Surviving floods: leaf gas films improve O2 and CO2 exchange, root aeration, and growth of completely submerged rice

When completely submerged, the leaves of some species retain a surface gas film. Leaf gas films on submerged plants have recently been termed 'plant plastrons', analogous with the plastrons of aquatic insects. In aquatic insects, surface gas layers (i.e. plastrons) enlarge the gas–water interface to promote O₂ uptake when under water; however, the function of leaf gas films has rarely been considered. The present study demonstrates that gas films on leaves of completely submerged rice facilitate entry of O₂ from floodwaters when in darkness and CO₂ entry when in light. O₂ microprofiles showed that the improved gas exchange was not caused by differences in diffusive boundary layers adjacent to submerged leaves with or without gas films; instead, reduced resistance to gas exchange was probably due to the enlarged water–gas interface (cf. aquatic insects). When gas films were removed artificially, underwater net photosynthesis declined to only 20% of the rate with gas films present, such that, after 7 days of complete submergence, tissue sugar levels declined, and both shoot and root growth were reduced. Internal aeration of roots in anoxic medium, when shoots were in aerobic floodwater in darkness or when in light, was improved considerably when leaf gas films were present. Thus, leaf gas films contribute to the submergence tolerance of rice, in addition to those traits already recognized, such as the shoot-elongation response, aerenchyma and metabolic adjustments to O₂ deficiency and oxidative stress.     

Atmospheric nitrogenous compounds and ozone – is NOx fixation by plants a possible solution?

Air quality thresholds for O₃ for the protection of human health and vegetation set by the European Union (EU) have been exceeded in Europe regularly in the 1990s. Because target reductions for oxides of nitrogen (NO_x) set for the year 2000 are unlikely to be achieved, these O₃ exceedances are likely to continue into the next millenium. Improvements of plant tolerance towards O₃ are being investigated but very little work has been done to explore NO_x tolerance and plant acclimation to NO₂ and NO. However, it is clear that within the populations of some plant species there is wide variation, and some individuals can fix NO_x and use the nitrogen directly from the atmosphere, rather than rely upon, for example, root uptake of nitrate. It is possible that individuals capable of fixing NO_x could be selected for a range of species, and genotypes with high rates of uptake could be of value as crops or for forestation in polluted areas (e.g. landscaping in the vicinity of motorways) to reduce tropospheric concentrations of NO_x significantly and also to decrease the potential for O₃ production.     

Sensing soil oxygen

Abstract. Under natural conditions where gaseous exchange between soil and atmosphere is restricted by excess water, the concentration of O₂ in the rooting zone can become very low while reduced ions and organic compounds that are potentially phytoxic may accumulate. Mechanisms by which shoots and roots detect, and adjust to, this O₂-deficient environment are reviewed. Injury to roots and their inability to function because of insufficient O₂ is communicated to the shoot in a variety of ways, so that it adjusts physiologically. Roots may acclimate metabolically to a gradual fall in O₂ supply, so that they either improve their tolerance of anoxia, or partially avoid O₂-deficiency by structural changes that aid internal transfer of O₂ to the roots from the shoot. Molecular mechanisms regulating such metabolic changes, including environmental cues, are discussed.     

Routine metabolism of juvenile spot, Leiostomus xanthums (Lacépède), as a function of temperature, salinity and weight

Routine oxygen consumption rates of juvenile spot, Leiostomus xanthums , were measured over a range of temperatures, salinities and fish weights. As predicted, Q O₂ increased with temperature and decreased with body weight. However, Q O₂ decreased with decreasing salinity and did not show the expected minimum at isosmotic concentrations. The data are best described by the relationship: log₁₀ Q O₂ (mg O₂ g⁻¹ h⁻¹) = 0.129 log_lo salinity (%0) + 1.604 log₁₀ temperature (°C)-0.1401og₁₀(g)-2.767.     

Differential response of four tree species to ozone-induced acceleration of foliar senescence

Ozone (O₃)-induced accelerated senescence of leaves was measured in four tree species: black cherry ( Prunus serotina ), hybrid poplar ( Populus maximowizii x trichocarpa , clone 245), northern red oak ( Quercus rubra ) and sugar maple ( Acer saccharum ). Seedlings or ramets of the four species were subjected to chronic O₃ exposures and designated leaves harvested periodically from emergence to senescence. Gas exchange was analysed, and concentrations of total soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase were measured as indicators of leaf senescence. Total antioxidant potential and ascorbate peroxidase and glutathione reductase activities also were determined. Black cherry and hybrid poplar exhibited O₃-induced accelerated leaf senescence, whereas sugar maple and northern red oak did not. When the O₃ effects were related to cumulative uptake of the gas, black cherry was the most sensitive of the four species. Although hybrid poplar exhibited similar symptoms of O₃-induced accelerated senescence after the same exposure period as did black cherry, this species took up much greater quantities of O₃ to achieve the same response. The O₃-induced increase in glutathione reductase activity in hybrid poplar was consistent with the capacity of this species to take up high concentrations of the gas. Relative tolerance of northern red oak and sugar maple could be explained only in part by lower cumulative O₃ uptake and lower rate of uptake. Sugar maple had the highest antioxidant potential of all four species, which may have contributed to O₃ tolerance of this species. Ascorbate peroxidase activity, when expressed on a fresh weight basis, could not account for differential sensitivity among the four species.     

Oxygen uptake in coprophilous beetles (Aphodius, Geotrupes, Sphaeridium) at low oxygen and high carbon dioxide concentrations

Abstract. The rate of O₂ consumption was measured in five coprophilous beetle species (common in Denmark) at O₂ concentrations from 1–21%. With the exception of the mainly soil-living Geotrupes spiniger (Marsham) (Geotrupidae), these beetles are probably exposed to severe hypoxia in fresh cattle pats. Aphodius fossor (Linnaeus), A. contaminatus (Herbst) (Aphodiidae) and Sphaeridium lunatum Fabricius (Hydrophilidae) maintained normal movements and a normal rate of 0₂ uptake (for at least 30 min) at only 1% O₂. There is no evidence, therefore, that the beetles switch to anaerobic metabolism under these conditions. This ability to regulate respiration, and hence to extract 0₂ at very low concentrations, is exceptional even among terrestrial arthropods living in soil or other potentially hypoxic substrates. In A. rufipes (Linnaeus), respiration declined at ambient concentrations below 2% O₂, and in G. spiniger the ability to regulate respiration seemed to fail at even higher concentrations. In four of the species (G. spiniger was not tested), about 11% CO₂ (the level in a dung pat at 2% O₂) did not affect the O₂ uptake at 2% O₂.     

Effects of elevated oxygen tensions on acetylene reduction in Alnus incana-Frankia symbioses

An open flow-through gas system was used to determine the effect of C₂H₂ and elevated O₂ on acetylene reduction activity (ARA) and respiration of the intact, potted root system of Alnus incana (L.) Moench in symbiosis with Frankia Avcll or with a local source of Frankia . Both symbiotic systems responded to C₂H₂ by an immediate plateau range in ARA. The Plateau in ARA was in some cases followed by a decline of less extent than reported for many legumes. A concurrent decline in net respiration of the root system was on average 8% of the CO₂ efflux prior to C₂H₂ introduction.
Respiration of the root systems in both symbioses responded to elevated oxygen levels in the 10 kPa C₂H₂ atmosphere by an increase of up to 17% of the net respiration prior to C₂H₂ introduction in 21 kPa O₂. In contrast, the elevated oxygen levels resulted in an immediate drop in ARA followed by a minor increase to a stable level lower than that at the preceding, lower oxygen tension. The symbiosis with the local Frankia had lost all ARA when the partial pressure of O₂ exceeded 50 kPa, whereas the symbiosis with Avcll still had some activity at 80 kPa O₂. This difference in tolerance of elevated O₂ clearly shows that the oxygen exclusion mechanisms may be controlled by the microsymbiont in Alnus-Frankia symbioses. The symbiotic systems recovered ARA to a similar extent when returned from elevated O₂ levels to 21 kPa O₂.     

Species-specific responses to atmospheric carbon dioxide and tropospheric ozone mediate changes in soil carbon

We repeatedly sampled the surface mineral soil (0–20 cm depth) in three northern temperate forest communities over an 11-year experimental fumigation to understand the effects of elevated carbon dioxide (CO₂) and/or elevated phyto-toxic ozone (O₃) on soil carbon (C). After 11 years, there was no significant main effect of CO₂ or O₃ on soil C. However, within the community containing only aspen ( Populus tremuloides Michx.), elevated CO₂ caused a significant decrease in soil C content. Together with the observations of increased litter inputs, this result strongly suggests accelerated decomposition under elevated CO_2. In addition, an initial reduction in the formation of new (fumigation-derived) soil C by O₃ under elevated CO₂ proved to be only a temporary effect, mirroring trends in fine root biomass. Our results contradict predictions of increased soil C under elevated CO₂ and decreased soil C under elevated O₃ and should be considered in models simulating the effects of Earth's altered atmosphere.     

Soil oxygen flux measured polarographically in an Alaskan tussock tundra

Soil oxygen flux was measured polarographically in undisturbed tussock tundra and in an old vehicle track at Eagle Creek, Alaska. Because some polarograms did not show effective voltage plateaus, the polarographic method was tested for use in the tundra by experimental manipulation of soil water content and microbial oxygen demand. The response of soil oxygen flux was as predicted for artificially waterlogged systems, and the response to increased microbial demand indicated relatively high oxygen availability. Soil oxygen flux ranged from 25.9 ng O₂ cm⁻² min⁻¹ in Sphagnum mats to 3.9 ng O₂ cm⁻² min⁻¹ in unvegetated ruts of the vehicle track. Most sites in unvegetated ruts did not have measurable soil oxygen flux. In contrast, oxygen flux beneath Eriophorum vaginatum tussocks growing in the vehicle track averaged 10.7 ng O₂ cm⁻² min⁻¹.     

Influence of soil O2 and CO2 on root respiration for Agave deserti

Respiration measured as CO₂ efflux was determined at various soil O₂ and CO₂ concentrations for individual, attached roots of a succulent perennial from the Sonoran Desert, Agave deserti Engelm. The respiration rate increased with increasing O₂ concentration up to about 16% O₂ for established roots and 5% O₂ for rain roots (fine branch roots on established roots induced by wetting of the soil) and then remained fairly constant up to 21% O₂. When O₂ was decreased from 21 to 0%, the respiration rates were similar to those obtained with increasing O₂ concentration. The CO₂ concentration in the root zone, which for the shallow-rooted A. deserti in the field was about 1 000 μl l^-1, did not affect root respiration at concentrations up to 2 000 μl l^-1, but higher concentrations reduced it, respiration being abolished at 20 000 μl l^-1 (2%) CO₂ for both established and rain roots. Upon lowering CO₂ to 1 000 μl l^-1 after exposure to concentrations up to 10000 μl l^-1 CO₂, inhibition of respiration was reversible. Uptake of the vital stain neutral red by root cortical cells was reduced to zero, indicating cell death, in about 4 h at 2% CO₂, substantiating the detrimental effects of high soil CO₂ concentrations on roots of A. deserti . This CO₂ response may explain why roots of desert succulents tend to occur in porous, well-aerated soils.     

Why do fast- and slow-growing grass species differ so little in their rate of root respiration,considering the large differences in rate of growth and ion uptake?

Herbaceous plants grown with free access to nutrients exhibit inherent differences in maximum relative growth rate (RGR) and rate of nutrient uptake. Measured rates of root respiration are higher in fast-growing species than in slow-growing ones. Fast-growing herbaceous species, however, exhibit lower rates of respiration than would be expected from their high rates of growth and nitrate uptake. We investigated why the difference in root O₂ uptake between fast- and slow-growing species is relatively small. Inhibition of respiration by the build-up of CO₂ in closed cuvettes, diurnal variation in respiration rates or an increasing ratio of respiratory CO₂ release to O₂ uptake (RQ) with increasing RGR failed to explain the relatively low root respiration rates in fast-growing grasses. Furthermore, differences in alternative pathway activity can at most only partly explain why the difference in root respiration between fast- and slow-growing grasses is relatively small. Although specific respiratory costs for maintenance of biomass are slightly higher in the fast-growing Dactylis glomerata L. than those in the slow-growing Festuca ovina L., they account for 50% of total root respiration in both species. The specific respiratory costs for ion uptake in the fast-growing grass are one-third of those in the slow-growing grass [0·41 versus 1·22 mol O₂ mol (NO₃^–)^–1]. We conclude that this is the major cause of the relatively low rates of root respiration in fast-growing grasses.     

Kinetics of leaf oxygen uptake represent in planta activities of respiratory electron transport and terminal oxidases

We present, for the first time, the oxygen response kinetics of mitochondrial respiration measured in intact leaves (sunflower and aspen). Low O₂ concentrations in N₂ (9–1500 ppm) were preset in a flow-through gas exchange measurement system, and the decrease in O₂ concentration and the increase in CO₂ concentration as result of leaf respiration were measured by a zirconium cell O₂ analyser and infrared-absorption CO₂ analyser, respectively. The low O₂ concentrations little influenced the rate of CO₂ evolution during the 60-s exposure. The initial slope of the O₂ uptake curve on the dissolved O₂ concentration basis was relatively constant in leaves of a single species, 1.5 mm s⁻¹ in sunflower and 1.8 mm s⁻¹ in aspen. The apparent K _0.5(O₂) values ranged from 0.33 to 0.67 μ M in sunflower and from 0.33 to 1.1 μ M in aspen, mainly because of the variation of the maximum rate, V _max (leaf temperature 22°C). The initial slope of the O₂ response of respiration characterizes the catalytic efficiency of terminal oxidases, an important parameter of the respiratory machinery in leaves. The plateau of the response characterizes the activity of the mitochondrial electron transport chain and is subject to regulations in accordance with the necessity for ATP production. The relatively low oxygen conductivity of terminal oxidases means that in leaves, less than 10% of the photosynthetic oxygen can be reassimilated by mitochondria.     

Oxygen consumption in Oreochromis niloticus (L.) in relation to development, salinity, temperature and time of day

Oxygen consumption of Oreochromis niloticus at different stages of development was studied in relation to salinity, temperature and time of day, using a Warburg apparatus. The oxygen consumption of newly hatched (0–14 h) larvae was 3.40 μl O₂ larva⁻¹ h⁻¹, of older yolk sac larvae 10.09 μl O₂ larva⁻¹ h⁻¹, and of one-month-old fry 32.99 μl O₂ larva⁻¹ h⁻¹. The QO₂ values showed a decrease with development and growth, ranging from 21.2–26.0 μl O₂ mg⁻¹ h⁻¹ in newly hatched larvae to 2.97 μl mg⁻¹ h⁻¹ in one-month-old fry. Changes in oxygen consumption occurred with salinity, the highest being at 17%o. Active larvae (12-24 mm T.L.) showed a doubling of consumption with a 10° C rise in temperature, and their Q₁₀ factor increased from 2.25 to 3.43 with increasing size. Day-old yolk-sac larvae, late yolk-sac larvae (5 days old) and fry of 12 14 mm length all showed a depression in oxygen consumption at midnight followed by a dawn rise.     

Nitric oxide modulates ozone-induced cell death, hormone biosynthesis and gene expression in Arabidopsis thaliana

Nitric oxide (NO) is involved together with reactive oxygen species (ROS) in the activation of various stress responses in plants. We have used ozone (O₃) as a tool to elicit ROS-activated stress responses, and to activate cell death in plant leaves. Here, we have investigated the roles and interactions of ROS and NO in the induction and regulation of O₃-induced cell death. Treatment with O₃ induced a rapid accumulation of NO, which started from guard cells, spread to adjacent epidermal cells and eventually moved to mesophyll cells. During the later time points, NO production coincided with the formation of hypersensitive response (HR)-like lesions. The NO donor sodium nitroprusside (SNP) and O₃ individually induced a large set of defence-related genes; however, in a combined treatment SNP attenuated the O₃ induction of salicylic acid (SA) biosynthesis and other defence-related genes. Consistent with this, SNP treatment also decreased O₃-induced SA accumulation. The O₃-sensitive mutant rcd1 was found to be an NO overproducer; in contrast, Atnoa1/rif1 ( Arabidopsis nitric oxide associated 1/resistant to inhibition by FSM1 ), a mutant with decreased production of NO, was also O₃ sensitive. This, together with experiments combining O₃ and the NO donor SNP suggested that NO can modify signalling, hormone biosynthesis and gene expression in plants during O₃ exposure, and that a functional NO production is needed for a proper O₃ response. In summary, NO is an important signalling molecule in the response to O₃.     

Amphipod gigantism dictated by oxygen availability?

A recent study of gigantism in amphipods proposes a novel mechanism which could be used to explain Bergmann's rule, namely differences in environmental O₂ concentration. Consideration of the respiratory biology of this group indicates significant flaws in this proposal. Nonetheless, it is not unreasonable to predict a relationship between ectotherm body size and O₂ partial pressure ( p O₂) in some systems, where the partial pressure gradient is steep, either in time (comparison of Carboniferous and Permian atmospheres) or space (animals inhabiting oceanic O₂ minimum layers, and, particularly in freshwaters, living at high altitude). Animal gigantism can be correlated with periods of very high environmental p O₂ over geological time. Unfortunately, data for present day spatial patterns are scant.     

Can genotypes of soybean (Glycine max) selected for nitrate tolerance provide good "models" for studying the mechanism of nitrate inhibition of nitrogenase activity?

In soybeans ( Glycine max L. Merr.), high levels of soil nitrate inhibit N₂ fixation, and nitrate-tolerant symbioses have been identified within a chemically mutagenized line of cv. Bragg denoted nts382 and within the line K466, a genotype representative of a number of Korean soybean cultivars. The genotypes nts382 and K466 were examined to see if they could be used as a model system for studying the mechanism responsible for the short-term (i.e. 3-day) inhibition of specific nitrogenase activity, especially the mechanism behind the greater O₂ limitation of nodule metabolism that is characteristic of nitrate inhibition of N₂ fixation in soybean. In nts382, total nitrogenase activity (TNA = H₂ production in Ar:O₂) was inhibited to a lesser degree (48% of control) relative to Bragg (30% of control), and the nitrate-treated symbioses showed less of an O₂ limitation of nodule metabolism in nts382 than in Bragg. However, the relative proportion of O₂ limitation to the total nitrate inhibition was similar (40 and 41%) in nts382 and Bragg, respectively. Therefore, the nts382 symbioses may be useful in elucidating the general mechanism for down-regulation of nitrogenase activity in soybean, but would not be a useful model system for studying the control of O₂-limited metabolism following nitrate exposure. The effects of nitrate on TNA and on the degree of O₂ limitation of nodule metabolism were the same in K466 and a reference cultivar Maple Arrow. Consequently, the tolerance of K466 to nitrate reported previously was attributed to the ability of this symbiosis to maintain nodule biomass in the presence of nitrate, not to any ability to maintain specific nitrogenase activity in the presence of nitrate.     

Can genotypes of soybean (Glycine max) selected for nitrate tolerance provide good “models” for studying the mechanism of nitrate inhibition of nitrogenase activity?

In soybeans ( Glycine max L. Merr.), high levels of soil nitrate inhibit N₂ fixation, and nitrate-tolerant symbioses have been identified within a chemically mutagenized line of cv. Bragg denoted nts382 and within the line K466, a genotype representative of a number of Korean soybean cultivars. The genotypes nts382 and K466 were examined to see if they could be used as a model system for studying the mechanism responsible for the short-term (i.e. 3-day) inhibition of specific nitrogenase activity, especially the mechanism behind the greater O₂ limitation of nodule metabolism that is characteristic of nitrate inhibition of N₂ fixation in soybean. In nts382, total nitrogenase activity (TNA = H₂ production in Ar:O₂) was inhibited to a lesser degree (48% of control) relative to Bragg (30% of control), and the nitrate-treated symbioses showed less of an O₂ limitation of nodule metabolism in nts382 than in Bragg. However, the relative proportion of O₂ limitation to the total nitrate inhibition was similar (40 and 41%) in nts382 and Bragg, respectively. Therefore, the nts382 symbioses may be useful in elucidating the general mechanism for down-regulation of nitrogenase activity in soybean, but would not be a useful model system for studying the control of O₂-limited metabolism following nitrate exposure. The effects of nitrate on TNA and on the degree of O₂ limitation of nodule metabolism were the same in K466 and a reference cultivar Maple Arrow. Consequently, the tolerance of K466 to nitrate reported previously was attributed to the ability of this symbiosis to maintain nodule biomass in the presence of nitrate, not to any ability to maintain specific nitrogenase activity in the presence of nitrate.     

A comparative study of the effects of NaCl salinity on respiration, photosynthesis, and leaf extension growth in Beta vulgaris L. (sugar beet)

Abstract. Three parameters influencing the capacity for carbon accumulation, i.e. photosynthesis, respiration, and leaf extension growth, were studied in Beta vulgaris L. (sugar beet) cultured in nutrient solution containing 0.5 to 500 mol m⁻³ NaCl. Leaf extension growth was the parameter most sensitive to salinity: the initial rate of leaf extension and final leaf length each declined linearly with increase in external NaCl concentration. Photosynthetic O₂ evolution of thin leaf slices did not decline until salinity levels reached 350 to 500 mol m⁻³ NaCl, while respiratory O₂ consumption was not affected by salinity throughout the range. The results suggest that the influence of salinity on the capacity for carbon accumulation in B. vulgaris occurs primarily through reduction in the area of photosynthetic surface.     

Photosynthesis in aquatic adventitious roots of the halophytic stem-succulent Tecticornia pergranulata (formerly Halosarcia pergranulata)

In flood-tolerant species, a common response to inundation is growth of adventitious roots into the water column. The capacity for these roots to become photosynthetically active has received scant attention. The experiments presented here show the aquatic adventitious roots of the flood-tolerant, halophytic stem-succulent, Tecticornia pergranulata (subfamily Salicornioideae, Chenopodiaceae) are photosynthetic and quantify for the first time the photosynthetic capacity of aquatic roots for a terrestrial species. Fluorescence microscopy was used to determine the presence of chloroplasts within cells of aquatic roots. Net O₂ production by excised aquatic roots, when underwater, was measured with varying light and CO₂ regimes; the apparent maximum capacity ( P _max) for underwater net photosynthesis in aquatic roots was 0.45  µ mol O₂ m⁻² s⁻¹. The photosynthetic potential of these roots was supported by the immunolocalization of PsbA, the major protein of photosystem II, and ribulose-1-5-bisphosphate carboxylase/oxygenase (Rubisco) in root protein extracts. Chlorophyllous aquatic roots of T. pergranulata are photosynthetically active, and such activity is a previously unrecognized source of O₂, and potentially carbohydrates, in flooded and submerged plants.     

Protective mechanisms of nitrogenase against oxygen excess and partially-reduced oxygen intermediates

Diazotrophic systems have developed a number of strategies to protect nitrogenase (N₂ase; EC 1.18.6.1) from O₂ excess and active-oxygen species (AOS). Protection against O₂ excess is given by biochemical modifications of N₂ase, increased rates of low-efficiency respiration, temporal segregation of N₂ fixation and photosynthesis, physical barriers to O₂ diffusion, and hemoglobins. On the other hand, AOS may originate from oxidation of N₂ase components, ferredoxins, flavodoxins and hemoglobins; interaction among the AOS themselves, or between H₂O₂ and hemoglobins; and during reactions catalyzed by hydrogenase (EC 1.18.99.1), xanthine oxidase (EC 1.1.3.22) and uricase (EC 1.7.3.3). Active-oxygen species are scavenged enzymatically [superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6). peroxidase (EC 1.11.1.7), ascorbate peroxidase (EC 1.11.1.11)] or through non-enzymic reaction with low-molecular-weight compounds (ascorbate, α-tocopherol, glutathione).