Radial oxygen loss and physical barriers in relation to root tissue age in species with different types of aerenchyma.

Plant root aeration relies on aerenchyma and barrier formation in outer cortex influencing the radial oxygen loss (ROL) from roots towards the rhizosphere. Plant species display large variation in strategies for both responses. We investigated the impacts of root-zone hypoxia on aerenchyma formation and development of ROL apoplastic barriers in the outer cortex as a function of root tissue age using three lowland grassland species, each with alternative aerenchyma structure. All species increased root aerenchyma and continued with root elongation after imposing hypoxia. However, ROL barrier development differed: (i) Rumex crispus L. displayed only 'partial' barrier to ROL evidenced at older tissue ages, (ii) Cyperus eragrostis Lam. initiated a 'tighter' barrier to ROL following exposure to hypoxia in tissues older than 3 days, and (iii) Paspalidium geminatum (Forssk.) Stapf demonstrated highly effective inhibition of ROL under aerated and hypoxic conditions at all tissue ages related to constitutive 'tight' apoplastic barriers in outer cortex. Thus, hypoxic conditions affected root elongation and 'tightness' of apoplastic barriers depending on species. The physiological implications of the different ROL responses among species in relation to the differential formation of barriers are discussed.

Facilitative effect of Lotustenuis on Paspalumdilatatum in a lowland grassland of Argentina.

We studied the responses in growth and N content of the perennial grass Paspalum dilatatum to the substitution of Lotus tenuis for a whole group of species, the dicotyledons of a natural grassland community, in the Salado lowland Pampas of Argentina. Two kinds of manipulations were performed in the field: removal of alien dicots with herbicide application, and introduction of L. tenuis, resulting in a combination of four treatments, arranged in a 2 × 2 factorial randomized block design. Leaf area per tiller of P. dilatatum was higher when it was growing near L. tenuis; this increase was the result of a greater leaf elongation rate and slower leaf senescence. In the vicinity of L. tenuis, P. dilatatum exhibited an increase in tiller production and a decrease in tiller death. More tillers were functional at the end of the growing season and their aboveground biomass was 5␣times higher than for plants growing in plots where the community dicots were removed. This increment was accompanied by a higher N content. Growth enhancement of P. dilatatum plants when L. tenuis was the␣immediate neighbour is interpreted as the result of facilitation mediated by higher N availability, and not as a consequence of a release from competition exerted by the community dicots. Competition and facilitation did not interact to produce an increase in the vegetative output of Paspalum dilatatum plants growing under these field conditions. It is on these grounds that Lotus tenuis might be considered as a keystone species in the managed grassland.

Effects of flood-influenced factors on seed germination of Ambrosia tenuifolia.

Strong fluctuations are exhibited by populations of the perennial herb Ambrosia tenuifolia in the grasslands of the Salado basin (Province of Buenos Aires, Argentina), an area frequently enduring prolonged floods. Flooding causes the death of most dicotyledon plants of the community, A. tenuifolia among them, opening numerous gaps of various sizes. After the recession of the flood the density of A. tenuifolia seedlings was higher in flooded than in non-flooded plots and it was larger in wider gaps. Canopy removal in non-flooded plots increased field seedling emergence of A. tenuifolia up to the levels found in flooded plots. Responses of the seeds in the soil to gap-associated environmental factors such as light quality and temperature regime were studied both in the field and under controlled were studied both in the field and under controlled conditions. Seedling emergence was significantly enhanced when the red:far-red ratio of natural light reaching the soil surface under the canopy of nonflooded plots was increased by means of copper sulfate filters. The influence of light quality and temperature on germination of the soil seed population was also tested using grassland soil monoliths or mesocosms, transported from the field to the laboratory, in which the canopy was clipped and the soil exposed to either red or far-red light and kept at constant or fluctuating temperatures. Significant seedling emergence was observed only when the soil samples were exposed to red light and incubated at alternating temperatures. No emergence was recorded in samples exposed to far-red light or incubated at a constant 25°C. Seeds stored dry in the laboratory were also stimulated to germinate by red light and alternating temperatures but only after dormancy was sufficiently decreased by low temperature stratification or by low temperature under immersion. The results are consistent with the hypothesis that primary dormancy of A. tenuifolia seeds is decreased by low temperatures in winter even if the seeds are submerged as happens when floods occur. The decrease in dormancy makes the seeds prone to be stimulated to germinate by the Pfr form of phytochrome in combination with alternating temperatures. These conditions are likely to be met in the gaps opened by the flood-caused death of dicotyledon plants.