Pedro Insausti

Escape from water or remain quiescent? Lotus tenuis changes its strategy depending on depth of submergence.

BACKGROUND AND AIMS Two main strategies that allow plants to cope with soil waterlogging or deeper submergence are: (1) escaping by means of upward shoot elongation or (2) remaining quiescent underwater. This study investigates these strategies in Lotus tenuis, a forage legume of increasing importance in areas prone to soil waterlogging, shallow submergence or complete submergence. METHODS Plants of L. tenuis were subjected for 30 d to well-drained (control), waterlogged (water-saturated soil), partially submerged (6 cm water depth) and completely submerged conditions. Plant responses assessed were tissue porosity, shoot number and length, biomass and utilization of water-soluble carbohydrates (WSCs) and starch in the crown. KEY RESULTS Lotus tenuis adjusted its strategy depending on the depth of submergence. Root growth of partially submerged plants ceased and carbon allocation prioritized shoot lengthening (32 cm vs. 24.5 cm under other treatments), without depleting carbohydrate reserves to sustain the faster growth. These plants also developed more shoot and root porosity. In contrast, completely submerged plants became quiescent, with no associated biomass accumulation, new shoot production or shoot elongation. In addition, tissue porosity was not enhanced. The survival of completely submerged plants is attributed to consumption of WSCs and starch reserves from crowns (concentrations 50-75 % less than in other treatments). CONCLUSIONS The forage legume L. tenuis has the flexibility either to escape from partial submergence by elongating its shoot more vigorously to avoid becoming totally submerged or to adopt a non-elongating quiescent strategy when completely immersed that is based on utilizing stored reserves. The possession of these alternative survival strategies helps to explain the success of L. tenuis in environments subjected to unpredictable flooding depths.

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.

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

Abstract 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.

Flooding Effects on Plants Recovering from Defoliation in Paspalum dilatatum and Lotus tenuis

BACKGROUND AND AIMS Flooding and grazing are major disturbances that simultaneously affect plant performance in many humid grassland ecosystems. The effects of flooding on plant recovery from defoliation were studied in two species: the grass Paspalum dilatatum, regrowing primarily from current assimilation; and the legume, Lotus tenuis, which can use crown reserves during regrowth. METHODS Plants of both species were subjected to intense defoliation in combination with 15 d of flooding at 6 cm water depth. Plant recovery was evaluated during a subsequent 30-d growth period under well-watered conditions. Plant responses in tissue porosity, height, tiller or shoot number and biomass of the different organs were assessed. KEY RESULTS Flooding increased porosity in both P. dilatatum and L. tenuis, as expected in flood-tolerant species. In P. dilatatum, defoliation of flooded plants induced a reduction in plant height, thus encouraging the prostrated-growth response typical of defoliated plants rather than the restoration of contact with atmospheric oxygen, and most tillers remained submerged until the end of the flooding period. In contrast, in L. tenuis, plant height was not reduced when defoliated and flooded, a high proportion of shoots being presented emerging above water (72 %). In consequence, flooding plus defoliation did not depress plant recovery from defoliation in the legume species, which showed high sprouting and use of crown biomass during regrowth, whereas in the grass species it negatively affected plant recovery, achieving 32 % lower biomass than plants subjected to flooding or defoliation as single treatments. CONCLUSIONS The interactive effect of flooding and defoliation determines a reduction in the regrowth of P. dilatatum that was not detected in L. tenuis. In the legume, the use of crown reserves seems to be a key factor in plant recovery from defoliation under flooding conditions.

CONSTITUTIVE AND PLASTIC ROOT TRAITS AND THEIR ROLE IN DIFFERENTIAL TOLERANCE TO SOIL FLOODING AMONG COEXISTING SPECIES OF A LOWLAND GRASSLAND

Natural flooding is a major component of the disturbance regime in many grassland ecosystems. The objective of this study was to analyze the relationship among constitutive and plastic root traits and tolerance to flooding in coexisting perennial species of the flooding pampa grasslands (Argentina). A mesocosm experiment was designed for five native species (Paspalidium paludivagum, Paspalum dilatatum, Bothriochloa laguroides, Eryngium ebracteatum, and Eclipta bellidioides) and two exotic ones (Mentha pulegium and Plantago lanceolata). Across species, constitutive root porosity was positively correlated with the tolerance to soil flooding. Moreover, the generation of additional aerenchyma was larger in species with intermediate values of constitutive root porosity and lower in species with low or high constitutive root porosity. This differential increase in the root porosity of each species, combined with the values of constitutive root porosity, resulted in a stronger correlation between final root porosity and tolerance under flooding conditions. Native grasses increased the proportion of root aerenchyma, showing a small change in the number of lysed cells but a significant increase in the cortex proportion and diameter of roots. Exotic dicots generated lysigenous aerenchyma throughout their cortex; in contrast, native dicot species maintained the cell layers adjacent to the stele. A lag in the development of secondary growth during flooding was detected in both groups of dicots, a response that was particularly evident in the exotic species, contrasting with their prominent growth under nonflooded conditions. In general, our results indicate that constitutive and plastic root traits are very significant in terms of the effects of periodic flooding on the abundance of coexisting species of the flooding pampa grasslands.

Flooding induces secondary dormancy in Setaria parviflora seeds.

The effect of flooding on the dormancy level of Setaria parviflora seeds from a non-flooded upland and a seasonally (winter–spring) flooded lowland in the Pampa grasslands of Argentina was investigated. Seeds from both communities were subjected to reciprocal burial treatments in the two habitats, and exhumed during and after the flooding season. Effect of immersion in water at 5°C was compared to incubation of seeds on the surface of water-saturated paper at the same temperature. After exhumation of the buried seeds or immersion treatments, germination was assayed at 25°C and at 20/30°C in the dark or in combination with light. Burial in the lowland, which was flooded in winter–spring, significantly reduced germination, while burial in the non-flooded upland did not reduce germination. Similarly, immersion in water at 5°C significantly reduced germination compared to non-immersed controls. During summer, seeds buried in the lowland showed increased capacity to germinate, particularly when exposed to fluctuating temperatures or light. Thus, flooding induced secondary dormancy in S. parviflora seeds, and it was broken during the non-flooding season. These responses of the seeds would prevent germination until there was no further risk of flooding. Remarkably, in S. parviflora seeds harvested from both habitats, we observed essentially the same germination requirements after flooding. However, some slight differences were detected between the seed populations exhumed from the lowland site, indicating that flooding had larger effects on the seeds from the upland community. This suggests some differentiation of these populations evident only after flooding in the field.

Dormancy and germination responses of kiwifruit (Actinidia deliciosa) seeds to environmental cues

Seed germination of the kiwifruit (Actinidia deliciosa A. Chev. C.F. Liang and A.R. Ferguson), a deciduous, perennial vine, is low because of seed dormancy. The main purpose of this study was to characterize kiwifruit seed dormancy and germination in response to environmental factors such as temperature and light. Dormancy of hydrated seeds is alleviated by the perception of a period at low temperatures (stratification) of at least 3 weeks at 2 or 5°C. Alleviation of dormancy is accomplished by incubation at fluctuating temperatures (20/30°C). A red light pulse did not affect germination, whereas a far-red light pulse strongly inhibited germination. This inhibition was readily reverted by a second pulse of red light, indicating that phytochromes are also involved in dormancy alleviation. Although seed germination was inhibited by the fruit pulp, the latter prevented neither perception of low temperature nor changes in the light sensitivity of the seeds. Therefore, it can be advanced that kiwifruit seeds will only germinate after dispersal if the time–temperature requirement for dormancy alleviation and fluctuating temperatures for dormancy termination are fulfilled. Perception of a closed canopy might interfere with dormancy termination.