The Significance of Oxygen Transport and of Metabolic Adaptation in Flood-Tolerance of Senecio Species

Experiments were designed to provide information about the physiological basis of flood-tolerance in Senecio species. The oxygen concentration in roots of S. jacobaea L., S. viscosus L. and S. vulgaris L. became almost zero after transplantation to a solution of low oxygen concentration, and it was concluded that the flood-sensitivity of these Senecio species could be due to insufficient oxygen transport from the shoots to the roots. The oxygen concentration in the roots of the flood-tolerant S. congestus (R.Br.) DC., growing in a solution of low oxygen tension, was almost sufficient to maintain oxygen utilization at the rate observed in roots of plants, grown in an air-saturated solution. Oxygen utilization by roots of the flood-tolerant S. aquaticus Hill, growing in a solution of low oxygen tension, was inhibited 50%. However, the oxygen concentration in the roots of this species remained high enough to maintain cytochrome oxidase activity and oxidative phosphorylation at the rate observed in roots from an air-saturated environment. The activity of a second (“alternative”) oxidase must have been drastically reduced. Alternative NADH-oxidizing enzymes, like nitrate reductase which was induced by anaerobiosis in roots of S. aquaticus, might replace the regulatory function of the alternative oxidase. — Thus, in S. aquaticus root porosity and root length contributed to the maintenance of an oxygen concentration which was sufficient for uninhibited cytochrome oxidase activity and oxidative phosphorylation rate in roots growing in a solution of low oxygen tension.     

Efficiency of Root Respiration of a Flood-Tolerant and a Flood-Intolerant Senecio Species as Affected by Low Oxygen Tension

A method was described to determine root growth respiration and root maintenance respiration rate of plants, grown in culture solutions of high and low oxygen concentrations, during linear growth. Root growth respiration of aerobically grown plants was three to four times lower in the flood-intolerant Senecio jacobaea L. than in the flood-tolerant species. Senecio aquaticus Hill. Root growth respiration of Senecio aquaticus Hill decreased by a factor two to three upon transplantation to a culture solution of low oxygen tension. The difference in root growth respiration between aerobically and anaerobically grown Senecio aquaticus Hill was ascribed to the presence of a highly active non-phosphorylating oxidase.     

Interactions between osmoregulation and the alternative respiratory pathway in Plantago coronopus as affected by salinity

Plantago coronopus L., a species from the coastal zone, was grown in culture solution with and without 50 mM NaCl. In addition it was transferred from a non-saline solution to a solution containing 50 mM NaCl. Short term effects of NaCl on growth and various aspects of energy metabolism, including photosynthesis, shoot dark respiration, root respiration and the contribution of the SHAM-sensitive alternative pathway to root respiration were investigated. The concentrations of soluble and insoluble non-structural carbohydrates and of sorbitol a compatible osmotic solute in Plantago, in both shoots and roots were also determined. Growth of shoots and roots was largely unaffected by addition of 50 mM NaCl. Net photosynthesis, shoot dark respiration and the concentration of non-structural carbohydrates in both shoots and roots were also unaffected by salinity. The rate of root respiration immediately decreased upon addition of 50 mM NaCl. This decrease was almost exclusively attributed to a decreased activity of the SHAM-sensitive alternative pathway. The concentration of sorbitol in the roots increased quickly after addition of 50 mM NaCl, whilst the increase in sorbitol concentration in the shoots started later. The time course of the increase of sorbitol concentration was similar to that of the decrease in activity of the alternative pathway. During the first 12 h after exposure to 50 mM NaCl, the amount of carbohydrates which was saved in respiration, due to the decreased activity of the alternative pathway, was the same as that used for sorbitol synthesis in the roots. It is concluded that the activity of the alternative pathway decreased due to increased utilization of carbohydrates for sorbitol synthesis, according to a proposed ‘energy overflow model’. After 24 h, the sorbitol concentration in the cytoplasm of the root cells of plants transferred to a saline solution reached a level that was sufficient to compensate for 50 mM NaCl, assuming a cytoplasmic volume of ca. 10% of the total cell volume. The sorbitol concentration in roots of plants grown in a saline environment for several weeks was lower than that in roots of plants transferred to a saline environment for c. 24 h. It is suggested that sorbitol accumulated in roots of Plantago coronopus as an immediate reaction upon salinity, whilst other adaptations may occur thereafter.     

Changes in Respiration of Seedling Roots Inoculated with Phytophthora cinnamomi

An increase in rate of respiration was recorded for intact roots of seven native Australian species 16 h after inoculation with Phytophthora cinnamomi. By 24 h the magnitude of the increase ranged from 2—159% above that of the uninoculated controls and was evidently not related to host susceptibility. A time sequence study of lesion extension and the associated increased respiration rates for both susceptible and tolerant eucalypts demonstrated a difference in response. The rate of respiration in the tolerant species increased 2 % and only at the site of inoculation, whereas in the susceptible species the respiration rate increased in a wave which began at the inoculation site and continued along the root with the advancing fungal invasion. Respiration rate only increased in regions of the root actually inhabited by the pathogen. The fungal contribution to the total respiration of infected roots was less than 1 % and was determined by measuring respiration of inoculated killed roots. Respiration rates were measured in the presence of potassium cyanide (KCN) and salicylhydroxamic acid (SHAM). Both KCN-sensitive and SHAM-sensitive respiration occurred in normal uninfected E. marginate seedlings. A large proportion of the increase in total respiration rate of infected seedlings compared with uninoculated controls was due to the alternate, SHAM-sensitive pathway. The physiological implications of these results are discussed.     

Efficiency of Root Respiration in Relation to Growth Rate, Morphology and Soil Composition

Root growth respiration and root maintenance respiration rate of the following species were determined: Hypochaeris radicata L. ssp. radicata L., H. radicata ssp. ericetorum Van Soest, Plantago lanceolata L., P. major L. ssp. major, P. major ssp. pleiosperma Pilgcr, P. maritime L., Senecio viscosus L., S. vulgaris L. and Urtica dioica L. A high root growth respiration (i.e. the amount of oxygen consumed for synthesis of a given weight of root material) implied a high maintenance respiration rate (i.e. the amount of oxygen consumed per unit of time and dry weight, but not connected with growth). High values of both components reflect a low efficiency of root respiratory processes. The efficiency of root respiration, as determined by the values for root growth respiration and root maintenance respiration rate could not be demonstrated to be of advantage in adaptation to soil conditions, as e.g. nitrogen content, moisture content and pH. It is concluded that (he degree of ‘wasteful utilization of sugars’ in roots, i.e. such consumption of sugars as cannot be related to structural growth, storage of carbohydrates or maintenance processes, depends on imbalance of transport of sugars from the shoot to the roots with utilization of sugars for synthesis of root material. The results are discussed in relation to Brouwer's explanation for the equilibrium between the growth of shoots and of roots. Root growth rate in the present species appears limited by a factor produced in the shoot under light conditions, and which factor is distinct from carbohydrates. The evidence presented shows that relatively inefficient root respiration does not imply a low growth rate. In regulation of plant growth the growth rate itself and also the shoot to-root ratio may be more important than the regulation of the efficiency of energy metabolism.     

Efficiency and regulation of root respiration in a legume: Effects of the N source

A comparison was made of energy metabolism of nodulated N₂ fixing plants and non-nodulated NO₃-fed plants of Lupinus albus L. Growth, N-increment, root respiration (O₂ uptake and CO² production) and the contribution of a SHAM-sensitive oxidative pathway (the alternative pathway) in root respiration were measured. Both growth rate and the rate of N-increment were the same in both series of plants. The rate of root respiration, both O₂ uptake and CO₂ production, and the activity of the SHAM-sensitive pathway were higher in NO₃-fed plants than in N₂ fixing plants. The rate of ATP production in oxidative phosphorylation was computed also to be higher in NO₃-fed plants. It is concluded that both carbohydrate costings and ATP costings for synthesis + maintenance of root material were lower in N₂ fixing than in NO₃-fed plants. The respiratory quotient of root respiration was 1.6 in N₂-fixing plants and 1.4 in NO₃-fed plants. These values were slightly higher than the values calculated on the basis of CO₂ output due to N-assimilation and the experimental values of O₂ uptake, but showed the same trend: highest in N₂ fixing plants. Root respiration of NO₃-fed plants showed a diurnal pattern (both O₂ uptake, CO₂ production and the activity of the SHAM-sensitive pathway), whilst no diurnal variation in root respiration was found in N₂ fixing plants. However, C₂H₂ reduction did show a diurnal rhythm, which is suggested to be related to the diurnal variation in transpiration. Addition of NO₃ to N₂ fixing plants increased the rate of root respiration and the activity of the alternative pathway. This treatment did not decrease C₂H₂ reduction and H₂ evolution within 4 days. Withdrawal of NO₃-supply from NO₃-fed plants decreased the rate of root respiration but had no effect on the relative activity of the alternative pathway. It is suggested that the higher rate of root respiration and the higher activity of the SHAM-sensitive pathway in NO₃-fed plants is due to a larger supply of carbohydrates to the roots, partly due to a better photosynthetic performance of the shoots and partly due to a higher capacity of the roots to attract carbohydrates.     

Respiration of Senecio Shoots: Inhibition during Photosynthesis, Resistance to Cyanide and Relation to Growth and Maintenance

Respiration and dry matter producation were measured in shoots of senecia aquaticus Hill, which is flood tolerant and in shoots of S. jacobaea L., which is flood- sensitive. Both species were grown in culture solutions of high and of low oxygen concentration Growth of food of S. jacobaca was unaffected by a low oxygen supply bur growth of S. jacobaca was severly hampered by a low oxygen concentration in the root medium. Kinetic data about the rate of apparent photosynthesis at low oxygen conetration and different carbon dioxide concentrations indicated that at light saturation respiration was strongly repressed during photosynthesis. Shoot growth respiration, i.e. the amount of carbon dioxide produced for synthesis of shoot dry, matter appeared to be absent on S. jacobaea and to be very low (13.mg CO₂/g dry shoots) in S. aquaticus. In comparison with values prepiration rate was 2.8. 2.0. 1.5 and 1.3mg CO₂/h.g dry shoots in aerobically and anaerobically growth S. jacobaea and in aerobically and anaerobically growth S. aquabaea respectively. These values were also low in comprision with values previously found for roots of the same species. Shoot dark respiration on S. aquaticus was inbihitedd by a com bination on CN and salicylhydroxamic acid (SHAM), but not by application on one of these inhibitors alone. It was therefore concluded that an alternative oxidative pathway was present but not active in shoots of S. aquaticus. In the absence of inhibited of the cylochorome pathway. The low value of growth respiration and maintenance respiration rate in the shoots as compared with those in the roots of the investigated Sencio species are discussed in relation to the activity of the alternative oxidative pathway and to the possibilbity of a direct supply of ATP by photosynthesis intead of respiratory meta bolism.     

Oxygen uptake by roots of Rumex species at different temperatures: the relative importance of diffusive resistance and enzyme kinetics

Abstract. Oxygen uptake characteristics of the roots of three Rumex species were compared, and related to kinetics of the respiratory system and to root anatomy. The observed differences could not be explained by differences in fundamental characteristics of the oxygen uptake system: with all three species, cytochrome-mediated respiration contributed 70% and cyanide-insensitive (alternative) respiration 30% of the total respiration rate, and apparent K_m values of cytochrome oxidase were lower than those obtained for the alternative oxidase in all cases. However, differences in critical oxygen pressure for respiration (COPR) and in apparent K_m for oxygen, were strongly correlated with differences in root porosity and root diameter. K_m(O₂) values at high and low temperatures were determined, and from Arrhenius plots of oxygen uptake rates between 11 and 32°C, the role of diffusional impedance could be estimated. Root respiration of Rumex maritimus and R. crispus , both with high root porosity, but differing in root diameter, had a low K_m for oxygen (3–7 mmol m⁻³). In contrast with this were the responses of R. thvrsiflorus , which has thin roots but low root porosity: a high K_m (10-20 mmol m⁻³) was found at all temperatures. The role of diffusional impedance as a function of temperature in oxygen uptake rate by the three species is discussed and related to the differential resistance of the species towards flooding.     

Growth Respiration of a Flood-Tolerant and a Flood-Intolerant Senecio Species: Correlation between Calculated and Experimental Values

Root growth respiration of Senecio aquaticus Hill (flood-tolerant) and Senecio jacobaea L. (flood-sensitive) was calculated, assuming different P: O ratios. The growth respiration values were calculated on the basis of the chemical composition of root and shoot dry matter, in combination with published data on the energy costs of biosynthetic and transport processes. The comparison between calculated and experimental values suggests a relatively low efficiency of ATP utilization in the roots of the flood-tolerant species. Root growth respiration of S. congestus (R.Br.)DC., which is also flood-tolerant, and Plantago lanceolata L. were also determined. The data showed that not all the flood-tolerant species investigated had high root growth respiration values. An “overflow model’ is proposed to explain observed differences in root growth respiration between species.     

Internal oxygen transport inRumex species and its significance for respiration under hypoxic conditions

Rumex thyrsiflorus, Rumex crispus andRumex maritimus show a differential flood-tolerance in the river ecosystem in the Netherlands.R. thyrsiflorus occurs at high-elevated habitats and is flood-intolerant, the other two species occur at lower-elevated habitats and are flood-tolerant. We compared their respiratory activity under aerobic and anaerobic conditions in the root environment and quantified the internal gas transport. The results indicate that aerial oxygen can be used for root respiration in both aerobically and anaerobically grown plants. The amount of oxygen used via internal aeration increased with decreasing oxygen concentration in the root environment. Aerobically grown plants ofR. maritimus andR. crispus already showed a high internal aeration, but there was a significant increase in internal oxygen transport in anaerobic plants, where new, aerenchymatous roots had formed. This indicates the functional significance of new root formation for respiration in these species upon hypoxia. After two weeks of anaerobiosis, more than 50% of the total respiration of the roots of young plants ofR. maritumus and 40% of roots of young plants ofR. crispus was due to internal aeration at low oxygen concentrations in the root environment. InR. maritimus both young and old plants performed in this way, inR. crispus only young plants, whileR. thyrsiflorus showed some internal aeration, but this was hardly detectable. These differences can be explained on the basis of a different morphology and concomitant diffusive resistance of both root and shoot system. In experiments with different submergence levels of the shoot, the amount of internal aeration was positively correlated to the total leaf area protruding above the water surface inR. maritimus. This indicates a functional significance of the petiole and leaf elongation response upon total submergence of this species.     

Growth,biomass distribution and CO2 exchange of northern hardwood seedlings in high and low light: relationships with successional status and shade tolerance

The physiology, morphology and growth of first-year Betula papyrifera Marsh., Betula alleghaniensis Britton, Ostrya virginiana (Mill.) K. Koch, Acer saccharum Marsh., and Quercus rubra L. seedlings, which differ widely in reported successional affinity and shade tolerance, were compared in a controlled high-resource environment. Relative to late-successional, shade-tolerant Acer and Ostrya species, early-successional, shade-intolerant Betula species had high relative growth rates (RGR) and high rates of photosynthesis, nitrogen uptake and respiration when grown in high light. Fire-adapted Quercus rubra had intermediate photosynthetic rates, but had the lowest RGR and leaf area ratio and the highest root weight ratio of any species. Interspecific variation in RGR in high light was positively correlated with allocation to leaves and rates of photosynthesis and respiration, and negatively related to seed mass and leaf mass per unit area. Despite higher respiration rates, early-successional Betula papyrifera lost a lower percentage of daily photosynthetic CO₂ gain to respiration than other species in high light. A subset comprised of the three Betulaceae family members was also grown in low light. As in high light, low-light grown Betula species had higher growth rates than tolerant Ostrya virainiana. The rapid growth habit of sarly-successional species in low light was associated with a higher proportion of biomass distributed to leaves, lower leaf mass per unit area, a lower proportion of biomass in roots, and a greater height per unit stem mass. Variation in these traits is discussed in terms of reported species ecologies in a resource availability context.     

Temporal changes in the growth, saponin content and antioxidant defense in the adventitious roots of Panax ginseng subjected to nitric oxide elicitation

Nitric oxide (NO) is a diffusible, gaseous signaling molecule. In plants, NO influences growth and development, and it can also affect plant responses to various stresses. Because NO induces root differentiation and interacts with reactive oxygen species, we examined the temporal effect of NO elicitation on root growth, saponin accumulation and antioxidant defense responses in the adventitious roots of mountain ginseng (Panax ginseng). The observations revealed that NO is involved in root growth and saponin production. Elicitation with sodium nitroprusside (SNP) activated O₂ ⁻-generating NADPH oxidase (NOX) activity, which most probably subsequently enhanced growth of adventitious roots of mountain ginseng. A severe inhibition of NOX activity and decline in dry weight of SNP elicited adventitious roots in the presence of NOX inhibitor (diphenyl iodonium, DPI), which further supports involvement of NOX in root growth. Enhanced activities of antioxidant enzymes by SNP appear to be responsible for low H₂O₂, less lipid peroxidation, and modulation of ascorbate and non-protein thiol statuses in the adventitious roots of mountain ginseng. Dry mass, saponin content and NOX activity was related with NO content present in adventitious roots of mountain ginseng.     

The effects of elevated CO2 on root respiration rates of two Mojave Desert shrubs

Although desert ecosystems are predicted to be the most responsive to elevated CO₂, low nutrient availability may limit increases in productivity and cause plants in deserts to allocate more resources to root biomass or activity for increased nutrient acquisition. We measured root respiration of two Mojave Desert shrubs, Ambrosia dumosa and Larrea tridentata, grown under ambient (～375 ppm) and elevated (～517 ppm) CO₂ concentrations at the Nevada Desert FACE Facility (NDFF) over five growing seasons. In addition, we grew L. tridentata seedlings in a greenhouse with similar CO₂ treatments to determine responses of primary and lateral roots to an increase in CO₂. In both field and greenhouse studies, root respiration was not significantly affected by elevated CO₂. However, respiration of A. dumosa roots <1 month old was significantly greater than respiration of A. dumosa roots between 1 and 4 months old. For both shrub species, respiration rates of very fine (<1.0 mm diameter) roots were significantly greater than those of fine (1–2 mm diameter) roots, and root respiration decreased as soil water decreased. Because specific root length was not significantly affected by CO₂ and because field minirhizotron measurements of root production were not significantly different, we infer that root growth at the NDFF has not increased with elevated CO₂. Furthermore, other studies at the NDFF have shown increased nutrient availability under elevated CO₂, which reduces the need for roots to increase scavenging for nutrients. Thus, we conclude that A. dumosa and L. tridentata root systems have not increased in size or activity, and increased shoot production observed under elevated CO₂ for these species does not appear to be constrained by the plant's root growth or activity.     

Elongation by primary lateral roots and adventitious roots during conditions of hypoxia and high ethylene concentrations

Soil flooding results in unusually low oxygen concentrations and high ethylene concentrations in the roots of plants. This gas composition had a strongly negative effect on root elongation of two Rumex species. The effect of low oxygen concentrations was less severe when roots contained aerenchymatous tissues, such as in R. palustris Sm. R. thyrsiflorus Fingerh., which has little root porosity, was much more affected. Ethylene had an even stronger effect on root elongation than hypoxia, since very small concentrations (0.1 cm³ m^?3) reduced root extension in the two species, and higher concentrations inhibited elongation more severely than did anoxia in the culture medium. Thus, ethylene contributes strongly to the negative effects of flooding on root growth. An exception may be the highly aerenchymatous, adventitious roots of R. palustris. Aerenchyma in these roots provides a low-resistance diffusion pathway for both endogenously produced ethylene and shoot-derived oxygen. This paper shows that extension by roots of R. palustris in flooded soil depends almost completely on this shoot-derived oxygen, and that aerenchyma prevents accumulation of growth-inhibiting levels of ethylene in the root.     

Temperature effects on the reactive oxygen species formation and antioxidant defence in roots of two cucurbit species with contrasting root zone temperature optima

Suboptimal root zone temperature (14°C) was imposed on chilling-sensitive cucumber (Cucumis sativus L.) and chilling-tolerant figleaf gourd (Cucurbita ficifolia Bouché) plants. Exposure of roots to low temperature for up to 10 days caused a strong growth inhibition in cucumber compared with figleaf gourd. Physiological analysis showed that generation of reactive oxygen species (ROS) such as hydrogen peroxide and superoxide anion was significantly induced in cucumber plants as fast as 1 day after low root zone temperature treatment. In addition to the significant induction of antioxidant superoxide dismutase activity, low root zone temperature also increased the mitochondrial electron transport allocated to alternative pathway while decreased cytochrome pathway salicylhydroxamic acid-resistant respiration. However, these defense responses could not compensate for the ROS production, resulting in membrane lipid peroxidation and loss of root cell viability in the low root zone temperature treated cucumber roots. In contrast, 14°C root zone temperature had no significant effects on figleaf gourd plant growth, antioxidant enzymes, ROS levels and alternative respiratory pathway. Hence, difference in ROS metabolism would be associated with the remarkable difference in adaptability of cucumber and figleaf gourd plants in response to suboptimal root zone temperature condition.     

Functional linkage between N acquisition strategies and aeration capacities of hydrophytes for efficient oxygen consumption in roots

We evaluated the specific strategies of hydrophytes for root O₂ consumption in relation to N acquisition and investigated whether the strategies varied depending on the aeration capacity. Aeration capacity of roots is an important factor for determining hypoxia tolerance in plants. However, some hydrophytes possessing quite different aeration capacities often co‐occur in wetlands, suggesting that root O₂ consumption also strongly affects hypoxia tolerance. We cultivated Phragmites australis with high aeration capacity and Zizania latifolia with low aeration capacity in hypoxic conditions with NH or NO treatment and compared the growth, N uptake, N assimilation and root respiration between the two species. In Z. latifolia grown with NH treatment, high N uptake activity and restrained root growth led to sufficient N acquisition and decrease in whole‐root respiration rate. These characteristics consequently compensated for the low aeration capacity. In contrast, in P. australis, low N uptake activity was compensated by active root growth, but the whole‐root respiration rate was high. This high root respiration rate was allowed by the high aeration capacity. The O₂ consumption‐related traits of hydrophyte roots were closely correlated with N acquisition strategies, which consequently led to a compensational relationship with the root aeration capacity. It is likely that this functional linkage plays an important role as a core mechanism in the adaptation of plants to hypoxic soils.     

Inhibition and stimulation of root respiration in pisum and plantago by hydroxamate : its consequences for the assessment of alternative path activity

The contribution of the alternative pathway in root respiration of Pisum sativum L. cv Rondo, Plantago lanceolata L., and Plantago major L. ssp major was determined by titration with salicylhydroxamate (SHAM) in the absence and presence of cyanide. SHAM completely inhibited the cyanide-resistant component of root respiration at 5 to 10 millimolar with an apparent K_i of 600 micromolar. In contrast, SHAM enhanced pea root respiration by 30% at most, at concentrations below 15 millimolar. An unknown oxidase appeared to be responsible for this stimulation. Its maximum activity in the presence of low SHAM concentrations (1-5 millimolar) was 40% of control respiration rate in pea roots, since 25 millimolar SHAM resulted in 10% inhibition. In plantain roots, the maximum activity was found to be 15%. This hydroxamate-activated oxidase was distinct from the cytochrome path by its resistance to antimycin. The results of titrations with cyanide and antimycin indicated that high SHAM concentrations (up to 25 millimolar) block the hydroxamate-activated oxidase, but do not affect the cytochrome path and, therefore, are a reliable tool for estimating the activity of the alternative path in vivo. A considerable fraction of root respiration was mediated by the alternative path in plantain (45%) and pea (15%), in the latter because of the saturation of the cytochrome path.     

The NADPH oxidase activity of pea seedling roots in rhizobial infection depending on abiotic and biotic factors

The changes in NADPH activity was studied in the roots of 3–4-day-old etiolated pea (cultivar Aksaiskii usatyi) seedlings depending on plant inoculation with Rhizobium leguminosarum bv. viceae (strain CIAM 1026), adverse environmental factors (low temperature and high dose of a mineral nitrogen fertilizer), chemical substances (sodium nitroprusside and methyl viologen, or paraquat), and a biotic factor—the bacterium Escherichia coli (strain XL-1Blue). It was demonstrated that all exogenous factors increased the activity of microsomal NADPH oxidase. Rhizobial infection removed the activation caused by exogenous factors only in the case of high nitrogen content in the medium, thereby displaying an antagonistic effect. A synergistic action on the enzyme activity was observed in the variants with combined action of rhizobia + paraquat and rhizobia + E. coli. An increased NADPH oxidase activity coincided with a growth inhibition of pea seedling roots. The results are discussed from the standpoint of the roles of NADPH oxidase and reactive oxygen species in the legume-rhizobium symbiosis.     

Respiration of crop species under CO2 enrichment

Respiratory characteristics of wheat (Triticum aestivum L. cvs Gabo and WW15), mung bean (Vigna radiata L. Wilczek cv. Celera) and sunflower (Helianthus annuus L. cv. Sunfola) were studied in plants grown under a normal CO₂ concentration and in air containing an additional 340 (or 250) μl l^?1 CO₂. Such an increase in global atmospheric CO₂ concentration has been forecast for about the middle of the next century. The aim was to measure the effect of high CO₂ on respiration and its components. Polarographic and, with wheat, CO₂ exchange techniques were used. The capacity of the alternative pathway of respiration in roots was determined polarographically in the presence of 0.1 mM KCN. The actual rate of alternative pathway respiration was assessed by reduction in oxygen consumption caused by 10 mM salicylhydroxamic acid. Each species responded differently. In wheat, growth in high atmospheric CO₂ was associated with up to 45% reduction in respiration by both roots and whole plants. Use of respiratory inhibitors in polarographic measurements on wheat roots implicated reduction in the degree of engagement of the alternative pathway as a major contributor to this reduced respiratory activity of high-CO₂ plants. No change was found in the total sugar content per unit wheat root dry weight as a result of high CO₂. In none of the species was there an increase in the absolute, or relative, contribution by the alternative pathway to total respiration of the root systems. Thus the improved photosynthetic assimilate supply of plants grown in high CO₂ did not lead to increased diversion of carbon through the non-phosphorylating alternative pathway of respiration in the root. On the contrary, in wheat grown in high CO₂ the reduced loss of carbon through that route must have contributed to their larger dry weight.