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The International Commission on Radiological Protection (ICRP) has embarked on a review and revision of the system of Radiological Protection that will update the 2007 general recommendations in ICRP Publication 103. This is the beginning of a process that will take several years, involving open and transparent engagement with organisations and individuals around the world. While the system is robust and has performed well, it must adapt to address changes in science and society to remain fit for purpose. The aim of this paper is to encourage discussions on which areas of the system might gain the greatest benefit from review, and to initiate collaborative efforts. Increased clarity and consistency are high priorities. The better the system is understood, the more effectively it can be applied, resulting in improved protection and increased harmonisation. Many areas are identified for potential review including: classification of effects, with particular focus on tissue reactions; reformulation of detriment, potentially including non-cancer diseases; re-evaluation of the relationship between detriment and effective dose, and the possibility of defining detriments for males and females of different ages; individual variation in the response to radiation exposure; heritable effects; and effects and risks in non-human biota and ecosystems. Some of the basic concepts are also being considered, including the framework for bringing together protection of people and the environment, incremental improvements to the fundamental principles of justification and optimisation, a broader approach to protection of individuals, and clarification of the exposure situations introduced in 2007. In addition, ICRP is considering identifying where explicit incorporation of the ethical basis of the system would be beneficial, how to better reflect the importance of communications and stakeholder involvement, and further advice on education and training. ICRP invites responses on these and other areas relating to the review of the System of Radiological Protection.

Australia’s regulatory framework has evolved over the past decade from the assumption that protection of humans implies protection of the environment to the situation now where radiological impacts on non-human species (wildlife) are considered in their own right. In an Australian context, there was a recognised need for specific national guidance on protection of non-human species, for which the uranium mining industry provides the major backdrop. National guidance supported by publications of the Australian Radiation Protection and Nuclear Safety Agency (Radiation Protection Series) provides clear and consistent advice to operators and regulators on protection of non-human species, including advice on specific assessment methods and models, and how these might be applied in an Australian context. These approaches and the supporting assessment tools provide a mechanism for industry to assess and demonstrate compliance with the environmental protection objectives of relevant legislation, and to meet stakeholder expectations that radiological protection of the environment is taken into consideration in accordance with international best practice. Experiences from the past 5–10 years, and examples of where the approach to radiation protection of the environment has been well integrated or presented some challenges will be discussed. Future challenges in addressing protection of the environment in existing exposure situations will also be discussed.

Protection of the environment is integral to the system of radiological protection, as outlined in the 2007 Recommendations of the International Commission on Radiological Protection (ICRP, Publication 103). The Commission’s activities in this area are mainly pursued by Committee 5 and its associated Task Groups. Publication 91 broadly outlines the approach to radiological protection of the environment, and its alignment with approaches to environmental protection from hazardous substances in general. Publications 108 and 114 provide the cornerstones of the environmental protection system and relevant databases. Publication 124 considers its application in planned, existing, and emergency exposure situations. The system centres on 12 Reference Animals and Plants (RAPs) with broad relevance for environmental protection based on their ubiquity and significance as well as other criteria, as described in Publication 108. The databases comprise general biology of the RAPs, transfer parameters, dose conversion coefficients, and effects data. Derived Consideration Reference Levels (DCRLs) were established for each RAP; a DCRL represents a band of dose rates that might result in some deleterious effects in individuals of that type of RAP. Newly established Task Group 99 will compile the RAP-specific reference information into monographs, with the view of updating information and improving the applicability of the system in different exposure situations. For certain scenarios, more precise and ecosystem-specific protection benchmarks may be justified, which would have to be informed by consideration of representative organisms (i.e. representative of a particular ecosystem and relevant to the specific scenario; Publication 124). Committee 5 will explore this further, making use of a limited number of case studies.

The International Commission on Radiological Protection’s (ICRP) system to protect the living components of the environment is designed to provide a broad and practical framework across all exposure situations. The objectives of ICRP are therefore also set in fairly broad terms, recognising that national and local environmental protection requirements may need to be set within them. The framework recognises the need to be able to demonstrate an adequate level of protection in relation to planned exposure situations, whilst also providing an ability to manage existing situations and accidents, as well as emergency situations, in a rational way. The objects of protection are always real biota in real exposure situations, and the scientific basis for their protection needs to be based on data originating from studies on the relationships between exposure and dose, dose and effects, and effects and consequences in real animals and plants. The framework that has been developed has therefore had to take such realities into account to make the optimum use of the data currently available, whilst being sufficiently flexible to accommodate new scientific information as it arises without having to alter the framework as a whole.

The International Commission on Radiological Protection (ICRP) described its approach to the protection of the environment and how it should be applied in Publication 124. The report expanded on the Commission’s objectives for environmental protection, and how the Derived Consideration Reference Levels (DCRLs) apply within different exposure situations. DCRLs relate radiation effects to doses over and above their normal local background radiation levels, and consider different potential pathways of exposure for animals and plants. This paper will describe how the DCRLs may be used within existing exposure situations to better understand the potential impacts on animals and plants. In these circumstances, the Commission recommends that the aim be to reduce exposures to levels that are within the DCRL bands (or even below, depending upon the potential cost/benefits), but with full consideration of the radiological and non-radiological consequences of doing so. Using examples, this paper will demonstrate how this may be achieved in practice, bearing in mind the potential exposure of humans, animals and plants during and following any remediation attempted.

The IRPA13 Congress took place from 14-18 May 2012 in Glasgow, Scotland, UK, and was attended by almost 1500 radiological protection professionals. The scientific programme of the Congress was designed to capture a snapshot of the profession's views of the current state of knowledge, and of the challenges seen for the coming years. This paper provides a summary of these results of the Congress in twelve key scientific areas that served as the structural backbone of IRPA13.

The ERICA project (environmental risks from ionising contaminants: assessment and management, EC contract no. FI6R-CT-2004-508847) concluded with the publication of two main outputs: the ERICA Integrated Approach to the assessment and management of environmental risks from ionising radiation, of which also introduces the user to the second main output, the ERICA Tool, which is a software programme with supporting databases, that together with its associated help will guide users through the assessment process. More than 60 European scientists contributed to the ERICA Integrated Approach. In addition, a large number of experts, policy makers, and decision-makers in different areas have contributed views on the ERICA Integrated Approach and its associated Tool from the user's perspective, through participation in the End-Users Group set up under the ERICA project. Databases on transfer, dose conversion coefficients and radiation effects on biota have been developed specifically for the purpose of the Integrated Approach, and incorporated into, or interacting with, the Tool. Species sensitivity distributions of biological effects data have been performed and did not reveal, for chronic exposure, any statistical grounds for separation between terrestrial, marine and freshwater ecosystems in terms of species sensitivity to radiation; on the basis of such analysis a universal screening dose rate criterion of 10 microGy h(-1) incremental dose rate is suggested for exiting the assessment procedure while being confident that environmental risks are negligible. This criterion is used for the two first tiers (conservative assessment with limited data requirement and various possibilities of incorporating user-defined parameter values, including the screening dose rate criterion) of the assessment methodology. Exposure situations of concern are carried through a third tier, making use of all relevant databases and with a number of issues and options listed to support and guide decision-making. This article provides an overview to the ERICA Integrated Approach, whereas further articles of this special issue describe in-depth different vital aspects of the Integrated Approach.

In this report, the Commission describes its framework for protection of the environment and how it should be applied within the Commission’s system of protection. The report expands upon its objectives in relation to protection of the environment, in so far as it relates to the protection of animals and plants (biota) in their natural environment, and how these can be met by the use of Reference Animals and Plants (RAPs); their Derived Consideration Reference Levels (DCRLs), which relate radiation effects to doses over and above their normal local background natural radiation levels; and different potential pathways of exposure. The report explains the different types of exposure situations to which its recommendations apply; the key principles that are relevant to protection of the environment; and hence how reference values based on the use of DCRLs can be used to inform on the appropriate level of effort relevant to different exposure situations. Further recommendations are made with regard to how the Commission’s recommendations can be implemented to satisfy different forms of environmental protection objectives, which may require the use of representative organisms specific to a site, and how these may be compared with the reference values. Additional information is also given with regard to, in particular, communication with other interested parties and stakeholders. Issues that may arise in relation to compliance are also discussed, and the final chapter discusses the overall implications of the Commission’s work in this area to date. Appendices A and B provide some numerical information relating to the RAPs. Annex C considers various existing types of environmental protection legislation currently in place in relation to large industrial sites and practices, and the various ways in which wildlife are protected from various threats arising from such sites.

The International Commission on Radiological Protection (ICRP) established Committee 5 in 2005 in response to the need to provide direct demonstration of environmental protection from radiation in accordance with national law and international agreements. The development of the ICRP system for environmental protection was facilitated by research over the previous decades, as well as by ICRP’s evaluation of the ethical and philosophical basis for environmental protection as laid out in ICRP Publication 91. The 2007 Recommendations ( Publication 103) incorporated environmental protection as one of the integral elements of the radiation protection system. Over a relatively short time, the system has evolved to incorporate a set of 12 Reference Animals and Plants (RAPs), which is a small enough number to develop comprehensive databases for each RAP, but wide ranging enough to provide some insight into radiation impact and protection against such impact, as appropriate, in terrestrial, freshwater, and marine ecosystems. As necessary, the databases can be used to derive supplementary databases for Representative Organisms typical for a particular exposure situation of concern or under study. The system, to date, details biology of the RAPs ( Publication 108); outlines transfer factors for estimation of internal concentrations of radionuclides of environmental significance under different situations ( Publication 114); provides further information ( Publication 108) on dosimetry, biological effects, and derived consideration reference levels (bands of environmental dose rates where potential detrimental effects may deserve attention); and provides information on application of the system in planned, emergency, and existing exposure situations ( Publication 124). Currently, a review of experimental determinations of relative biological effectiveness, to guide derivation of specific weighting factors for use in environmental radiation protection if possible and necessary, is being concluded, as is work on improved dosimetry. Further work in this area involves consolidation of databases, recommendations for derivation of specific databases for Representative Organisms on the basis of the RAP data, and recommendations for application of the system to environmental protection in relation to certain human activities of potential environmental concern. Consideration needs to be made for the wider range of ecosystem effects that may be covered in ecological risk assessments, which incorporate the complete suite of stressors that result from human activity, and their effects, to understand the role of radiation effects in this context.

The current system of radiological protection is aimed at protecting human health, and largely neglects both the effects of radiation on the environment and the managerial aspects of environmental protection. The Swedish Radiation Protection Act was revised in 1988 and includes environmental protection as one of its aims. In practice, little guidance had been given in the regulations based on the Act until 1998, when the Swedish Radiation Protection Authority (SSI) formulated environmental aims in its regulations concerning protection of human health and the environment in connection to the final management of spent nuclear fuel and waste. These regulations focus on protection of biodiversity and biological resources, based on ecosystem characterisation. In a broader perspective, the Swedish Parliament established 15 national environmental quality objectives in 1999, covering all aspects of protecting the environment, including the effects of radiation. This paper reviews the background for radiological protection of the environment from both an international and a Swedish perspective, describing the aims and current activities in establishing a system for assessing environmental effects and their consequences that can be used in decision-making. Such activities are largely a result of the European Union research project FASSET (Framework for Assessment of Environmental Impact), carried out under the 5th Framework Programme of the Union. This work is complemented at the Swedish national level by government support to initiate a national environmental monitoring and assessment programme for characterising the radiation environment, which will provide the foundation for decision-making.

The impact of low humidity in ambient air on water relations, nitrate uptake, and translocation of recently absorbed nitrogen, was investigated in 5-week-old tomato (Lycopersicon esculentum Mill cv. Ailsa Craig) plants grown hydroponically in a complete nutrient solution. Plants were subjected to dry air (relative humidity 2-4%) for 6 h. The transpiration rate increased several-fold and the shoot water content decreased by almost 20%, whereas root water content was unaffected. No effect on in vitro nitrate reductase (NR) activity was detected when using an EDTA-containing assay buffer. Replacement of EDTA with Mg 2+ revealed a significant decline in shoot NR activity, which suggests phosphorylation of the enzyme during the stress treatment. Plants were grown in a split-root system, in which one root half was fed 15 N-nitrate during the treatment, in order to determine nitrate uptake and translocation of recently absorbed nitrogen in the plants. Uptake of nitrate was substantially inhibited, but the proportion of absorbed 15 N that was translocated to the shoots was only slightly affected. In untreated plants, 71% of the 15 N recovered in roots had been retranslocated from the shoots, whereas in plants subjected to stress the delivery of 15 N from shoots to roots appeared to be completely inhibited. The data show that lowered humidity in air has significant effects on both uptake of nitrate as well as translocation of nitrogen within the plants. Some of these effects appear to be common with those observed in plants subjected to reduced water potentials in the root environment and point to the possibility of the shoot water relations being highly influential on nitrogen uptake and translocation.

Spring barley (Hordeum vulgare L. cv. Golf) was grown at different nitrate supply rates, controlled by using the relative addition rate technique, in order to elucidate the relationship between nitrate-N supply and root and shoot levels of abscisic acid (ABA). The plants were maintained as (1) standard cultures where nitrate was supplied at relative addition rates (RAs) of 0.03, 0.09 and 0.18 day -1 , and (2) split-root cultures at RA 0.09 day -1 but with the nitrate distributed between the two root parts in ratios of 100 :0, 80 :20 and 60 :40. Time-dependent changes in root and shoot concentrations of ABA (determined by radioimmunoassay using a monoclonal antibody) were observed in both standard and split-root cultures during 12 days of acclimation to the different nitrate regimes. However, the ABA responses were similar at all nitrate supply rates. Further experiments were performed with split-root cultures where the distribution of nitrate between the two root parts was reversed from 80 :20 to 20 :80 so that short-term effects to local perturbations of nitrate supply could be studied without altering whole-plant N absorption. Transient increases in ABA concentrations (maximum of 25 to 40% after 3 to 4 h) were observed in both subroot parts, as well as in xylem sap and shoot tissue. By pruning the root system it was demonstrated that the change in ABA had its origin in the subroot part receiving the increased nitrate supply (i.e. switched from 20 to 80% of the total nitrate supply). The data indicate that ABA responses are easily transmitted between different organs, including transmission from one set of seminal roots to another via the shoot. The data do not provide any indication that long-term nitrate supplies or general nitrogen status of barley plants affect, or are otherwise related to, the average tissue ABA concentrations of roots and shoots.

Spring wheat (Triticum aestivum L.) was grown with daily additions of nitrate-N. The relative addition rate of nitrate-N was decreased stepwise, and after 125 days of growth, 58 mg N plant-1 had been introduced. The fate and effect of an extra addition of nitrate (20 mg N plant-1) at six different times during the ontogeny (37, 54, 66, 79, 94 and 108 days from sowing) on grain yield and grain protein concentration was investigated. The plants absorbed all or most of the extra nitrate at all stages of development evaluated. Dry matter production of both aerial vegetative parts and grains, but not roots, generally increased as a result of the extra nitrate addition. The increase in grain dry matter was mainly an effect of an increased number of grains per plant. Extra nitrate applications had large effects on grain nitrogen content at all stages, but the effect on main shoot and tiller ears varied depending on the time of application. Early applications, i.e. before anthesis, mainly led to increased yield with unchanged protein concentration whereas late applications also led to increased grain protein concentration. The largest effect on grain nitrogen concentration (25–30% increase) was obtained when the extra nitrate was applied late after sowing, i.e. less than four weeks before final harvest. As the extra dose of nitrate was labelled with 15N, it was possible to follow the movement of the extra nitrogen addition within the plant. Samples were taken at one and five days after 15N-addition and at final harvest. There were differences in the movement of 15N depending on when it was introduced. Generally, net movement of the 15N-labelled N into the grain increased with age at application until 94 days after sowing when a maximum of 90% of the added 15N ended up in the grain.

Barley (Hordeum vulgare L. cv. Golf) was cultured using the relative addition rate technique, where nitrogen is added in a fixed relation to the nitrogen already bound in biomass. The relative rate of total nitrogen addition was 0.09 day -1 (growth limiting by 35%), while the nitrate addition was varied by means of different nitrate :ammonium ratios. In 3- to 4-week-old plants, these ratios of nitrate to ammonium supported nitrate fluxes ranging from 0 to 22 μmol g -1 root dry weight h-', whereas the total N flux was 21.8 ± 0.25 μmol g -1 root dry weight h-' for all treatments. The external nitrate concentrations varied between 0.18 and 1.5 μM. The relative growth rate, root to total biomass dry weight ratios, as well as Kjeldahl nitrogen in roots and shoots were unaffected by the nitrate :ammonium ratio. Tissue nitrate concentration in roots were comparable in all treatments. Shoot nitrate concentration increased with increasing nitrate supply, indicating increased translocation of nitrate to the shoot. The apparent V max for net nitrate uptake increased with increased nitrate fluxes. Uptake activity was recorded also after growth at zero nitrate addition. This activity may have been induced by the small, but detectable, nitrate concentration in the medium under these conditions. In contrast, nitrate reductase (NR) activity in roots was unaffected by different nitrate fluxes, whereas NR activity in the shoot increased with increased nitrate supply. NR-mRNA was detected in roots from all cultures and showed no significant response to the nitrate flux, corroborating the data for NR activity. The data show that an extremely low amount of nitrate is required to elicit expression of NR and uptake activity. However, the uptake system and root NR respond differentially to increased nitrate flux at constant total N nutrition. It appears that root NR expression under these conditions is additionally controlled by factors related to the total N flux or the internal N status of the root and/or plant. The method used in this study may facilitate separation of nitrate-specific responses from the nutritional effect of nitrate.

The responses of nitrate reductase (NR) activity and levels of NR-mRNA to environmental nitrate and exogenous cytokinins are characterised in roots and shoots of barley (Hordeum vulgare L., cv. Golf), using a chemostate-like culture system for controlling nitrate nutrition. Experiments were mainly performed with split root cultures where nitrate-N was supplied at a constant relative addition rate of 0.09 day −1 , and distributed between the subroots in a ratio of 20%:80%. The subroot NR-mRNA level and NR activity, as well as the endogenous level of zeatin riboside (ZR), increased when the local nitrate supply to one of the subroots was increased 4-fold by reversing the nitrate addition ratio (i.e. from 20%:80% to 80%:20%). Also shoot levels of ZR, NR-mRNA and NR activity increased in response to this treatment, even though the total nitrate supply remained unaltered. External supply of ZR at 0.1 μM caused an approximately 3-fold increase in root ZR levels within 6 h, which is comparable to the nitrate-induced increase in root ZR. External application of ZR, zeatin, isopentenyl adenine or isopentenyl adenosine at 0.1 μM caused from insignificant to 25% increases in NR-mRNA and activity in roots and up to 100% stimulation in shoots, whereas adenine or adenosine had no effect. No synergistic effects of perturbed nitrate supply and cytokinin application were detected in either roots or shoots. The translocation of nitrate from the root to the shoot was unaffected by application of ZR or switching the nitrate distribution ratio between subroots. The data give arguments for a physiological role of cytokinins in the response of root and shoot NR to environmental nitrate availability. The nature and limitations of the physiological role of cytokinins are discussed

The approach to protection of the environment may vary considerably depending on ethical basis, methodological approach, and identification of endpoints and protective targets. The International Commission on Radiological Protection (ICRP) reviewed these issues in Publication 91, ‘A framework for assessing the impact of ionising radiation on non-human species’, published in 2003. At the same time, ICRP proposed that a possible future ICRP system addressing environmental assessment and protection would focus on biota, that the system should be effect-based so that any reasoning about adequate protection would be derived from firm understanding of harm at different exposure levels, and that the system should be based on data sets for Reference Animals and Plants. ICRP has thus chosen to approach environmental protection on the basis of biology, and further developed the approach in Publications 103, 108 and 114. This paper explores the biological basis for the ICRP system of environmental protection from the viewpoints of: the effects endpoints of concern; the hierarchy of biological organisation; adequate and appropriate protective targets; and the derivation of benchmark dose (rates) to guide protective efforts.

The relationship between availability of external nitrate and N translocation between root and shoot was studied in N-limited barley (Hordeum vulgare L. cv. Golf). Nitrate-N was added at a relative rate (i.e. N added per unit time and unit N in plant biomass) of (1.09 da-1, and distributed between the subroots at ratios of 50:50 or 80:20. The plants were grown for 13 days under these conditions of nitrate nutrition, and for another three days with the nitrate distribution reversed from 80:20 to 20:80. The nitrale-N doses thus experienced by individual subroots ranged from 2 to 11 mg N g-1 root dry weight day-1 . 15N-Nitrate labellings were performed after 2 to 3 and 12 to 13 days of nitrate nutrition. and 2 to 3 days after reversal of nitrate additions. For all treatments, between 60 and 82% of the absorbed label initially left the root, and between 25 and 55% of the label recovered in roots had been supplied (cycled) via the shoot. Labelling of xylem N at the end of the 24-h labelling period ranged from to 36 to 46% indicating that a substantial fraction of the N in the xylem had been absorbed by the plant prior to labelling. It is concluded that cycling of N to roots, and cycling of N in the plant as a whole, is substantial also during N-limited growth. N allocation to roots increased with external nitrate dose. An increased utilization of non-translocated N as well as an increased translocation of N from the shoot contributed to this effect. Thus, the results indicate that increased external availability of N also increased the sink strength of the root for cycling N.

Differential flows of photoassimilate result in patterns of dry matter distribution among plant organs. The extent to which these patterns are dependent on the flux of different mineral nutrients entering the root and the extent to which the distribution of dry matter in the whole plant is affected by differentials in nutrient (primarily nitrate) flux among parts of the root system is considered. It is concluded that patterns of dry matter distribution and nutritional status may depend on how nutrient supply has been manipulated about the root. Where the flux density of nutrient has been decreased and has become limiting to plant growth, two categories of response have been observed. In the case of N, P or S, limiting flux density results in a proportionately greater amount of plant dry matter in roots than is found at higher flux densities. This contrasts with the case of limiting K, Mg or Mn supply, where proportionately less plant dry matter is found in roots at lower nutrient flux densities than at higher flux densities. In the case of N, particular attention is paid as to how sink strength may be related to differences between root and leaf cells in their capacity for loosening and synthesis processes in the primary cell wall.

Barley (Hordeum vulgare L., cvs Golf and Laevigatum) was grown under nitrogen limitation, controlled by the relative rate of nitrate-N addition (RA), in solution culture. The seminal and crown root systems were kept apart, but in contact with the same nutrient solution throughout culturing. Growth, nitrate uptake, and in vitro nitrate reductase (NR) activity in the different root parts were studied at plant ages from 40 (late vegetative stage) to 110 (mid grain-filling) days. The RA was during this time interval stepwise decreased from 0.08 day−1 to 0.005 day−1. The ratio between seminal root dry weight and total plant dry weight decreased drastically during post-anthesis growth, whereas the contribution by crown roots remained unchanged. Tissue nitrogen concentrations in seminal roots did not change with time, but decreased in crown roots after day 80. The NR activity decreased with age in both seminal and crown roots. The Vmax for net nitrate uptake decreased throughout the experiment in the seminal root system, but not in the crown root system. The kinetic properties (Vmax and KM) were used to calculate the nitrate concentration required to maintain a relative rate of nitrate-N uptake that equals the relative addition rate. These concentrations (2 to 5 mmol m−3) were found to closely match actually measured nitrate concentrations in the nutrient solution (1 to 6 mmol m−3). From uptake kinetics, it was deduced that the contribution by seminal roots to total nitrate uptake at these concentrations decreased from more than 50% in vegetative plants, to about 20% just after main shoot anthesis, and to less than 5% during grain-filling. ei]Section editor: H Lambers

Barley (Hordeum vulgare L., cvs Golf and Laevigatum) was grown under nitrogen limitation in solution culture until near maturity. Three different nitrogen addition regimes were used: in the «HN» culture, the relative rate of nitrate-N addition was 0-08 d -1 until day 48 and then stepwise decreased to, finally, 0.005 d -1 during late grain-filling; the «LN» culture received 45% of the nitrogen added in HN; the «CN» culture was maintained at RA 0.0375 d -1 throughout growth. At four different growth stages (vegetative, anthesis, and twice during grain-filling), 15 N-nitrate was fed to the plants

The zeatin riboside (ZR) response to environmental nitrate is characterized in barley roots, using a chemostate-like culture system for controlling nitrate nutrition. Experiments were mainly performed with two-compartment split-root cultures, where nitrate-N was added at a constant relative addition rate of 0.09/day, and distributed between the subroots in a ratio of 80:20 . ZR responses were recorded following reversal of the addition ratio, i.e., the local nitrate dose was increased by a factor of 4 in one subroot, and decreased to one quarter of the original value in the other subroot. The endogenous levels of ZR in the subroot subjected to an elevated nitrate dose increased three-fold in less than 6 h. A decreased nitrate dose was without effect. Pretreatment with tungstate, methionine sulfoximine or cycloheximide inhibited the nitrate-induced ZR response, but there was no decline in the level of ZR compared to the control. The data indicate that the root ZR level of N-limited barley is largely non-responsive to nitrate dose in the long term, but transiently responding (positively) to increased nitrate supply. An accelerated rate of N flux via glutamine synthetase appears to be required for this response. A further, nitrate-specific, effect (as compared to ammonium) is also observed.

The effect of nitrate availability on characteristics of the nitrate assimilatory system was investigated in N-limited barley (Hordeum valgare L. cv. Golf), grown with the seminal root system split into initially equal-sized halves. The cultures were continuously supplied with nitrate-N at a relative addition rate (RA) of 0.09 day−1, which resulted in relative growth rates (RG) that were ca 85% of those observed under surplus nitrate nutrition. The total N addition was divided between the subroots in ratios of 100:0, 80:20, 70:30, 60:40, and 50:50. For comparison, standard cultures were grown at RAs ranging from 0.03 to 0.18 day−1. Initially, biomass and N partitioning to the subroots responded strongly and proportionally to the nitrate distribution ratio. After 12-14 days no further effect was observed. The Vmax for net nitrate uptake and in vitro nitrate reductase (NR) activity were measured in acclimated plants, i.e., after > 14 days under a certain nitrate regime. In subroots fed from 20 to 100% of the total N addition, Vmax for net nitrate uptake increased slightly, whereas NR activity was unaffected. Uptake and NR activities were insignificant in the 0%-subroot. Uneven nitrate supply to individual subroots had negligible effect on the whole-plant ability for nitrate uptake, and the relative Vmax (unit N taken up per unit N in whole plant tissue and time) remained about 7-fold in excess of the demand set by growth. Balancing nitrate concentrations (the resulting external nitrate concentrations at a certain RA) generally ranged between 2 and 10 μM at growth-limiting RA, both when predicted from uptake kinetics and when actually measured. When comparing split root and standard cultures when acclimated, it appears that uptake and NR activities in roots respond more strongly to over-all nitrate availability than to nitrate availability to individual subroots.

The influence of nitrate availability on growth of seminal roots, and root cytokinin levels, was studied in barley (Hordeum vulgare L. cv Golf). Nitrate was continuously supplied to initially N-starved seedlings at relative addition rates (RA) of 0.03 to 0.21 per day (standard cultures) or at RA 0.09 per day in split root cultures with the nitrate additions distributed in ratios of 100:0 or 80:20 to the two subroots. Data were collected both during a phase of acclimation (first 10 days of N additions) and in the acclimated stage (>10 days after onset of N additions). Limitation of whole-plant growth was observed at RA

The 2007 Recommendations of the International Commission on Radiological Protection (ICRP) represent a change from a process-based to an exposure-based approach, where exposure situations are categorised as planned, emergency, and existing exposure situations. Although the new Recommendations contain further changes based on, inter alia, scientific developments since the publication of the 1990 Recommendations, they overall represent more continuity than change - a notable exception being the direct consideration of environmental protection. The implications of the new Recommendations for radioactive waste management are fairly marginal, as earlier recommendations in this area are still considered valid. This communication provides a brief overview of the current ICRP system for radiological protection as applied to radioactive waste management, as well as an outlook including the possibility of using the developing ICRP system for management of environmental effects as an 'alternative way of reasoning' when building the safety case for a specific waste disposal concept.

15N-Nitrate and 35S-sulphate labelling experiments were performed with spring wheat (Triticum aestivum L. cv. Timmo) 44. 64, 79, 95 and 115 days after sowing (growth stages arbitrarily denoted I to V). Label was fed to the plants via a fraction of the root system, termed “donor root”, whereas the rest of the root (“receiver root”) was fed non-labelled nutrient solution. Net uptake rates for both nitrate and sulphate per unit root weight changed little from growth stage I to IV, but were considerably lower at stage V. On a whole-plant weight basis, uptake declined from stage I to IV, because root contribution to total plant weight declined. Between 80 and 95% of absorbed label was translocated to the shoot at all growth stages. At stage V, up to 30% of absorbed label was recovered in the ears. Labelling of the receiver root indicated that, at all growth stages, 10 to 17% of N and 12 to 32% of S translocated to the shoot was retranslocated to the root. This corresponds to between 35 and 85% of the label actually recovered in the roots. Analysis of 15N-labelling of xylem sap collected from receiver roots at growth stages I to IV indicated that about half of the reduced N in the sap is derived from cycling through roots of recently assimilated N. Evidence of cycling was also obtained at stage V. Labelled sulphate was the only form of S cycled in the plant, but it accounted for only 1 to 7% of the sulphate in the xylem sap.

The performance and regulation of the nitrate uptake system under different degrees of long-term nitrogen limitation are reported for three cultivars of the annual crop, Hordeum vulgare, in the vegetative growth stage

The ERICA Integrated Approach requires that a risk assessment screening dose rate is defined for the risk characterisation within Tiers 1 and 2. At Tier 3, no numerical screening dose rate is used, and the risk characterisation is driven by methods that can evaluate the possible effects of ionising radiation on reproduction, mortality and morbidity. Species sensitivity distribution has been used to derive the ERICA risk assessment predicted no-effect dose rate (PNEDR). The method used was based on the mathematical processing of data from FRED (FASSET radiation effects database merged with the EPIC database to form FREDERICA) and resulted in a PNEDR of 10 μGy/h. This rate was assumed to ascribe sufficient protection of all ecosystems from detrimental effects on structure and function under chronic exposure. The value was weighed against a number of points of comparison: (i) PNEDR values obtained by application of the safety factor method, (ii) background levels, (iii) dose rates triggering effects on radioactively contaminated sites and (iv) former guidelines from literature reviews. In Tier 3, the effects analysis must be driven by the problem formulation and is thus highly case specific. Instead of specific recommendations on numeric values, guidance on the sorts of methods that may be applied for refined effect analysis is provided and illustrated. © 2008 Elsevier Ltd. All rights reserved.

By inspection of the FASSET database on radiation effects on non-human biota, one of the major difficulties in the implementation of ecological risk assessments for radioactive pollutants is found to be the lack of data for chronic low-level exposure. A critical review is provided of a number of extrapolation issues that arise in undertaking an ecological risk assessment: acute versus chronic exposure regime; radiation quality including relative biological effectiveness and radiation weighting factors; biological effects from an individual to a population level, including radiosensitivity and lifestyle variations throughout the life cycle; single radionuclide versus multi-contaminants. The specificities of the environmental situations of interest (mainly chronic low-level exposure regimes) emphasise the importance of reproductive parameters governing the demography of the population within a given ecosystem and, as a consequence, the structure and functioning of that ecosystem. As an operational conclusion to keep in mind for any site-specific risk assessment, the present state-of-the-art on extrapolation issues allows us to grade the magnitude of the uncertainties as follows: one species to another > acute to chronic = external to internal = mixture of stressors > individual to population > ecosystem structure to function.

A nitrogen addition regime is described for growth of barley to maturation in N-limited solution culture. In 3 cultivars of barley, growth, nitrogen partitioning and nitrate uptake properties were studied every 14 days. The three cultivars differed from each other in partitioning of biomass and nitrogen between different plant parts, but the nitrate uptake properties were very similar. An ontogeny-linked decline in nitrate uptake capacity was observed, and could be explained by a decreased contribution of the root to total biomass production.

Summary Cells of Scenedesmus obtusiusculus Chod. were grown in a phosphorous-free nutrient medium for 48 hours and then used for experiments concerning the effects of readdition of phosphate on energy metabolism. Addition of 0.25 mM phosphate, which is saturating for uptake, approximately doubled the respiratory oxygen consumption. This effect was accompanied by a steady rise in the cellular ATP pool. Both the stimulation of oxygen consumption and the increment in ATP was sensitive to antimycin A. Under dark anaerobic conditions, phosphate addition caused a transient peak in ATP. Presence of phosphate strongly affected the pattern of ATP pool fluctuations in dark-light and light-dark transitions. Net O 2 evolution in the light was decreased by phosphate addition. Providing phosphate at steady state conditions in the light in a N 2 atmosphere produced a rapid drop in the ATP content, followed by a recovery. In cells treated with DCMU (1 and 50 μM), addition of phosphate was accompanied by a rapid and permanent increase in the ATP pool. In the presence of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) or phloridzin at concentrations that block photophosphorylation, transient peaks in the ATP level occurred with maxima after 1 min. The results are discussed in terms of regulation of metabolic pathways and the action of the inhibitors on ATP synthesis and transport.

Synchronous cultures of Scenedesmus obtusiusculus Chod. were starved for phosphorus for 48 h. Such cells develop an efficient mechanism for phosphate binding which is very sensitive to metabolic inhibitions. Phosphate binding, fluctuations in the ATP pool during dark-light-dark transitions, and steady state levels of ATP, ADP and AMP were studied. The experiments were carried out in a CO2-free N2 atmosphere. DCMU, phloridzin and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) were used as inhibitors of photophosphorylation. Light-induced phosphate uptake was inhibited to various extents by all the inhibitions. The dark-light-dark transition experiments show that neither the light-induced increment in ATP nor the decrease at darkening are affected by DCMU, but DBMIB and phloridzin inhibit both processes. DCMU seems to affect the regulation of the ATP pool size. The steady state levels of the adenylate pools were almost the same in the light as in the dark, and they were also little sensitive to the inhibitors. In unpoisoned cells in the light the steady state ATP/ADP ratio was 1.7 and the energy charge was 0.66. The rates of phosphate binding are not correlated to any of the adenylate parameters studied. This is probably due to the diverse effects of the inhibitors on light-stimulated production of reducing equivalents, photophosphorylation and transfer of energy from the chloroplast to the cytoplasm.

In N-starved (−N) fronds of Lemna gibba L. G 1, NH4+ uptake rates were several-fold those of NO3−-supplied (+N) fronds. NO3−, uptake in +N-plants was slow and not inhibited by addition of NH4+. However, in −N-plants with higher NO3− and still higher NH4+ uptake rates, addition of NH4+ immediately reduced the NO3− uptake rates to about one third until the NH4+ was consumed. The membrane potential (Em) decreased immediately upon addition of NH4+ in all fronds, but whereas depolarisation was moderate and transient in +N-plants, it was strong, up to 150 mV, in N-starved plants, where Em remained at the level of the K+ diffusion potential (ED) until NH4+ was removed. In N-starved plants NH4+ uptake and membrane depolarisation showed the same concentration dependence, except for an apparent linear component for uptake. Phosphate uptake was inhibited by NH4+ similarly to NO3− uptake, but only in P- and N-starved plants, not after mere P starvation. Influx of NO3− and H2PO 4− into the negatively charged cells of Lemna is mediated by anion/H+ cotransport, but NH4+ influx can follow the electrochemical gradient. Its saturating component may reflect a carrier-mediated NH4+ uniport, the linear component diffusion of NH4+ or NH3. Inhibition of anion/H+ cotransport by high NH4+ influx rates may be due to loss of the proton-driving force, ΔμH+, across the plasmalemma. Reversible inhibition by NH4+ of the H+ extrusion pump may contribute to the finding that ΔμH+ cannot be reconstituted in the presence of higher NH4+ concentrations.

The relations between growth and internal nitrogen concentrations were investigated in nonnodulated Pisum sativum L. cv. Marma and Lemna gibba L. grown at relative rates of nitrate-N additions (RA) varying from 0.03 to 0.27 d 1(Pisum) and 0.05 to 0.40 d 1 (Lemna). At RA≤0.21 d 1(Pisum) and ≤0.30 d 1 (Lemna), the relative growth rate (RGR) correlated well with RA whereas higher RA was not met by any further increawse in growth rate. The tissue nitrogen concentrations at growth-limiting RA increased linearly with RGR. The slope of these lines indicate a maximum nitrogen productivity (amount of biomass formed per unit nitrogen and time) of 14.4 g DW g 1 Nd 1 for Pisum and 15.9 g DW g 1 N d 1 for Lemna. Extrapolation of the plots to RGR=0 yielded intercepts of 10–15 mg N g−1 DW for Pisum tissue, whereas for Lemna the intercepts were closer to the origin than for Pisum. These intercepts formally define a fraction of the total plant nitrogen that appears not to be active in production of new biomass, her termed ‘non-growth nitrogen’. The partitioning of nitrogen as well as biomass to the roots increased at low RA, and is discussed in relation to activity of shoots and roots, respectively.

Summary Lemna gibba L. G3, L. minor L. and L. paucicostata Hegelm. 6746 were initially cultured in a nitrogen-sufficient nutrient medium, where they maintained a relative growth rate, RGR, over 50 [ R G R = 100 ( ln W 2 − ln W 1 ) ( t 2 − t 1 ) , where (ln W2- ln W1) represents the fresh weight increase over the time interval (t2 - t1)]. After one week of preculture in nitrogen-sufficient medium, the plants were transferred to a nitrogen-free nutrient medium. Growth-limiting amounts of nitrogen, together with ample amounts of all other essential elements, were added once daily in exponentially increasing amounts. The relative rate of nitrogen addition, RN, was varied between 15 and 45 [ R N = 100 ( ln N 2 − ln N 1 ) t 2 − t 1 , where (ln N2-l1 Ni) represents the increase in nitrogen bound in plant material over the time (t2-t1)]. During an adaptation phase of 10 to 15 days, the RGR gradually decreased until it reached the value set by RN. The plants then continued to grow at a rate fluctuating around the rate set by RN. Just prior to the daily nutrient additions, only traces of nitrogen were detected in the nutrient medium, indicating that RN was the decisive factor regulating growth. The concentrations of K, P, Ca and Mg did not fall to growth-limiting values. The contents of nitrogen and chlorophyll in the plants were proportional to RN, whereas the surface area of the plants expressed on a dry weight basis was fairly unaffected by RN. It seems that adaptation to nitrogen limitation involves reduced net CO2 assimilation rate on areal basis, which is also reflected in the decrease in chlorophyll per area unit. It is concluded that daily, exponentially increasing additions of nitrogen is a convenient method to obtain plants with stable growth rates under sub-optimal nitrogen nutrition, where the supply rate, not the concentration, is the decisive factor for growth.

Influx, efflux and net uptake of NO 3 − was studied in Pisum sativum L. cv. Marma in short-term experiments where 13NO 3 − was used to trace influx. The influx rate in N-limited plants was similar both during net uptake at external concentrations of around 50 μM, and at low external NO 3 − concentrations (4–6 μM) when net uptake was practically zero. Efflux could be inferred from discrepancies between influx and net uptake but was never very high in the N-limited plants during net uptake. Close to the threshold concentration for not NO 3 − uptake, efflux was high and equalled influx. Thus, the threshold concentration can be regarded as a NO 3 − compensation point. The inclusion of NH 4 + in the outer medium decreased influx by about 40% but did not significantly affect efflux. The roles of NO 3 − fluxes and nitrate-reductase activity in regulating/limiting NO 3 − utilization are discussed.

NO3−-dependent O2 in synchronous Scenedesmus obtusiusculus Chod. in the absence of CO2 is stoichiometric with NH4+ excretion, indicating a close coupling of NO3− reduction to non-cyclic electron flow. Also in the presence of CO2, NO3− stimulates O2 evolution as manifested by an increase in the O2/CO2 ratio from 0.96 to 1.11. This quotient was increased to 1.36 by addition of NO2−, without competitive interaction with CO2 fixation, indicating that the capacity for non-cyclic electron transport at saturating light is non-limiting for simultaneous reduction of NO3− and CO2 at high rates. During incubation with NO3−+ CO2, no NH4+ is released to the outer medium, whereas during incubation with NO2−+ CO2, excess NH4+ is formed and excreted. NO3− uptake is stimulated by CO2, and this stimulation is also significant when the cellular energy metabolism is restricted by moderate concentrations of carbonyl cyanide-p-trifluoromethoxyphenylhydrazone, whereas NO3− uptake in the absence of CO2 is severely inhibited by the uncoupler. Also under energy-restricted conditions NO3− uptake is not competitive with CO2 fixation. Antimycin A is inhibitory for NO3− uptake in the absence of CO2, and there is no enhancement of NO3− uptake by CO2 in the presence of antimycin A. It is assumed that the energy demand for NO3− uptake is met by energy fixed as triosephosphates in the Calvin cycle. Antimycin A possibly affects the transfer of reduced triose phosphates from the chloroplast to the cytoplasm. Active carbon metabolism also seems to exert a control effect on NO3− assimilation, inducing complete incorporation of all NO3− taken up into amino acids. This control effect is not functional when NO2− is the nitrogen source. Active carbon metabolism thus seems to be essential both for provision of energy for NO3− uptake and for regulation of the process.

Methionine sulfoximine induced release of ammonia from illuminated cells of Ankistrodesmus braunii (Naegeli) Brunnth, in normal air, but less in air enriched to 3% CO2. In normal air, methionine sulfoximine also induced glycolate release. Addition of either glutamate, glycine, or serine suppressed glycolate release, whereas glutamate and glycine at the same time stimulated ammonia release. The results indicate that inhibition of glutamine synthetase and thereby inhibition of photorespiratory nitrogen cycling restricts the sink capacity for glycolate in the photorespiratory carbon cycle. An external supply of glutamate, glycine, or serine seems to stimulate glyoxylate transamination and thus partly restores the sink capacity. Calculations of total glycolate formation rates in air from glycolate and ammonia release rates in the presence of methionine sulfoximine and glutamate revealed values of approximately 20 micromoles glycolate per milligram chlorophyll per hour on the average. Similar calculations led to an estimated rate of photorespiratory ammonia release in air, in the absence of methionine sulfoximine, of about 10 micromoles per milligram chlorophyll per hour on the average, a value comparable to the primary nitrogen assimilation rate of 8 micromoles per milligram chlorophyll per hour.

Uptake and utilization of nitrate were investigated in Hordeum vulgare L. cvs Mette and Golf in the vegetative stage, 2 and 4 weeks after sowing. The plants were subjected to a light/dark cycle of 16/8 h (18/12°C). Results obtained with the two genotypes were essentially similar. In the light, xylem nitrate transport and shoot nitrate reduction approximately equalled the amount of nitrate absorbed by the root. A drastic decline in translocation to the shoot in darkness was entirely attributable to decreased transpiration since no major changes in xylem nitrate concentration were observed. Darkening caused only a slight decrease in nitrate uptake, while root nitrate reduction was enhanced. Nitrate starvation for 2 days did not significantlly affect dry matter increment, but resulted in a drastic drop in previously accumulated nitrate, indicating that the stored nitrate is accessible and can sustain unrestricted growth. Uptake increased upon re-addition of nitrate and after 8 h it was about twice that of non-starved plants. During recovery, restoration of root nitrate pools and root nitrate reduction took precedence over shoot nitrate accumulation and reduction. Net nitrate uptake and removal of nitrate from the root to the transpiration stream seem to be decisive for the rate of root nitrate reduction.

(13)N-labeled nitrate was used to trace short-term nitrate influx into Lemna gibba L. G3 in experiments where disappearance of both radioactivity and total nitrate from the incubation medium was measured continuously and simultaneously. In plants performing net nitrate uptake from an initial nitrate concentration of 40 to 60 micromolar, there was no discrepancy between net uptake and influx, irrespective of the N status of the plants, indicating that concomitant nitrate efflux was low or nil. Plants treated with tungstate to inactivate nitrate reductase were able to take up nitrate following induction of the uptake system by exposure to a low amount of nitrate. Also, in this case, net uptake was equivalent to influx. In tungstate-treated plants preloaded with nitrate, both net uptake and influx were nil. In contrast to these observations, a clear discrepancy between net uptake and influx was observed when the plants were incubated at an initial nitrate concentration of approximately 5 micromolar, where net uptake is low and eventually ceases. It is concluded that plasmalemma nitrate transport is essentially unidirectional in plants performing net uptake at a concentration of 40 to 60 micromolar, and that transport is nil when internal nitrate sinks (vacuole, metabolism) are eliminated. The efflux component becomes increasingly important when the external concentration approaches the threshold value for net nitrate uptake (the nitrate compensation point) where considerable exchange between internal and external nitrate occurs.

The commercial polychlorinated biphenyl (PCB) mixture Aroclor 1242 was tested regarding its effects on the uni-cellular green alga Scenedesmus obtusiasculus Chod. The algae were grown in liquid medium at PCB concentrations between 10 and 1000 ppb. Growth was inhibited at 300 ppb and above, viability was only affected at the highest concentration, 1000 ppb. Reduction of phosphate uptake was seen at 300 ppb and above. The inhibition of phosphate uptake had almost exactly the same appearance in light as in darkness. It is suggested that this inhibition is mainly caused by PCB action on the plasmalemma. At 800 ppb some experiments indicated an uncoupling of the oxidative phosphorylation. 1000 ppb inhibited both respiration and O2 evolution.

The utilization and translocation of nitrogen was investigated in exponentially growing, nitrogen-limited Pisum sativum L. cv. Marma. The plants were given N daily at exponentially increasing, although suboptimal, relative nitrogen addition rates (RN) calculated to yield a relative increment in N of 0.06 day−1 and 0.12 day−1. After 10 days of NO−3 additions (26 days after sowing), the relative growth rate more or less equaled RN. Uptake of NO−3 was several-fold higher than the N requirement for the growth rate set by RN. The daily addition of NO−3 was taken up after 7 to 8 h, resulting in a cyclic behaviour in the NO−3 utilization. During the phase of net NO−3 influx, the filling phase (0 to 8 h), in vitro nitrate reductase activity (NR activity) and intracellular levels of soluble N in the root increased. In the phase of no net influx of NO−3 the depletion phase (8 to 24 h), the plants were entirely dependent on stored N. During this phase both in vitro NR activity and intracellular levels of soluble N decreased. Also the calculated actual rate of NO−3 reduction was high in the filling phase, while it was close to zero in the depletion phase. The pattern of these fluctuations indicates that the regulation of NO−3 utilization involves an interplay between transmembrane fluxes of NO−3, the cytosolic NO−3 concentration and NR activity. Cyclic fluctuations in N-containing compounds were also found in the xylem. Nitrogen was mainly transported as amino acids. The pattern of NO−3 transport in the xylem and the fluctuations in the shoot of in vitro NR activity indicate that a reasoning similar to that for the regulation of NO−3 assimilation in the root also applies for the shoot. The results also indicate a substantial supply of amino acids to the xylem through recirculation from the shoot.

The photosynthetic performance and nitrogen utilization of Lemna gibba L. G3 adapted to limited nitrogen supply was studied. The plants were adapted to two levels of nitrogen limitation where the nitrogen addition rates were calculated to sustain relative growth rates (RGR) of 0.15 day−1 and 0.25 day−1, respectively. The photosynthetic performance of these cultures was compared to nitrogen-sufficient cultures with an average RGR of 0.32 day−1. Plants transferred from nitrogen-sufficient conditions attained RGR values corresponding to the nitrogen addition rates after 6 to 10 days. Light-saturated net photosynthesis declined during adaptation according to the drop in growth rate, and a concomitant decrease in the respiration rate was recorded. The efficiency of net photosynthesis on a dry weight basis increased with increased nitrogen supply, whereas it was the same in all cultures when expressed on a chlorophyll basis. The light compensation point was unaffected by the nitrogen regime. Limited nitrogen supply resulted in an increased proportion of dry matter in the roots, which led to decreased leaf area ratios. The net assimilation rates also decreased, but not to the same extent as the leaf area ratios. Growth-limiting amounts of nitrogen were added to the cultures once daily, and the net influx of N was higher than the requirement for N, also in adapted cultures with a steady growth rate. This resulted in transient, periodic fluctuations in the NO3−, NH4+ and amino acid pools. Also the rates of NO3− reduction and NH4+ assimilation fluctuated as did the amino acid assimilation which paralleled NH4+ assimilation. The role of flux rates over the plasmalemma and tonoplast for control of nitrogen assimilation rates are discussed.

Pyridine nucleotides in the cyanobacterium Synechococcus leopoliensis (Racib) Komarek and in the green algae Scenedesmus obtusiusulus Chod. and Ankistrodesmus braunii (Naegeli) Brunnth. were extracted with either hot NaOH, hot ethanol, hot acidic or alkaline methanol, perchloric acid (PCA) or trichloroacetic acid (TCA). Reduced pyridine nucleotides were determined with NADH- and NADPH-dependent bacterial luciferase. Oxidized nucleotides were determined after enzymatic conversion to the reduced forms. The yields of pyridine nucleotides in the extracts were compared to the relative extraction efficiency for ATP. Extraction with hot NaOH appeared satisfactory for reduced nucleotides in the green algae but less so in Synechococcus. Extraction with PCA seemed preferable for the oxidized nucleotides. The sensitivity of the bacterial luciferase assay was lowered by all extractants. Criteria for assessment of optimal extraction procedures for pyridine nucleotides are discussed.

The relation between light-induced electron transport with NO3−, NO2− or CO2 as acceptors, ATP pools and transients in dark-light-dark transitions, and phosphate uptake was examined in phosphorus-starved cells of Scenedesmus obtusiusculus Chod. Net O2 evolution at saturating light was around 6 μmol × (mg chlorophyll × h)−1 in the absence of any acceptor, but reached average rates of 21, 65 and 145 μmol × (mg chlorophyll × h)−1 upon additions of 5 mM KNO3, KNO2 and KHCO3, respectively. The apparent rate of photophosphorylation in transition experiments was only a few percent of the rate calculated from CO2-dependent O2 evolution. Blocking non-cyclic electron transport with DCMU inhibited phosphate assimilation, but acceleration of non-cyclic electron flow by addition of NO3− or NO2− did not stimulate phosphate assimilation as compared to the situation without an acceptor. A functional non-cyclic system might primarily be needed for an efficient shuttle transfer of ATP from the chloroplast to the cytoplasm. An inhibition of the non-cyclic system due to lack of reducible substrates accelerates the cyclic system and thus indicates a regulation mechanism between the two systems.