Internal Nutrient Concentrations Research Articles

Improving Plant Health Through Nutrient Remineralization in Aquaponic Systems.

The exploitation of readily bioavailable fish excreta as a source of plant nutrients lies at the cornerstone of aquaponics farming. Research on nutrient cycling in aquaponic systems has devoted considerable attention to the plant uptake of dissolved nutrients in fish excreta, however, the integration of particulate-bound nutrients into downstream hydroponic farming has remained elusive. The high amount of organic carbon present in fish sludge may lead to biofouling if directly incorporated into hydroponic circulation systems, reducing the utility of incorporating fish solids on a large scale. In this study, we implemented a novel treatment system capable of reducing the carbon and nitrogen load of fish solids to produce a liquid fertilizer for a downstream hydroponics unit. Lettuce (Lactuca sativa) fertilized with exclusively a commercial nutrient solution, the biofilter effluent (coupled aquaponic system), effluent from the solids treatment system, or the latter two combined were grown in nutrient flow technique gutters downstream of a recirculating aquaculture system stocked with rainbow trout (Oncorhynchus mykiss). While crop yields were lower for the aquaponic treatments compared to lettuce grown in a commercial nutrient solution, plant sap analysis demonstrated a contrasting picture with respect to internal nutrient concentrations. Lettuce grown in the commercial hydroponic solution were deficient in several mineral nutrients (Mg, Ca, Na, and Si) nor did they have higher iron concentrations despite the significantly higher EDTA-chelated aqueous iron (460 × greater than other treatments) in the nutrient solution. Nutrient uptake in the rhizosphere was not investigated on a molecular level, although stunted rhizosphere growth in the commercial nutrient solution control suggests a weakened capacity for nutrient uptake in comparison to other treatments. Alongside the remineralization of micronutrients, the solids treatment system addressed the common issue of excess carbon leading to biofouling via a total suspended solids reduction of 87.27% ± 9.95 during the coupled aquaponics cultivation period. Ultimately, these data lead to two important conclusions. Firstly, optimizing nutrient bioavailability is not synonymous to increasing the presence of a nutrient in the water column. Secondly, estimating ideal nutrient solution concentrations involves both preventing nutrient blocking and improving bioavailability.

Growth and nitrogen uptake of jack pine seedlings in response to exponential fertilization and weed control in reclaimed soil

We evaluated the impact of exponential fertilization in nursery and weed removal in the field on growth and nitrogen (N) retranslocation and uptake from the soil of jack pine (Pinus banksiana Lamb.) seedlings planted on an oil sands reclaimed soil. Exponential fertilization is a method of supplying nutrients at an exponential rate to achieve constant internal nutrient concentrations in seedlings without changing their size during their growth in the nursery. The N retranslocation in seedlings was traced using 15N isotope labeling. Exponential fertilization increased nutrient reserve in the seedling in nursery production, and increased height (P = 0.003), root collar diameter (P < 0.001), total biomass (P < 0.001), and N content (P < 0.001) of seedlings at the end of first growing season in the field growth. Conventionally fertilized seedlings allocated a greater percent of biomass to roots than to current-year needles. The 15N isotope analysis showed that 59 to 82% of total N demand of new growth was met by retranslocation from old tissues. Exponential fertilization increased N retranslocation by 147% (P < 0.001) and N uptake from the soil by 175% (P = 0.012). Weed removal marginally increased (P = 0.077) N uptake from the soil but decreased (P = 0.046) N retranslocation with no net effect on total N content in new tissues. We conclude that exponential fertilization improves the early growth of jack pine and can help improve revegetation in reclaiming disturbed oil sands sites.

Achieving global perfect homeostasis through transporter regulation.

Nutrient homeostasis—the maintenance of relatively constant internal nutrient concentrations in fluctuating external environments—is essential to the survival of most organisms. Transcriptional regulation of plasma membrane transporters by internal nutrient concentrations is typically assumed to be the main mechanism by which homeostasis is achieved. While this mechanism is homeostatic we show that it does not achieve global perfect homeostasis—a condition where internal nutrient concentrations are completely independent of external nutrient concentrations for all external nutrient concentrations. We show that the criterion for global perfect homeostasis is that transporter levels must be inversely proportional to net nutrient flux into the cell and that downregulation of active transporters (activity-dependent regulation) is a simple and biologically plausible mechanism that meets this criterion. Activity-dependent transporter regulation creates a trade-off between robustness and efficiency, i.e., the system's ability to withstand perturbation in external nutrients and the transporter production rate needed to maintain homeostasis. Additionally, we show that a system that utilizes both activity-dependent transporter downregulation and regulation of transporter synthesis by internal nutrient levels can create a system that mitigates the shortcomings of each of the individual mechanisms. This analysis highlights the utility of activity-dependent regulation in achieving homeostasis and calls for a re-examination of the mechanisms of regulation of other homeostatic systems.

Stoichiometric homeostasis: a test to predict tundra vascular plant species and community-level responses to climate change

Climate change is having profound influences on Arctic tundra plant composition, community dynamics, and ecosystem processes. Stoichiometric homeostasis (H), the degree to which a plant maintains its internal nutrient concentrations independent of nutrient variations in its environment, may be a useful approach to predict the impacts of these influences. In this case study, we used fertilization manipulation data to calculate homeostasis indices based on nitrogen (HN), phosphorus (HP), and nitrogen to phosphorus ratios (HN:P) of aboveground tissues for seven common tundra vascular species belonging to three growth forms. We then analyzed species H relationships with dominance, spatial stability, and responsiveness to various experimental manipulations. Each of the H indices was correlated amongst tissue types within each species and was generally highest in ericoid mycorrhizal host species and lowest in the ectomycorrhizal birch. Species HP and HN:P were consistently positively correlated with aboveground biomass within the controls and across all manipulations. Furthermore, these same species were spatially stable across experimentally warmed field plots. Stoichiometric homeostasis theory has been successful in predicting grassland community dynamics. This first test of its applicability across a variety of Arctic plant growth forms highlights its considerable potential in predicting tundra plant community structure and responses to environmental change.

Internal efficiency of nutrient utilization: what is it and how to measure it during vegetative plant growth?

Efficient use of the resources required by plants to sustain crop production is considered an important objective in agriculture. In this context, the idea of developing crops with an enhanced ability to utilize mineral nutrients already taken up by roots has been proposed. In recent years powerful tools that allow the association of phenotypic variation with high-resolution genetic maps of crop plants have also emerged. To take advantage of these tools, accurate methods are needed to estimate the internal efficiency of nutrient utilization (ENU) at the whole-plant level, which requires using suitable conceptual and experimental approaches. Here we highlight some inconsistencies in the definitions of ENU commonly used for ENU 'phenotyping' at the vegetative stage and suggest that it would be convenient to adopt a dynamic definition. The idea that ENU should provide information about the relationship between carbon and mineral nutrient economies mainly during the period under which growth is actually affected by low internal nutrient concentration is here advocated as a guide for the selection of adequate operational ENU formulae for the vegetative stage. The desirability of using experimental approaches that allow removal of the influence of nutrient acquisition efficiency on ENU estimations is highlighted. It is proposed that the use of simulation models could help refine the conclusions obtained through these experimental procedures. Some potential limitations in breeding for high ENU are also considered.

Nutritional regulation of root development.

Mineral nutrients such as nitrogen (N), phosphorus (P), and iron (Fe) are essential for plant growth, development, and reproduction. Adequate provision of nutrients via the root system impacts greatly on shoot biomass and plant productivity and is therefore of crucial importance for agriculture. Nutrients are taken up at the root surface in ionic form, which is mediated by specific transport proteins. Noteworthy, root tips are able to sense the local and internal concentrations of nutrients to adjust growth and developmental processes, and ultimately, to increase or decrease the exploratory capacity of the root system. Recently, important progress has been achieved in identifying the mechanisms of nutrient sensing in wild- and cultivated species, including Arabidopsis, bean, maize, rice, lupin as well as in members of the Proteaceae and Cyperaceae families, which develop highly sophisticated root clusters as adaptations to survive in soils with very low fertility. Major findings include identification of transporter proteins and transcription factors regulating nutrient sensing, miRNAs as mobile signals and peptides as repressors of lateral root development under heterogeneous nutrient supply. Understanding the roles played by N, P, and Fe in gene expression and biochemical characterization of proteins involved in root developmental responses to homogeneous or heterogeneous N and P sources has gained additional interest due to its potential for improving fertilizer acquisition efficiency in crops.

Effects of sodium chloride stress on gas exchange, chlorophyll content and nutrient concentrations of nine citrus rootstocks

We investigated the influence of salinity (0, 25, 50, or 75 mM NaCl) on gas exchange and physiological characteristics of nine citrus rootstocks (Cleopatra mandarin, Carrizo citrange, Macrophylla, Iranian mandarin Bakraii, Rangpur lime, Rough lemon, Sour orange, Swingle citrumelo, and Trifoliate orange) in a greenhouse experiment. Total plant dry mass, total chlorophyll (Chl) content, and gas-exchange variables, such as net photosynthetic rate (P N), stomatal conductance (g s), intercellular CO2 concentration, were negatively affected by salinity. In addition, ion concentrations of Cl− and Na+ increased by salinity treatments. Salinity also increased Mg2+ content in roots and reduced Ca2+ and Mg2+ concentrations in leaves. The K+ concentration in leaves was enhanced at low salinity (25 mM NaCl), whereas it decreased with increasing salinity stress. Salinity caused a decline in K+ contents in roots. The rootstocks showed major differences in the extent of Cl− and Na+ accumulation in leaves and in their ability to maintain the internal concentrations of essential nutrients in response to different salinity. Therefore, in addition to inhibitory effects of high concentrations of Cl− and Na+, an imbalance of essential nutrients may also contribute to the reduction in gas exchange under saline conditions. Higher tolerance of rootstocks to salinity could be associated with the reduction of Cl− and Na+ uptake and transport to leaves, ability to keep higher Chl, g s, P N, and better maintenance of nutrient uptake even under high salinity. We found that Sour orange and Cleopatra mandarin were the rootstocks most tolerant to salinity of all nine studied. In addition, Trifoliate orange, Carrizo citrange, and Swingle citrumelo were the rootstocks most sensitive to salt stress followed by the Rough lemon and Macrophylla that showed a low-to-moderate tolerance, and Rangpur lime and Bakraii, with a moderate-to-high tolerance to high salinity.

Signatures of nutrient limitation and co‐limitation: responses of autotroph internal nutrient concentrations to nitrogen and phosphorus additions

Humans are modifying the availability of nutrients such as nitrogen (N) and phosphorus (P), and it is therefore important to understand how these nutrients, independently or in combination, influence the growth and nutrient content of primary producers. Using meta‐analysis of 118 field and laboratory experiments in freshwater, marine and terrestrial ecosystems, we tested hypotheses about co‐limitation of N and P by comparing the effects of adding N alone, P alone, and both N and P together on internal N (e.g. %N, C:N) and P (e.g. %P, C:P) concentrations in autotroph communities. In particular, we tested the following predictions. First, if only one nutrient was limiting, addition of that nutrient should decrease the concentration of the other nutrient, but addition of the non‐limiting nutrient would have no effect on the internal concentration of the limiting nutrient. If community co‐limitation was occurring then addition of either nutrient should result in a decrease in the internal concentration of the other nutrient. Community co‐limitation could also result in no change – or even an increase – in N concentrations in response to P addition if P stimulated growth of N fixers. Finally, if biochemically dependent co‐limitation was occurring, addition of a limiting nutrient would not decrease, and could even increase, the concentration of the other, co‐limited nutrient. We found no general evidence for the decrease in the internal concentration of one nutrient due to addition of another nutrient. The one exception to this overall pattern was marine systems, where N addition decreased internal P concentrations. In contrast, P addition increased internal N concentrations across all experiments, consistent with co‐limitation. These results have important implications for understanding the roles that N and P play in controlling producer growth and internal nutrient accumulation as well as for managing the effects of nutrient enrichment in ecosystems.SynthesisOn a global scale, humans have doubled nitrogen (N) inputs and quadrupled phosphorus (P) inputs relative to pre‐industrial levels. N and P fertilization influences autotroph internal nutrient concentrations and ratios and thereby affects a variety of community and ecosystem processes, including decomposition and consumer population dynamics. It is therefore critical to understand the effects of nutrient additions on the growth and nutrient concentrations of primary producers. We used meta‐analysis to evaluate the responses of autotroph internal N and P concentrations to additions of N, P, and N+P and make inferences about limitation and co‐limitation of N and P across marine, terrestrial, and freshwater ecosystems. We found little evidence for single‐nutrient limitation, highlighting the fact that multiple nutrients generally limit primary production.

Effect of nutrient deficiencies on the water transport properties in figleaf gourd plants

Effects of nitrogen, phosphorus, and potassium deficiencies on water transport properties in figleaf gourd plants were studied. Plants were treated for different period of deficiency and physiological parameters such as stomatal conductance, photosynthesis and transpiration were measured. Cell and root pressure probes were utilized to measure turgor and root pressures, half-times of water exchange and hydraulic conductivities to analyze water transport properties. When plants were grown in nitrogen or phosphorus deficient nutrient solutions, they became insensitive to mercury, suggesting that aquaporin was closed resulting in reduced hydraulic conductivity. Inclusion of tungstate, however, restored the sensitivity of cells to mercury, indicating the importance of internal nutrient concentration, not the incoming nutrient supply. The hydrostatic hydraulic conductivity of roots grown in nitrogen deficient solution, representing apoplastic pathway of water transport, was reduced but this reduction was dramatically recovered by the application of tungstate, indicating the importance of nutrient availability from storage pools in relation to water status of plants.

Forest fertilization: Trends in knowledge and practice compared to agriculture

Plantation forestry continues to intensify and grow in area, with a concomitant increase in fertilizer demand. Virtually no fertilizer is used on non-plantation forest systems. The scale of fertilizer use per ha per year in a small proportion of plantation systems is now similar to some agricultural production systems, but the total area of plantation forestry remains only a few percent of that used for agriculture. Hence, in a global context, forestry is a minor user of fertilizers. In relation to the knowledge base for fertilizer management, forestry and agriculture have similar practical questions that drive research, i.e. nutritional diagnosis and the development of fertilizer prescriptions that optimize production, environmental and economic goals. Much of this research is soil-climate-species-management specific. During the past few decades, solution culture methods were developed that maintain stable internal nutrient concentrations, which were essential for improving our understanding of nutrient-growth relationships. The development of plant production models that include the mechanistic simulation of nutrient supply and uptake are at an early stage of development. Plantation forestry and agriculture lack a mechanistic basis for evaluating base cation availability that accounts for Al-pH-root interactions. Further developments in this field could assist in rationalizing the use of lime. There is a lack of resources available in plantation forestry, and probably also in agriculture in some countries, to develop and refine calibrations of traditional types of soil and foliar analyses. Further testing of soil solution approaches is warranted. Further research on resource use efficiency, wood quality, rhizosphere relations, and mixed-species systems in relation to fertilization is also warranted.

Mathematical analysis on nonautonomous droop model for phytoplankton growth in a chemostat: boundedness, permanence and extinction

In this paper, we considered the non-autonomous Droop model for phytoplankton growth in a chemostat in which the nutrient input varies non-periodically. It is assumed that growth rate varies with the internal nutrient level of the cell and the uptake rate of phytoplankton depends on both the external and the internal nutrient concentrations. A series of new criteria on the positivity, boundedness, permanence and extinction of the population is established.

‘Kardinal’ Rose Exhibits Growth Plasticity and Enhanced Nutrient Absorption Kinetics Following Nitrate, Phosphate, and Potassium Deprivation

Plant internal nutrient status is known to influence the kinetics of nutrient absorption, but little on this relationship has been reported for roses (Rosa spp. L.). The objectives of this experiment were to determine the influence of NO3, PO4, and K deprivation on plant tissue concentrations and relative growth rates and to quantify the influence of nutrient deprivation on absorption kinetic parameters. Rose plants growing in solution culture were deprived of N, P, or K for 0 to 20 days to establish differing tissue concentrations. Absorption kinetics were then determined based on the rate of NO3, PO4, or K depletion from solution over a range of concentrations. The data were fit to a modified Michaelis-Menten equation to account for the influence of internal nutrient status on absorption kinetics. Plants deprived of the nutrients for up to 20 d did not show significantly reduced root or plant fresh weight as compared with control plants. Plant tissue concentrations differed significantly by deprivation treatment and varied from 1.4% to 2.3% for N, 0.22% to 0.35% for P, and from 1.0% to 2.0% for K. Plants deprived of NO3, PO4, and K subsequently showed increased absorption rates. This was primarily expressed as an increased maximum absorption rate for NO3 and PO4. In contrast, K-deprived plants primarily exhibited an increased affinity (decreased Km) for K. The results demonstrate the plasticity of rose plants to grow and absorb nutrients under varying internal nutrient concentrations. This work quantifies the influence of rose plant nutritional status on the kinetics of NO3, PO4, and K absorption. The knowledge would be useful to improve models for providing decision support for fertilization based on plant growth rates and internal nutrient status.

Modelling in vitro growth of dense root networks

Hairy roots are plants genetically transformed by Agrobacterium rhizogenes, which do not produce shoots and are composed mainly by roots. Hairy roots of Ophiorrhiza mungos Linn. are currently gaining interest of pharmacologists, since a secondary product of their metabolism, camptothecin, is used in chemotherapy. To optimize the production of valuable secondary metabolites it is necessary to understand the metabolism and growth of these roots systems. In this work, a mathematical model for description of apical growth of a dense root network (e.g. hairy roots) is derived. A continuous approach is used to define densities of root tips and root volume. Equations are posed to describe the evolution of these and are coupled to the distribution of nutrient concentration in the medium and inside the network. Following the principles of irreversible thermodynamics, growth velocity is defined as the sum over three different driving forces: nutrient concentration gradients, space gradients and root tip diffusion. A finite volume scheme was used for the simulation and parameters were chosen to fit experimental data from O. mungos Linn. hairy roots. Internal nutrient concentration determines short-term growth. Long-term behavior is limited by the total nutrient amount in the medium. Therefore, mass yield could be increased by guaranteeing a constant supply of nutrients. Increasing the initial mass of inoculation did not result in higher mass yields, since nutrient consumption due to metabolism also rose. Four different growth strategies are compared and their properties discussed. This allowed to understand which strategy might be the best to increase mass production optimally. The model is able to describe very well the temporal evolution of mass increase and nutrient uptake. Our results provide further understanding of growth and density distribution of hairy root network and therefore it is a sound base for future applications to describe, e.g., secondary metabolite production.

Monitoring nutrient release from fish farms with macroalgal and phytoplankton bioassays

The release of nutrients from fish farms may cause environmental problems and is traditionally monitored by analysis of nutrient concentrations in the surrounding waters. As concentrations vary strongly over the day this requires intense and expensive sampling. The aim of this study was to demonstrate the use of bioassays as an alternative low cost monitoring strategy that may document the distance from the farms where pelagic primary production is affected. We used a macroalgal and a phytoplankton bioassay to monitor the release of nutrients from four fish farms located in Cyprus, Greece, Italy and Spain. The macroalgal assay consisted of Ulva sp. contained in net cages and the phytoplankton assay was surface water, filtered through a 50 μm filter to remove grazers and then incubated in dialysis bags. Growth in both types of bioassay was highest closest to the fish cages as a result of nutrient release from the cages. The 2 types of assays agreed in most cases and growth exceeding background levels in both types was only found less than 150 m from the fish cages. This indicates that pelagic primary production was stimulated by nutrient release from these 4 fish farms up to a distance of 150 m downstream in the dominant current direction. The content of N, but not of P, in Ulva tissue was highest closest to the cages. This indicates transfer of N to the primary producers. The N : P ratio of Ulva tissue was > 40 in all cases, indicating a severe phosphorous limitation in agreement with the general notion of the Mediterranean being P limited. The content of N and P in Ulva tissue was at or below literature values of the minimum nutrient content necessary for growth. However, Ulva did grow and subsistence quota from the literature may not hold for Mediterranean Ulva. It is thus recommended that the variation in threshold concentrations for a given Ulva population is investigated, before a firm translation of internal nutrient concentrations to growth conditions is made.

Critical influence of Eschscholzia californica cells nutritional state on secondary metabolite production

In this study, we investigated the influence of initial internal nutrient concentrations at the time of elicitation on the ability of Eschscholzia californica (EC) cells to produce alkaloids. Three EC cell suspensions cultivated in culture media differing in their PO4(3-) and NO3- contents were sampled daily for 12 days and analyzed for extracellular and intracellular nutrient concentrations. The ability of the cells to produce alkaloids was tested along the three cell suspension cultures. Sampled cells were then further cultured for 7 days in a production medium containing the elicitor and an extraction resin. The alkaloid production of the cells was measured 7 days post-elicitation. In the low-N medium, starch, glucose, and phosphate contents in the biomass was increased by 470, 1624 and 70%, respectively, 10 days after inoculation compared to the control culture in standard B5 medium. Cell concentration was significantly reduced from 10.3 to 8.6 millions cell/mL on this low-N medium compared to the control, nevertheless alkaloid production was multiplied by 39 at day 10 when cells were elicited. Cells grown on the low-N or low-P media accumulated 83% and 188% more carbon, respectively, than control cells at the end of the culture. This intracellular C was mainly stored in the form of starch in low-P medium and both in the form of starch and glucose in the low-N medium. The ability of EC cells to produce alkaloids upon elicitation was shown to be strongly dependent on the initial intracellular C and P content at the time of elicitation. This suggests that reproducibility and productivity during EC cell culture could be enhanced by manipulating the intracellular C and P content at the time of elicitation.

Molecular analysis and control of cysteine biosynthesis: integration of nitrogen and sulphur metabolism.

Since cysteine is the first committed molecule in plant metabolism containing both sulphur and nitrogen, the regulation of its biosynthesis is critically important. Cysteine itself is required for the production of an abundance of key metabolites in diverse pathways. Plants alter their metabolism to compensate for sulphur and nitrogen deficiencies as best as they can, but limitations in either nutrient not only curb a plant's ability to synthesize cysteine, but also restrict protein synthesis. Nutrients such as nitrate and sulphate (and carbon) act as signals; they trigger molecular mechanisms that modify biosynthetic pathways and thereby have a profound impact on metabolite fluxes. Cysteine biosynthesis is modified by regulators acting at the site of uptake and throughout the plant system. Recent data point to the existence of nutrient-specific signal transduction pathways that relay information about external and internal nutrient concentrations, resulting in alterations to cysteine biosynthesis. Progress in this field has led to the cloning of genes that play pivotal roles in nutrient-induced changes in cysteine formation.

Persistence in variable-yield nutrient-plankton models

Nutrient-phytoplankton-zooplankton models with general uptake functions in which only the internal nutrient concentration is capable of catalyzing cell growth and division for phytoplankton are proposed and analyzed. For the constant nutrient input model, it is shown that extinction or persistence of the population depends on its maximal growth rate relative to the total removal rate. The same biological conclusions hold for the periodic nutrient input model. However, while extinction and persistence are expressed in terms of convergence to steady states for the constant nutrient input model, these biological phenomena are exhibited in terms of asymptotic attraction to periodic solutions for the periodic nutrient input model.

Influence of initial tissue nutrient status of tropical marine algae on response to nitrogen and phosphorus additions

We conducted a 3-factor nutrient-enrichment experiment (factors: N, P, initial tissue- nutrient status) on common species of macroalgae collected from 2 sites along SW Puerto Rico. Our objective was to determine the relative importance of N- or P-limitation among species and sites and to investigate the role of tissue-nutrient status in the response to increased nutrient supply. Acan- thophora spicifera, Dictyota cervicornis, and Hypnea musciformis initially depleted in tissue nutrients responded strongly to either +N or +P alone and both nutrients together by increasing growth. In con- trast, growth of these macroalgae with enriched internal stores of nutrients was generally not as nutrient-limited. In our 3 d experiments, the calcified alga Halimeda incrassata never showed a growth response, perhaps due to the short duration or adaptation to sediment nutrient supply. Tissue N-stores in all algae with low internal nutrient concentrations increased in response to +N, whereas initially higher N-stores were diluted to support growth during the experiment. In addition, all algae depleted the added N and P from the water over the course of the experiment, regardless of tissue- nutrient status. Depleted algae also took up dissolved organic nitrogen (DON) as a secondary source of N, either directly or after mineralization, whereas enriched algae may have 'leaked' NH4. Our results demonstrated that prior storage of nutrients strongly influenced the response of a number of tropical algal species to increased nutrient supply. This variable response to nutrients within and among algae from 3 major algal divisions over small geographical scales may partially explain the mixed results in previous studies of N- and P-limitation.

Demarcating specific ion (NaCl, Cl −, Na +) and osmotic effects in the response of two citrus rootstocks to salinity

The present work differentiates the effects of NaCl, Cl −, Na + and concentrated macronutrients on two citrus species, sour orange ( Citrus aurantium L) and Macrophylla ( Citrus macrophylla Wester). Plants were grown in a base nutrient solution (0.07 MPa osmotic pressure) for 4 months before applying the treatments that consisted of isotonic solutions of 0.23 MPa osmotic pressure of Na + (40 mM, without Cl −), Cl − (40 mM, without Na +), NaCl (40 mM) and 3.5 times the concentration of macronutrients of the base solution. Plants were grown in the different treatment for 2 months before being examined for symptoms of toxicity. The two genotypes showed major differences in the extent of Cl − and Na + accumulation in leaves and in their ability to maintain the internal concentrations of essential nutrients in response to the different ionic compositions of the medium. Differences in mineral nutrient accumulation were observed among treatments in both rootstocks. It was concluded that growth response to the different treatments was primarily affected by an osmotic effect, although in Macrophylla, the ionic effects also seem to be present.

Modelling forest‐growth response to increasing CO2 concentration in relation to various factors affecting nutrient supply

AbstractIt is well recognized that photosynthesis of C3 plants is highly responsive to CO2 concentration. However, in natural ecosystems, plants are subject to a range of feed‐back effects that can interact with increased photosynthetic carbon gain in different ways so that it is not clear to what extent increased photosynthesis will translate into increased growth. To assess the probable growth response of nutrient‐limited forests to increasing CO2 concentration, we use a previously developed modelling framework and apply it under conditions where the supply of nutrients is affected by a range of different factors.Our analysis indicates that forest growth is likely to be highly stimulated by increasing CO2 concentration in forests with high fertility, in forests with nitrogen fixing plants, in those subject to fire or where nitrogen in wood is effectively removed from the biologically active cycle either through physical removal of stems in harvesting or through continued stem growth over long time periods. Forest growth is likely to be stimulated by CO2 concentration in both phosphorus‐ and sulphur‐limited forests provided nutrients in heartwood of trees are removed from the active nutrient cycle. Without this removal from the cycling system, however, sulphur‐limited forests should show little response to increasing CO2. In phosphorus‐limited forests without phosphorus removal, the response to increasing CO2 depends further on the equilibration state of the large pool of unavailable secondary phosphorus. Considered over periods of centuries during which the secondary pool has equilibrated, growth of phosphorus‐limited forests is likely to be only weakly stimulated by increasing CO2 concentration. However, over shorter periods, increasing CO2 concentration should lead to a substantial increase in productivity.In general, it can be concluded that systems that are more open with respect to nutrient gains and losses are likely to be more responsive to increasing CO2 concentration than systems where the amount of available nutrients is less variable. In more open systems, operation at a lower internal nutrient concentration as a result of increasing atmospheric CO2 concentration can lead to reduced nutrient losses per unit carbon gain. Our analysis shows that the effect of increasing CO2 on forest growth can differ substantially between forests due to interactions with a range of factors that affect nutrient supply. The response of a particular forest to increasing CO2 concentration can only be predicted if the main factors controlling nutrient supply and growth in that forest are understood and incorporated into an assessment.