Complete Submergence Research Articles

Survival tactics of Ranunculus species in river floodplains

The flooding resistance of four Ranunculus species was studied under controlled conditions and related to the tactics used by these species to survive in their natural habitat in river floodplains. R. bulbosus, a species from seldom-flooded river levées, was relatively intolerant of both waterlogging and complete submergence, due to a constitutively low level of aerenchyma in the root system. This lack of gas spaces resulted in high mortality rates during flooding treatments and an inability to use photosynthetically derived oxygen for root respiration during complete submergence. The pioneer R. sceleratus, predominantly abundant in low lying mudflats, was very resistant to waterlogging and shallow floods. Due to its constitutively high root porosity and its ability to greatly increase the elongation rate of petioles under water this species can ameliorate flooding stress. However, when leaf blades of R. sceleratus were unable to reach the water surface, this species died as quickly as the flooding-intolerant R. bulbosus. This indicates that fast elongation of petioles under water competes for energy and respirable reserves with maintenance processes. R. repens, a species from lower, frequently inundated floodplains, was very tolerant of prolonged waterlogging and submergence. Its high resistance to complete submergence under continuous darkness indicates that this species tolerates hypoxic and/or anoxic tissue conditions via metabolic adjustments. Lysigenous aerenchyma was also induced in the primary root system and in newly developed laterals, and it was able to use oxygen generated by underwater photosynthesis, for root respiration. R. acris, a species from less frequently flooded areas, was as resistant to waterlogging and submergence in the light as R. repens. However, it has a lower resistance than R. repens to complete submergence in the dark. A submergence pre-treatment increased the maximum net underwater photosynthetic rate in R. bulbosus, whereas a significant decrease of light compensation points was observed in R. repens when it had previously been submerged. This study shows that Ranunculus species exhibit various strategies to cope with different flooding conditions. R. repens responds to flooding by its tolerance mechanism and R. sceleratus by avoidance. R. acris ameliorates submergence and R. bulbosus was not able to adapt high water tables.

Survival of rice during complete submergence: effect of potassium bicarbonate application

The effect of potassium bicarbonate application to floodwater on the survival and growth of submergence-tolerant (FR13A) and -intolerant (IR42) rice cultivars during complete submergence was investigated. Potassium bicarbonate, applied at different rates to enhance floodwater carbon dioxide concentrations, increased the floodwater oxygen concentration. The treatment that had CuSO4, added alone to reduce algal growth showed the lowest O2 concentration at the time of submergence and after 10 d of submergence. Potassium bicarbonate at higher rates tended to maintain the floodwater pH near neutrality while copper sulfate affected pH increase during a 10-day period of complete submergence. Potassium bicarbonate addition led to 100% survival of tolerant FR13A. Potassium bicarbonate, even at 0.01 mol m–3 enhanced the survival of intolerant IR42 to 69% and at 0.1, 0.5 and 1.0 mol m–3, the survival was above 85%. Dry weights of submerged plants showed increases in both rice cultivars in floodwater treated with potassium bicarbonate. The dry weight and leaf chlorophyll concentration of both cultivars increased with increasing rates of potassium bicarbonate. Algal chlorophyll concentration of floodwater treated with potassium bicarbonate was comparable to that of the control without copper sulfate. The findings suggest a possibility of environmental manipulation of floodwater by potassium bicarbonate application to enhance the survival and growth of rice cultivars during complete submergence.

Stimulation of adenylate kinase in rice seedlings under submergence stress

Summary The enzymatic activity of adenylate kinase was stimulated in submerged rice seedlings. This activity was enhanced in every organ of the submerged plants. Treatment with N 2 gas had the same effect as submergence, a result which suggests that O 2 -depletion is a major cause of the stimulation of adenylate kinase activity. Northern blot analysis also indicated accumulation of adenylate kinase mRNA under submerged condition. Furthermore, different induction patterns of enzymatic activity were seen in two rice varieties, FR13A and IR42, which are respectively tolerant and intolerant of complete submergence.

Ethylene Sensitivity and Response Sensor Expression in Petioles of Rumex Species at Low O2 and High CO2 Concentrations.

Rumex palustris, a flooding-tolerant plant, elongates its petioles in response to complete submergence. This response can be partly mimicked by enhanced ethylene levels and low O2 concentrations. High levels of CO2 do not markedly affect petiole elongation in R. palustris. Experiments with ethylene synthesis and action inhibitors demonstrate that treatment with low O2 concentrations enhances petiole extension by shifting sensitivity to ethylene without changing the rate of ethylene production. The expression level of the R. palustris gene coding for the putative ethylene receptor (RP-ERS1) is up-regulated by 3% O2 and increases after 20 min of exposure to a low concentration of O2, thus preceding the first significant increase in elongation observable after 40 to 50 min. In the flooding-sensitive species Rumex acetosa, submergence results in a different response pattern: petiole growth of the submerged plants is the same as for control plants. Exposure of R. acetosa to enhanced ethylene levels strongly inhibits petiole growth. This inhibitory effect of ethylene on R. acetosa can be reduced by both low levels of O2 and/or high concentrations of CO2.

Mapping QTLs for submergence tolerance in rice by AFLP analysis and selective genotyping.

By combining the amplified fragment length polymorphism (AFLP) technique with selective genotyping, we constructed a linkage map for rice and assigned each linkage group to a corresponding chromosome. The AFLP map, consisting of 202 AFLP markers, was generated from 74 recombinant inbred lines (RIL) which were selected from both extremes of the population (250 lines) with respect to the response to complete submergence. Map length was 1756 cM, with an average interval size of 8.5 cM. To assign linkage groups to chromosomes, we used 50 previously mapped AFLP markers as anchor markers distributed over the 12 chromosomes. Other AFLP markers were then assigned to specific chromosomes based on their linkage to anchor markers. This AFLP map is equivalent to the RFLP/AFLP map constructed previously as the anchors were in the same order in both maps. Furthermore, tests with two restriction fragment length polymorphism (RFLP) markers and two sequence-tagged site (STS) markers showed that they mapped in the expected positions. Using this AFLP map, a major gene for submergence tolerance was localized on chromosome 9. Quantitative trait loci (QTL) associated with submergence tolerance were detected on chromosomes 6, 7, 11, and 12. We conclude that the combination of AFLP mapping and selective genotyping provides a much faster and easier approach to QTL identification than the use of RFLP markers.

Flooding resistance of Rumex species strongly depends on their response to ethylene: Rapid shoot elongation or foliar senescence

Rumex palustris is a flooding‐resistant amphibious species from frequently flooded riversides, whereas Rumex acetosella is flooding‐sensitive and grows on dry sandy soils. Upon complete submergence, both species accumulate ethylene to similar levels. After more than four days, however, the ethylene concentration in R. acetosella plants strongly rises to an extremely high level, whereas it remains much lower in R. palustris plants. This latter species responds to ethylene with enhanced leaf elongation, whereas elongation in R. acetosella is insensitive to ethylene. Elongation rates of leaves were measured continuously during the first 8 h of submergence. A comparison of the elongation rates of R. palustris, R. acetosella and silver‐treated R. palustris plants demonstrated that R. palustris plants responded to ethylene within 1 h of submergence. In R. acetosella, clear symptoms of senescence and decay were observed within two weeks of submergence. In R. palustris plants, only the oldest leaf was senescent. To investigate the role of ethylene in the senescence process, the effects of silver ions on submerged plants, and the effects of prolonged exposure to an extremely high ethylene level on drained plants were studied in both Rumex species. The results demonstrated that although ethylene accelerated senescence of submerged R. acetosella plants, the process may have been caused by other factors. The slower senescence of R. palustris plants could not be explained by their lower ethylene concentration. Rather, it was caused by a much lower sensitivity of the senescence process to ethylene. Moreover, other factors may be less unfavourable in R. palustris than in R. acetosella plants under submerged conditions.

Plant Adaptations to Anaerobic Stress

Inundation that gives rise to soil flooding, or more complete submergence, is the most common environmental cause of oxygen deprivation for vascular plants. Species differ considerably in their susceptibility to the stress. Tolerance can vary from only a few hours to many days or weeks depending on species, the organs directly affected, stage of development, and external conditions such as temperature. Mechanisms that underlie short- and long-term tolerance to external anaerobic conditions are reviewed. For roots, these include metabolic adaptations such as avoidance of self poisoning and cytoplasmic acidosis, maintenance of adequate supplies of energy and sugar, modifications to gene expression and metabolic acclimation to tissue anoxia by previous exposure to partial oxygen shortage. Morphological escape mechanisms based on aerenchyma development and internal aeration pathways are emphasised. Shoots are often less susceptible to oxygen deficiency than roots. Their mechanisms of tolerance can include metabolic adaptations and developmentally passive tolerance such as that seen in overwintering rhizomes of many wetland species. Escape mechanisms for shoots are based on active and, sometimes, increasingly rapid shoot extension in the presence or absence of oxygen, and formation of replacement roots through adventitious rooting at the shoot base. Systemic signalling between roots and shoots integrates root and shoot physiology and limits indirect damage to shoot tissues by soil flooding. The review is completed by an assessment of prospects for future research.

Physiology and Genetics of Submergence Tolerance in Rice

We review a multidisciplinary approach to improve flooding tolerance in rice and, specifically, tolerance of complete submergence. Environmental characterizations in India and Thailand suggest that limited gas diffusion and, sometimes, low irradiance are the most important factors contributing to plant mortality. This supports the view that submergence tolerance of rice seedlings is related to maintenance of energy supply partly through fast rates of alcoholic fermentation which require high levels of carbohydrates. In germinating seeds, rates of coleoptile elongation during anoxia are highly correlated with rates of alcoholic fermentation and carbohydrate supply for energy production. In older seedlings, survival during submergence is highly correlated with carbohydrate supply. Optimization of growth vs. maintenance processes affects survival because elongation growth competes for energy and carbohydrate reserves essential for maintenance processes. This was demonstrated by experiments using: (a) cultivar comparisons, (b) growth regulators and (c) dwarf-mutants. Hence, submergence tolerance of 14-d-old rice seedlings can increase by up to 98 % during 10 d submergence when elongation growth is reduced in these three ways. This is consistent with the observation that submergence tolerance and elongation ability rarely occur in the same genotype. Plant breeding has produced elite lines with up to four-fold greater yields and submergence tolerance equal to the world's most tolerant cultivars, but successful introduction of these elite lines in the field is elusive. Recent production of double haploid populations differing in submergence tolerance permitted testing of the physiological and genetic linkage of traits, or genes, with submergence tolerance. Genetics research with segregating populations of 15- to 50d-old seedlings demonstrated (a) there is one dominant gene for submergence tolerance and (b) this gene is present in three out of four of the world's most tolerant rice cultivars. This suggests that a common factor related to tolerance of limited gas diffusion, (e.g. one of the enzymes of alcoholic fermentation) may be responsible for genotypic differences in submergence tolerance of rice. An alternative possibility is that a gene for a transcription factor is involved in the expression of a multiple gene cascade that confers submergence tolerance. C 1997 Annals of Botany Company

The beneficial effect of reduced elongation growth on submergence tolerance of rice

Adverse effects of elongation growth on tolerance to complete submergence for up to 14 d were evaluated in rice seedlings of cultivars which differed in submergence tolerance. There is a good negative correlation between per cent survival and elongation growth of genotypes during complete submergence (r= -0.81). When elongation growth underwater is minimized by application of a gibberellin biosynthesis inhibitor, per cent survival increases by as much as 50 times for one cultivar. These effects are likely related to elongation growth since (i) addition of gibberellin had the opposite effect by reducing survival, and (ii) when the elongation inhibitor and gibberellin were added together, there was no effect on elongation growth and the per cent survival did not change. A GA-deficient mutant of rice which had little elongation ability during submergence showed a high level of submergence tolerance when plants were submerged at equal initial dry weights and carbohydrate levels relative to a submergence-tolerant cultivar. These results are consistent with the hypothesis that elongation growth competes with maintenance processes for energy and hence reduces survival during submergence. The impact of these findings is that in environments where elongation ability is not required, there is a potential to increase submergence tolerance of agriculturally important cultivars by selecting for least elongation, at least during periods of complete submergence. Furthermore, this trade-off between stimulated elongation growth and submergence tolerance will have important ecological consequences for the distribution of plant species in different flood-prone environments.

Flood‐induced leaf elongation in Rumex species: effects of water depth and water movements

SUMMARYSeveral species from the genus Rumex are found in Dutch river forelands. Species such as R. palustris Sm. From the low, frequently flooded areas are well adapted to wet conditions, Rumex species from higher and less frequently flooded sites are poorly adapted and therefore sensitive to flooding. One of the adaptations to flooding is enhanced shoot elongation upon complete submergence, enabling plants to restore leaf‐air contact, provided that the water is not too deep. This paper demonstrates the strong variation in the absolute extent of flood‐induced leaf elongation among species of the genus Rumex. The effects of flooding on shoot dry and fresh weight and internal gas volumes of an elongating and a non‐elongating species, R. palustris and R. acetosella L., were compared. Net water uptake in response to complete submergence was observed in the shoots of both species. Based on results presented here, we conclude that in plants of R. palustris water enters the cells and is used for cell expansion leading to petiole elongation, whereas in plants of R. acetosella at least part of the water taken up fills the intercellular gas spaces. Elongation of completely submerged Rumex plants does not vary with different depths of submergence, This was concluded from the observation that there is little effect of either hydrostatic pressure or irradiance on leaf elongation during complete submergence. However, when R. palustris plants were subjected to a changing water depth, i.e. alternate periods of complete submergence and waterlogging, they elongated less strongly than under permanent complete submergence. Water movements did not affect leaf elongation induced by submergence.

Short-term weathering rates of buried oils in experimental sand columns over north-temperate temperature range.

Petroleum residues are degraded (weathered) at different rates, depending on various conditions. Degradation in slicks generally has time scales of days to weeks for loss of low molecular weight alkanes and aromatics. This largely involves physical processes of wind-driven evaporation, and dissolution into the water column. Weathering in homogeneously oiled surface sediments, not having the benefit of evaporation, has time scales of months to years. Tidal submergence and temperature can influence this significantly. For example, Prudhoe Bay crude oil in sediments placed in trays on the bottom of a near-arctic bay showed virtually no weathering. Here we report on weathering rates and qualitative changes of oils, buried within constantly submerged and irrigated sandy beach sediments for up to 11 mon. Such conditions occur when coastal stranded oil becomes buried subtidally, or becomes mixed into intertidal beach sediments below the water table. Experimental conditions included continuous sea water irrigation, indigenous microbial populations, and continuous complete submergence. For preliminary microbial incubation and weathering results, see Thorpe et al. (1986). We expected that oil degradation within sandy sediments would be comparatively slow, as organic material and nutrients are more limited, thereby limiting microbial activity. Lower temperatures and submergence also should limit the weatheringmore » process.« less

Effects of the Deep Water Treatments on the Emergence of Tillers of Rice Plant.

We carried out two types of experiments to clarify the emergence of tillers of rice plants under deep water conditions. One (Exp. 1) was to keep the water level constant during the treatment, and the other (Exp. 2) was to raise the water depth in some grades. In Exp. 1, when the water level was deep enough to submerge the leaf sheath of the fully expanded leaf of main stem at about 5.0 plant age in leaf number, the emergence of 3 rd node tiller (T 3), which was growing inside the completely submerged leaf sheath, was inhibited. Though the emergence of T 3 was inhibited, the emergence of T 4 and T 5, which were growing inside the completely submerged leaf sheath at the start of treatment, was not inhibited. There was not always a relation between the inhibition of tiller emergence and the complete submergence of the leaf sheath. In Exp. 2, the emergence of T 4, T 5 and T 6 node tiller was inhibited when the water level was deep enough to submerge the leaf sheath of the fully expanded leaf of the main stem from the platn age of about 5.2 to 10.5 in succession. From the relation between the plant age at the time of the deep water treatment and the growth response of the tiller, we concluded that the growth response of the tiller under deep water varied with the growth stage of the tiller bud ; the most sensitive growth stage of the tiller bud to the deep water was just before emergence of the tiller which had four differentiated leaves. However, at the early growth stage just as the tiller bud differentiated 2-3 leaves and at the late growth stage just as the tiller began to emerge from the leaf sheath, the tiller growth was scarcely inhibited.

The Role of Flooding Resistance in the Establishment of Rumex Seedlings in River Flood Plains

This study deals with resistance to complete submergence of seedlings of Rumex acetosa, R. crispus and R. palustris. Seedlings of R. crispus and R. palustris appeared to be more resistant to flooding than those of R. acetosa under both field and greenhouse conditions. Younger seedlings of all species were able to survive a longer flooding period than older ones. When completely submerged the three Rumex species showed a low, but significant, photosynthetic activity. In R. crispus and R. palustris photosynthetically produced oxygen is released by the roots. Excised Rumex root tips have a low hypoxia tolerance, but that of R. acetosa was the lowest. This correlates with a high rate of ethanolic fermentation in both R. crispus and R. palustris and a low rate in R. acetosa (...)

Optimum sowing date and nitrogen fertilizer rate for rice varieties under intermediate deepwater conditions (15–50 cm)

SUMMARYIn an intermediate deepwater situation (15–50 cm) at Cuttack, India, three improved high-yielding rice varieties, Utkalprabha (semi-tall), CR 292–8051 (intermediate tall) and Gayatri (semi-dwarf) were sown on 20 May, 30 May and 10 June in 1988 and 1989 with an application of either 0, 20 or 40 kg N/ha at sowing. Tall varieties performed better than the dwarf type but grain yield decreased with delay in sowing. The early-sown crops (20–30 May) germinated with pre-monsoon rains and were established well before water accumulated in the field from mid-June onwards. The late-sown crops (10 June), despite good initial germination, could not withstand immediate waterlogging. The tall variety, Utkalprabha, elongated faster with rising water level and escaped complete submergence; whereas the semi-dwarf Gayatri failed to withstand such extreme excess water stress. Application of N fertilizer up to 40 kg/ha proved beneficial to the crops sown by the end of May. However, with crops sown on 10 June, the application of N could not compensate for the loss in yield due to an inadequate initial crop stand. Therefore, early sowing of tall varieties by the end of May along with a basal application of N fertilizer is recommended for higher productivity of rice under excess water conditions.

Flood tolerance of rice grown under intermediate deepwater conditions (15–50 cm) as affected by phosphorus fertilization

SUMMARYThe effect of phosphorus fertilizer on the performance of an improved semi-dwarf (100 cm), long duration (170 days), photosensitive rice variety CR 1016, grown under conditions of intermediate deepwater (15–50 cm) and simulated flash flooding (80 cm), was investigated at Cuttack, India in 1985, 1987 and 1988. Growth and yield of rice subjected to overhead flash flooding (complete submergence) at the vegetative stage for 10 days was decreased significantly compared with partial submergence in water of intermediate depth. The adverse effect of complete submergence was due to greater tiller mortality and lower number and/or weight of panicles at maturity. Rice plants fertilized with P tolerated flooding better and produced significantly higher grain yields than when no P was applied. There was no significant effect of P application on the crop grown under natural unsubmerged conditions, except when it was raised in the same plots as the previous year and may therefore have benefited from residual effects. Therefore, application of 8.7–17.5 kg P/ha along with 60 kg N and 16.7 kg K/ha at sowing was essential to overcome the deleterious effects of submergence on rice grown under waterlogged conditions.

Somaclonal variation in rice – Submergence tolerance and other agronomic characters

The ability to withstand complete submergence was studied in R2 seedlings raised by self‐pollination from 158 R1 plants regenerated from callus of mature rice seeds (cultivars FR13A and Calrose). Compared to parental controls, significant improvement in submergence tolerance as assessed by percentage survival, number of new leaves and chlorophyll content of the third leaf, was found in 5 of the 115 cv. FR13A somaclones and 3 of the 43 cv. Calrose somaclones tested using an aquarium system in a controlled‐temperature glasshouse. With some exceptions these improvements were also observed in the R3 generation when tested under field conditions in Thailand. Variation in other agronomically important characters was observed in the R2 plants grown in a temperature glasshouse under 2 contrasting environments.

Random Wave Slamming on Cylinders

This paper describes random wave forces acting on a fixed, slender, horizontal circular cylinder in the vicinity of the free surface, taking account of the intermittency of submergence and wave slamming. Expressions are derived for the probability density and the mean of the force maxima, and the calculation of the expected value of the single largest force maximum occurring within a given duration is also described. These various predictions are based on the assumptions of linear wave theory and a narrow-band wave spectrum. Depending on the elevation of the cylinder and the wave height, the force maxima in individual waves may be associated with either inertia, combined drag-inertia during complete submergence, the instant of complete submergence, partial submergence, or slamming. The probability density of force maxima incorporates force ranges within which the maxima are associated with one or other of these effects. Numerical results relating to these statistical properties are presented, and the effects of intermittent submergence and wave slamming are discussed.

Submergence of Rice. I. Growth and Photosynthetic Response to CO2 Enrichment of Floodwater

Growth and photosynthetic response of lowland rice following complete submergence is related to the concentration of CO2 dissolved in floodwater. Submergence of plants in stagnant solution at low CO2 concentration or solution gassed with air at 0.03 kPa CO2 (equilibrium of 0.01 mol m-3 dissolved CO2) decreased carbohydrates, and little or no growth occurred. Plants submerged in solutions gassed with 3-20 kPa CO2 in air (equilibrium of 0.9-6 mol m-3 CO2) showed at most small decreases in carbohydrates, and growth was up to 100% of the non-submerged plants. At pH 7.5, there was little net photosynthetic O2 evolution by detached submerged leaves even at high HCO3- concentrations, which suggests that these rice leaves could utilise only CO2 and not HCO3-. At pH 6.5, O2 evolution in solutions in equilibrium with 7.4 kPa CO2 was 3-4 fold higher than in solutions in equilibrium with 0.6 kPa CO2. Photorespiration was indicated by a decrease in the rate of net O2 evolution with increasing external O2. In stagnant solutions this reduction of O2 evolution was pronounced; at a CO2 concentration of 0.25 mol m-3 net O2 evolution ceased when the O2 concentration in the water had reached only 0.125 mol m-3. The requirement of photosynthesis for a combination of high CO2 concentrations and low external O2 was presumably due to slow diffusion of these gases in the unstirred layer of solution around the leaves.

Submergence of Rice. II. Adverse Effects of Low CO2 Concentrations

Submergence of rice in water at low CO2 concentrations was studied in phytotron experiments using plants in the 3rd to 4th leaf stage. Cultivars known to differ in tolerance to complete submergence were adversely affected by the same mechanisms but to a different degree. Submergence for 4-12 days either reduced dry weight production of the whole plant by 6 to 10 fold or even resulted in a loss of dry weight. Nevertheless, the emerging leaf elongated, and both ethanol insoluble material and protein content increased with time. These increases were associated with translocation of dry matter and nitrogen from expanded to expanding leaves. Submergence also reduced concentrations of soluble sugars and starch in all plant parts by 4 to 12 fold. In contrast, concentrations of potassium and free amino acids in shoots were either the same or, in the case of the emerging leaf, higher than in plants which were not submerged. These results indicate (i) these solutes were not limiting growth and (ii) the tissues retained their semipermeability to these solutes during submergence. Insufficient capacity of root metabolism in submerged plants was indicated by low rates of respiration, which persisted in the presence of glucose, and by a low ability to consume ethanol. A model is presented on the adverse effects of submergence of rice which considers possible interactions between CO2, low O2 and high ethylene concentrations.

Concentrations of CO2 and O2 in floodwater and in internodal lacunae of floating rice growing at 1–2 metre water depths

Abstract. Environment and plant measurements were made to determine what factors may limit growth of deepwater and floating rice plants during partial or complete submergence. Field surveys included measurements of temperature, pH, light, O2 and CO2 in floodwater in Thailand. In addition, measurements were made of O2 and CO2 concentrations inside internodal lacunae of deepwater and floating rice growing at 0.5–2.0 m water depths.The bulk of measurements were taken during periods when the changes in water level were less than 50 mm d−1. In the 0–0.02 m surface layer of floodwater at any location there were large changes in oxygen concentrations over diurnal cycles: there were decreases during the night down to 0.02–0.18 mol m−3 O2 at 0600 h and increases during the day to 0.13–0.28 mol m−3 O2 at 1500 h (0.28 mol m−3 being 120% of the O2 concentration of air saturated water at 30°C). During the day oxygen concentrations decreased with increasing water depth; concentrations just above the soil surface were occasionally zero. Most of this gradient disappeared during the night, and at dawn the 0.6 m surface layer of water had uniform low O2 concentrations.O2 concentrations were also measured during flash floods in Thailand. In contrast to the conditions with only small increases in water level, the O2 concentrations in the water during flash floods were more uniform with depth and changed little over a diurnal cycle, the O2 ranging between 0.14–0.19 mol m−3.In most locations floodwater contained 0.2–1.9 mol m−3 CO2 and 0.7–1.6 mol m−3 bicarbonate; however, in a location with acid sulphate soil CO2 was only 0.05–0.2 mol m−3, and bicarbonate concentrations were several fold lower. Concentrations of CO2 in floodwater increased with increasing water depth.O2 and CO2 concentrations inside internodal lacunae of rice were determined in the field when water depth were 1–2 m. Concentrations of O2 in internodes at the water surface were 16–20%, and decreased to 10% and 5% at 0.8 and 1.8 m water depth respectively. There was no diurnal cycle in O2 concentrations inside internodes. In contrast, CO2 concentrations in the lacunae increased with water depth and ranged from 1–3% in internodes at the water surface to 5–10% in internodes at 1.8 m water depth. There was evidence for a diurnal cycle in CO2 concentrations in the basal internode near the soil surface, CO2 increased during the day and decreased during the night.The above data are used to show that there is little or no relationship between gas concentrations in floodwater and internodal lacunae of rice plants. Results are discussed in relation to O2 supply to submerged portions of rice and metabolism of these tissues at low O2 concentrations.