20 cm tall) to tall shrubs (27-90 cm). All ground-level plants showed freezing tolerance as the main mechanism for resistance to freezing temperatures. Tall shrubs avoided freezing temperatures, mainly through supercooling. Supercooling was only present in plants occupying the lower elevation (i.e., 3200 m). Both avoidance and tolerance mechanisms are present in a single genus (i.e., Adesmia).