Enhanced ethylene production by primary roots of Zea mays L. in response to sub-ambient partial pressures of oxygen

Ethylene production by primary roots of 72–h-old intact seedlings of Zea mays L. cv. LG11 was studied under ambient and sub-ambient oxygen partial pressures (pO₂) using a gas flow-through system linked to a photoacoustic laser detector. Despite precautions to minimize physical perturbation to seedlings while setting-up, ethylene production in air was faster during the first 6h than later, in association with a small temporary swelling of the roots. When roots were switched from air (20–8kPa O₂) to 3 or 5kPa O₂ after 6h, ethylene production increased within 2—3 h. When, the roots were returned to air 16 h later, ethylene production decreased within 2—3 h. The presence of 10kPa CO₂ did not interfere with the effect of 3kPa O₂. Transferring roots from air to 12–5kPa did not change ethylene production, while a reduction to 1 kPa O₂ induced a small increase. The extra ethylene formed in 3 and 5 kPa O₂ was associated with plagiotropism, swelling, root hair production, and after 72 h, increased amounts of intercellular space (aerenchyma) in the root cortex. Root extension was also slowed down, but the pattern of response to oxygen shortage did not always match that of ethylene production. On return to air, subsequent growth patterns became normal within a few hours. In the complete absence of oxygen, no ethylene production was detected, even when anaerobic roots were returned to air after 16 h.     

Increased growth and recruitment of piscivorous perch, Perca fluviatilis, during a transient phase of expanding submerged vegetation in a shallow lake

1. In this study, we examine how a 7‐year period of expanding submerged stonewort (Chara spp.) vegetation during a shift from turbid to clear water in a shallow lake influenced individual growth and population size structure of perch (Perca fluviatilis). We expected that a shift from phytoplankton to macrophyte dominance and clear water would improve feeding conditions for perch during a critical benthivorous ontogenetic stage, and enhance the recruitment of piscivorous perch. 2. Growth analysis based on opercula showed that growth during the second year of life was significantly higher in years with abundant vegetation than in years with turbid water and sparse vegetation. Growth was not affected during the first, third and fourth year of life. Stable isotope analyses on opercula from 2‐year‐old perch showed that the increase in growth coincided with a change in carbon source in the diet. Stable nitrogen ratio did not change, indicating that the increased growth was not an effect of any change in trophic position. 3. Following the expansion of submerged vegetation, perch size range and abundance of piscivorous perch increased in central, unvegetated areas of the lake. In stands of stoneworts, however, mainly benthivorous perch were caught, and size range did not change with time. 4. Our findings provide empirical support for the notion that establishment of submerged vegetation may lead to increased recruitment of piscivorous perch, because of improved competitive conditions for perch during the benthivorous stage. This is likely to constitute a benthic‐pelagic feedback coupling, in which submerged vegetation and clear water promote the recruitment of piscivorous perch, which, in turn, may increase water clarity through top‐down effects in the pelagic.     

Growth and Survival Responses of Rumex Species to Flooded and Submerged Conditions: The Importance of Shoot Elongation, Underwater Photosynthesis and Reserve Carbohydrates

Plants of Rumex thyrsiflorus Fingerh., R. crispus L. and R.maritimus L., which are zoned along a gradient of elevationin a river foreland ecosystem, and differ in their flood-tolerance,were subjected to different flooding levels. Under conditionsof soil flooding, the growth rates of the flood-tolerant R.crispus and R. maritimus were as high as under drained conditions,but that of the flood-intolerant R. thyrsiflorus was halved.Upon submergence, the low elevation species R. maritimus showedrapid shoot elongation; when elongation resulted in a protrusionof leaves above the water surface, the plants survived. Alternatively,underwater photosynthesis also led to a 100% survival of submergedR. maritimus plants, provided that enough inorganic carbon wasmade available in the water. This could be attributed in partto the use of photosynthetically-derived oxygen for root respiration;in a hydroculture experiment, with 5.0 mM CO₂ in the water inthe shoot environment, photosynthetically-derived oxygen contributedmore than 50% to root oxygen consumption at low oxygen concentrationsin the root environment. The intermediately elevated species R. crispus appeared to bemuch more tolerant towards conditions of prolonged total submergence:older plants survived eight weeks submergence in the dark. Thisresponse was explicable in terms of a dormancy-strategy, whichis characterized by a slow consumption of carbohydrates storedin the tap-root. The differential responses of R. maritimusand R. crispus to total submergence reveal the limitations offlood-tolerance and reflect the different life-histories ofthe species. Key words: Photosynthesis, Rumex, submergence, carbohydrates, growth rate, shoot elongation     

Growth responses of Rumex species in relation to submergence and ethylene

Abstract. Submergence stimulates growth of the petioles of Rumex palustris and Rumex crispus under field, greenhouse and laboratory conditions. Growth of Rumex acetosa petioles was hardly influenced by submergence. These growth responses under flooded conditions can be partially mimicked by exposing non-submerged Rumex plants to ethylene-air mixtures. Submergence of intact plants in a solution of AgNO₃ inhibited the elongation of all petioles of R. palustris and the youngest petiole of R. crispus and stimulated growth of the youngest petiole of R. acetosa , The ethylene-air mixture experiments, the effect of AgNO₃ and observed increase of the endogenous ethylene concentration during submergence suggest that ethylene plays a regulatory role in the growth responses of these Rumex species under submerged conditions. The three Rumex species showed a gradient in elongation responses to submergence, which correlates with the field distribution of the three species in a flooding gradient.     

Thick Root Syndrome in Cucumber (Cucumis sativus L.): A Description of the Phenomenon and an Investigation of the Role of Ethylene

In commercial growth of horticultural plants in greenhouses,high financial losses are being suffered due to the so-calledthick root syndrome (TRS), a phenomenon characterized by severedeterioration of the root system. The early symptoms of TRSin cucumber (Cucumis sativus L.) are strongly curving rootsthat are swollen and superficially damaged. Research focusedon ethylene, as both the culture practice of cucumber and theknown effects of ethylene on root growth point to a possiblerole for this phytohormone in TRS. Ethylene induced root curvatureand swelling as well as damage of the epidermal layer and outercortex of roots of cucumber plants similar to TRS symptoms.Formation of root hairs was stimulated and root elongation wasalso severely inhibited by exogenous ethylene. However, basedon experiments with the ethylene inhibitor     

Vegetative reproduction by species with different adaptations to shallow-flooded habitats

In shallow flooded parts of rich fens Mentha aquatica might thrive in deeper water than Epilobium hirsutum but previous experiments have provided no clear indication that the flooding tolerance of these species differs. In this study we investigated, by measuring growth, biomass allocation and vegetative reproduction, whether the impact of water level on vegetative reproduction might produce different lower boundaries on water level gradients. There was a striking contrast between biomass production at high water levels and the field distribution of both species. After 18 wk, the mean biomass of E. hirsutum grown in waterlogged and flooded conditions was 82% and 54%, respectively, of the mean biomass production of drained plants. Biomass of waterlogged and flooded M. aquatica was reduced to 57% and 37% in drained conditions. Waterlogged and flooded E. hirsutum had swollen stem bases and invested a high proportion of biomass in adventitious roots. Stems of M. aquatica did not swell, formed few adventitious roots and maintained an equal proportion of below-ground roots at all water levels. The effect of water level on vegetative reproduction corresponded well with the lower hydrological boundaries of both species. When waterlogged and flooded, most rhizomes of E. hirsutum emerged from above-ground parts of the stem base and were oriented in an upward direction. Plants in flooded soil allocated less biomass to rhizomes and also reduced the number and size of rhizomes. Rhizome formation of M. aquatica on the other hand was not directly affected by water level and only depended on plant size. These differences in vegetative reproduction are discussed in relation to the different abilities of both species to oxygenate their below-ground roots. It was concluded that the mode of adaptation to soil flooding might also affect vegetative reproduction and, therefore, a species' ability for long-term persistence in soil-flooded habitats.     

Flooding tolerance of Carex species in relation to field distribution and aerenchyma formation

The flooding tolerance of Carex species was studied in relation to their field distribution and their capacity to form root aerenchyma under controlled conditions. In an alpine meadow, six Carex species were selected which were distributed in a clear zonation correlating with water content of the soil. Carex sempervirens and C. ferruginea were only found on nonflooded soil, the latter species preferring moister conditions. Carex davalliana and C. nigra were both associated with water-saturated soil, whereas C. limosa and C. rostrata preferred partially submerged conditions. Carex davalliana and C. limosa were bound to flooded soils with a relatively high redox potential and horizontally flowing groundwater. Carex rostrata and C. nigra grew in stagnant soil-flooded conditions with low soil redox potentials. The amount of aerenchyma in the roots of all species increased when grown in oxygen-deficient stagnant agar. This increase in root porosity, combined with increased root diameter, presumably improved internal aeration of the roots. Although all species survived experimental soil flooding, partial submergence was lethal to C. sempervirens and, surprisingly, also to the wetland species C. davalliana. Carex ferruginea showed a reduced growth rate during partial submergence. The three other species, all wetland plants, reached highest biomass production under soil-flooded and partially submerged conditions, with slower growth on free-draining soil. It is concluded that aerenchyma is not constitutive in the Carex species under study, and is best developed in Carex species from wetlands. Species with less aerenchyma perform poorly when soil-flooded, but conditions of partial submergence could even affect species with a considerable amount of root aerenchyma.     

Sensitivity to ethylene: the key factor in submergence-induced shoot elongation of Rumex

Rosettes of flooding-resistant Rumex palustris plants show a submergence-induced stimulation of elongation, which is confined to the petioles of young leaves. This response increases the probability of survival. It is induced by ethylene that accumulates in submerged tissues. Flooding-intolerant Rumex acetosella plants do not show this response. We investigated whether differences in shoot elongation between the species, between old and young leaves and between the petiole and leaf blade of a R. palustris plant result from differences in internal ethylene concentration or in sensitivity to the gas. Concentrations of free and conjugated ACC in petioles and leaf blades of R. palustris indicated that ethylene is synthesized throughout the submerged shoot, although production rates varied locally. Nevertheless, no differences in ethylene concentration were found between submerged leaves of various ages. In contrast, dose-response curves showed that only elongation of young petioles of R. palustris was sensitive to ethylene. In R. acetosella, elongation of all leaves was insensitive to ethylene. We conclude that variation in ethylene sensitivity rather than content explains the differences in submergence-induced shoot elongation between the two Rumex species and between leaves of R. palustris.     

The fragmented Atlantic rain forest of Brazil: size, shape and distribution of forest fragments

The geographical characteristics of a total of 1839 forest fragments surrounded by sugar cane fields were studied in the Brazilian Atlantic rain forest region of the northeastern state of Pernambuco. The size and shape of the fragments as well as inter-fragment distances and the effects of varying edge width were examined using a geographical information system. The analyses show that the fragments are relatively small and close to each other. Approximately 48% of the rain forest fragments are <10 hectares, while only about 7% are >100 hectares. Forest fragments are close to each other, as fragments located 50m or less apart formed groups that included ca. 50% of the total forest area. At 350m inter-fragment distance, 98% of the rain forest area was included in groups of fragments. Due to the small size and irregular shape of the fragments, the total area of edge zone exceeds that of the interior habitat when the edge width is ca. 60m. At an edge width of 300m ca. 94% of the total fragment area is edge zone. For conservation purposes, ways of establishing networks of forest fragments connected by corridors and stepping stone fragments are demonstrated using GIS. Simulations using these techniques show that reforestation of sugar cane fields between the forest fragments would considerably increase the area of interior forest habitat and connectivity between fragments.