Observations of neutral buoyancy in diapausing copepods Calanoides acutus during Antarctic winter

The herbivorous Antarctic copepod Calanoides acutus overwinters inactively in a resting stage (diapause) at depths below 500 m. It is assumed that during diapause C. acutus is neutrally buoyant in order to retain energy reserves otherwise depleted by swimming activities. However, so far, no experimental observations on its buoyancy have been reported and our knowledge of buoyancy regulation mechanisms is incomplete. In the present study, species-specific differences in buoyancy were assessed visually. Observations were made of specimens from the diapausing cohort of C. acutus and compared to another herbivorous copepod Calanus propinquus, which overwinters actively feeding in the upper water layers. Freshly caught copepods were anaesthetized in a 3-amino-benzoic acid ethyl ester (MS222) in seawater solution in order to exclude the influence of swimming movements on buoyancy control. It was shown that C. propinquus was negatively buoyant, whereas diapausing C. acutus remained neutrally buoyant. This is the first record that neutral buoyancy in diapausing copepods is maintained by the biochemical body composition without the additional need of swimming movements.     

Mild ammonium stress increases chlorophyll content in Arabidopsis thaliana

Nitrate (NO₃⁻) and ammonium (NH₄⁺) are the main forms of nitrogen available in the soil for plants. Excessive NH₄⁺ accumulation in tissues is toxic for plants and exclusive NH₄⁺-based nutrition enhances this effect. Ammonium toxicity syndrome commonly includes growth impairment, ion imbalance and chlorosis among others. In this work, we observed high intraspecific variability in chlorophyll content in 47 Arabidopsis thaliana natural accessions grown under 1 mM NH₄⁺ or 1 mM NO₃⁻ as N-source. Interestingly, chlorophyll content increased in every accession upon ammonium nutrition. Moreover, this increase was independent of ammonium tolerance capacity. Thus, chlorosis seems to be an exclusive effect of severe ammonium toxicity while mild ammonium stress induces chlorophyll accumulation.     

Regulation of NH4 + transport pathways in Pisum arvense roots

The effects of metabolic and protein synthesis inhibitors on NH₄ ⁺ uptake by Pisum arvense plants at low (0.05 mM) and high (1 mM) external ammonium concentration were studied. In short-time experiments cycloheximide decreased the ammonium uptake rate at low level of NH₄ ⁺ and increased the absorption of NH₄ ⁺ from uptake medium containing high ammonium concentration. Arsenate and azide supplied into uptake solutions at low ammonium concentration strongly decreased or completely suppressed the NH₄ ⁺ uptake rate, respectively. When the experiments were carried out at high level of ammonium only azide decreased the uptake rate of NH₄ ⁺ and arsenate stimulated this process. Dinitrophenol very strongly repressed the uptake rate of NH₄ ⁺ at both ammonium concentrations. After removing dinitrophenol from both solutions, neither at low nor high external ammonium level the recovery of NH₄ ⁺ uptake rate was achieved within 150 min or 3 h, respectively. The recovery of NH₄ ⁺ uptake rate after removing azide was observed within 90 min and 3 h at low and high ammonium concentrations, respectively. The regulation of NH₄ ⁺ uptake by some inhibitors at low external ammonium level was investigated using plasma membrane vesicles isolated from roots by two-phase partitioning. Orthovanadate completely suppressed the uptake of NH₄ ⁺ by vesicles and quinacrine decreased the NH₄ ⁺ uptake which 55 suggests that ammonium uptake depends on activities of plasma membrane-bound enzymes. On the other hand, it was found that dinitrophenol completely reduced the NH₄ ⁺ uptake by vesicles. The various effects of inhibitors on ammonium uptake dependent on external ammonium concentration suggest the action of different ammonium transport systems in Pisum arvense roots. The ammonium transport into root cells at low NH₄ ⁺ level requires energy and synthesis of protein in the cytoplasm. The research was supported by grant of KBN No. 6PO4C 068 08     

Acid-base regulation during ammonium assimilation in Hydrodictyon africanum

Abstract The acid-base balance during ammonium (used to mean NH ₄⁺ and/or NH₃) assimilation in Hydrodictyon africanum has been measured on cells growing with about 1 mol m^?3 ammonium at an external pH of about 6.5. Measurements made included (1) ash alkalinity (corrected for intracellular ammonium) which yields net organic negative charge, (2) the accumulation of organic N in the cells and (3) the change in extracellular H⁺ (from the pH change and the buffer capacity). These measurements showed that some 0.25 excess organic negative charge (half in the cell wall, half inside the plasmalemma) accumulates per organic N synthesized, while some 1.25H⁺ accumulate in the medium per organic N synthesized. Granted a permeability (PNH₃) of some 10^?3 cm s^?1, and a finite [NH₃] in the cytoplasm of these N-assimilating cells it is likely that most of the ammonium entering these growing cells is as NH ₄⁺. This means that most of the H ⁺ appearing in the medium must have originated from inside the cell and have been subjected to active efflux at the plasmalemma: H⁺ accumulates in the medium equivalent to any NH₃ entry by requilibration from exogenous NH ₄⁺. The cell composition (net organic negative charge, organic N content) is very similar in these ammonium-grown cells to that of NO₃⁺grown cells, suggesting that there is no action of a ‘biochemical pH stat’ during longterm assimilation of NO₃⁺in H. africanum. Short-term experiments were carried out at an external pH of 7.2 in which ammonium at various concentrations were supplied to NO₃⁺-grown cells. There was in all cases a rapid influx followed by a slower uptake; at least at the lower concentrations (less than 100 μmol dm^?3) the net influx was all attributable to NH₄⁺influx via a uniporter, probably partly short-circuited by a passive NH₃ efflux due to intrinsic membrane permeability to NH₃. The net ammonium influx was in all cases associated with H⁺ accumulation in the medium. (1.3-1.7 H ⁺ per ammonium taken up); as in the growth experiments, most of the ammonium taken up was assimilated. Determinations of cytoplasmic pH showed either no effect on, or a slight decrease in, pH during ammonium assimilation; the changes that occurred were in the direction expected for actuating a ‘pH-regulating’ change in H⁺ fluxes.     

Effects of Exogenously Supplied Ammonium on Root Development of Scots Pine (Pinus sylvestris L.) Seedlings

The effect of potentially toxic concentrations of ammonium on root development of Scots pine seedlings raised on Perlite was investigated during growth periods of 3 or 10 weeks after sowing. It was shown that imbalanced ammonium nutrition led to conspicuous changes of root morphology provided the pH value in the medium was allowed to decrease to 3.9 due to the NH⁺₄-dependent proton excretion into the rhizosphere. Ammonium toxicity could not be observed with seedlings treated either with ammonium nitrate or with ammonium chloride at pH 5.3 ? 6.8. While the supply of NH⁺₄ considerably inhibited root development the biomass production of the shoot was increased. Determination of the endogenous level of ammonium in roots and the leaf whorl exclude a simple causal correlation between ammonium toxicity and accumulated ammonium as has been postulated for herbaceous plants.     

Deciphering the Metabolic Changes Associated with Diapause Syndrome and Cold Acclimation in the Two-Spotted Spider Mite Tetranychus urticae

Diapause is a common feature in several arthropod species that are subject to unfavorable growing seasons. The range of environmental cues that trigger the onset and termination of diapause, in addition to associated hormonal, biochemical, and molecular changes, have been studied extensively in recent years; however, such information is only available for a few insect species. Diapause and cold hardening usually occur together in overwintering arthropods, and can be characterized by recording changes to the wealth of molecules present in the tissue, hemolymph, or whole body of organisms. Recent technological advances, such as high throughput screening and quantification of metabolites via chromatographic analyses, are able to identify such molecules. In the present work, we examined the survival ability of diapausing and non-diapausing females of the two-spotted spider mite, Tetranychus urticae, in the presence (0 or 5°C) or absence of cold acclimation. Furthermore, we examined the metabolic fingerprints of these specimens via gas chromatography-mass spectrophotometry (GC-MS). Partial Least Square Discriminant Analysis (PLS-DA) of metabolites revealed that major metabolic variations were related to diapause, indicating in a clear cut-off between diapausing and non-diapausing females, regardless of acclimation state. Signs of metabolic depression were evident in diapausing females, with most amino acids and TCA cycle intermediates being significantly reduced. Out of the 40 accurately quantified metabolites, seven metabolites remained elevated or were accumulated in diapausing mites, i.e. cadaverine, gluconolactone, glucose, inositol, maltose, mannitol and sorbitol. The capacity to accumulate winter polyols during cold-acclimation was restricted to diapausing females. We conclude that the induction of increased cold hardiness in this species is associated with the diapause syndrome, rather than being a direct effect of low temperature. Our results provide novel information about biochemical events related to the cold hardening process in the two-spotted spider mite.     

Ammonium under solution culture alleviates aluminum toxicity in rice and reduces aluminum accumulation in roots compared with nitrate

Al stress and ammonium–nitrogen nutrition often coexist in acidic soils due to their low pH and weak nitrification ability. Rice is the most Al-resistant species among small grain cereal crops and prefers NH₄ ⁺ as its major inorganic nitrogen source. This study investigates the effects of NH₄ ⁺ and NO₃ ^? on Al toxicity and Al accumulation in rice, and thereby associates rice Al resistance with its NH₄ ⁺ preference. Two rice subspecies, indica cv. Yangdao6 and japonica cv. Wuyunjing7, were used in this study. After treatment with or without Al under conditions of varying NH₄ ⁺ and NO₃ ^? supply, rice seedlings were harvested for the determination of root elongation, callose content, biomass, Al concentration and medium pH. The results indicated that Wuyunjing7 was more Al-resistant and NH₄ ⁺-preferring than Yangdao6. NH₄ ⁺ alleviated Al toxicity in two cultivars compared with NO₃ ^?. Both NH₄ ⁺-Al supply and pretreatment with NH₄ ⁺ reduced Al accumulation in roots and root tips compared with NO₃ ^?. NH₄ ⁺ decreased but NO₃ ^? increased the medium pH, and root tips accumulated more Al with a pH increase from 3.5 to 5.5. Increasing the NO₃ ^? concentration enhanced Al accumulation in root tips but increasing the NH₄ ⁺ concentration had the opposite effect. These results show NH₄ ⁺ alleviates Al toxicity for rice and reduces Al accumulation in roots compared with NO₃ ^?, possibly through medium pH changes and ionic competitive effects. Making use of the protective effect of NH₄ ⁺, in which the Al resistance increases, is advised for acidic soils, and the hypothesis that rice Al resistance is associated with the preferred utilization of NH₄ ⁺ is suggested.     

The effect of some ammonium salts on nitrate reductase level,onin vivo nitrate reduction and on nitrate content in excisedpisum sativum roots

The effect of some ammonium salts on nitrate reductase (NR) level, onin vivo nitrate reduction and on nitrate content was followed in the presence of nitrate in the medium, under changing experimental conditions, in excisedPisum sativum roots, and their effect was compared with that of KNO₃, Ca(NO₃)₂ and NaNO₃ at 15 mM NO₃ ^- concentration, i.e. at a concentration which considerably exceeded the level of saturation with nitrate with respect to nitrate reductase. The effect of ammonium salts on NR level is indirect and changes from a positive one to a strongly negative one which is dependent on the time of action of the salt, on the presence of other cations, on pH of the solution of the ammonium salt and on the nature of the anion of the ammonium salt. A positive effect on the enzyme level can be observed in the presence of other cations than NH₄ ⁺ at suitable concentrations of those ammonium salts, the solutions of which have their pH values in the acid region (i.e. NH₄H₂PO₄, (NH₄)₂SO₄ and NH₄NO₃). However their positive effect is independent of the presence of NH₄ ⁺ ions, and it is obviously the result of an increased concentration of H⁺ ions. A clear-cut negative effect on NR level can be observed after 24 h in one-salt NH₄NO₃ solution where NH₄ ⁺ is not balanced with other cations and thus certainly can adversely influence many metabolic processes, and in the solutions containing neutral (pH 6.2) and dibasic ammonium phosphates in which dissolved undissociated ammonia [(NH₃). (H₂O) which can also affect many metabolic processes incl. proteosynthesis] probably has a toxic influence. Thein vivo nitrate reduction is always depressed in excised pea roots in the presence of ammonium salts in the medium, regardless of the level of nitrate reductase. Under the described conditions, no relationship could be established between the enzyme level and the so-called metabolic NO₃ ^- pool (i.e. NO₂ ^- production under anaerobic conditions), nor between NR level and the total nitrate content in the roots. One-salt solutions of NaNO₃, Ca(NO₃)₂ and KNO₃ exert different effects on the level of nitrate reductase and on the content of NO₃ ^- in the roots, but the in vivo NO₃ ^- reduction shows the same trend as NR level in the roots influenced by these salts. Cl^- ions, supplied in NH₄C1, depress both NR level and NO₃ ^- content in the roots at higher concentrations, but they do not significantly affect the in vivo nitrate reduction in comparison with other ammonium salts. These results indicate that NR level,in vivo nitrate reduction, and nitrate uptake can be regulated in pea roots independently of each other.     

γ‐Aminobutyric acid addition alleviates ammonium toxicity by limiting ammonium accumulation in rice (Oryza sativa) seedlings

Excessive use of nitrogen (N) fertilizer has increased ammonium (NH₄⁺) accumulation in many paddy soils to levels that reduce rice vegetative biomass and yield. Based on studies of NH₄⁺ toxicity in rice (Oryza sativa, Nanjing 44) seedlings cultured in agar medium, we found that NH₄⁺ concentrations above 0.75 mM inhibited the growth of rice and caused NH₄⁺ accumulation in both shoots and roots. Use of excessive NH₄⁺ also induced rhizosphere acidification and inhibited the absorption of K, Ca, Mg, Fe and Zn in rice seedlings. Under excessive NH₄⁺ conditions, exogenous γ‐aminobutyric acid (GABA) treatment limited NH₄⁺ accumulation in rice seedlings, reduced NH₄⁺ toxicity symptoms and promoted plant growth. GABA addition also reduced rhizosphere acidification and alleviated the inhibition of Ca, Mg, Fe and Zn absorption caused by excessive NH₄⁺. Furthermore, we found that the activity of glutamine synthetase/NADH‐glutamate synthase (GS; EC 6.3.1.2/NADH‐GOGAT; EC1.4.1.14) in root increased gradually as the NH₄⁺ concentration increased. However, when the concentration of NH₄⁺ is more than 3 mM, GABA treatment inhibited NH₄⁺‐induced increases in GS/NADH‐GOGAT activity. The inhibition of ammonium assimilation may restore the elongation of seminal rice roots repressed by high NH₄⁺. These results suggest that mitigation of ammonium accumulation and assimilation is essential for GABA‐dependent alleviation of ammonium toxicity in rice seedlings.     

Free Ammonia Inhibition of Algal Photosynthesis in Intensive Cultures

The effect of free NH₃ inhibition on short-term photosynthesis was investigated in three microalgal species: the freshwater chlorophyte Scenedesmus obliquus, the marine diatom Phaeodactylum tricornutum and the marine chlorophyte Dunaliella tertiolecta. By performing a series of assays at various concentrations of added NH₄Cl and culture pH, we demonstrated that the inhibitory compound was free NH₃ and that pH played no role in determining the magnitude of inhibition, other than in establishing the degree of dissociation of nontoxic NH₄⁺ to toxic NH₃. When corrections were made for pH, all three species displayed the same sigmoidal response curve to free NH₃ concentration; 1.2 mM NH₃ led to 50% reduction in photoassimilation of ¹⁴C. Based on literature values, some marine phytoplankton appear to be significantly more sensitive to free NH₃ than were the test species, which are noted for their excellent growth characteristics. However, the combination of low algal biomass and strong pH buffering commonly found in most marine and many freshwater environments probably limits the possibilities for NH₃ toxicity to low alkalinity freshwaters and intensive algal cultures in which NH₄⁺ is the main source of N. Such conditions occur commonly in algal wastewater treatment systems.     

INTERACTIONS BETWEEN LIGHT,NH4+, AND CO2 IN BUOYANCY REGULATION OF ANABAENA FLOS-AQUAE (CYANOPHYCEAE)1

Buoyancy of the gas-vacuolate alga Anabaena flosaquae Brébisson was measured under various levels of light, NH₄⁺, and CO₂. At high irradiance (50 μE · m^?2·^?1) the alga was non-buoyant regardless of the availability of CO₂ and NH₄⁺. At low irradiance (≤10 μE · m ^?2· s^?1) buoyancy was controlled by the availability of NH₄⁺ and CO₂. When NH₄⁺ was abundant, algal buoyancy was high over a wide range of CO₂ concentrations. In the absence of NH₄⁺, algal buoyancy was reduced at high CO₂ concentrations, however as the CO₂ concentration declined below about 5 μmol · L^?1, algal buoyancy increased. These results help explain why gas vacuolate, nitrogen-fixing blue-green algae often form surface blooms in eutrophic lakes.     

Suppression of net transpiration by multiple mechanisms conserves water resources during pupal diapause in the corn earworm Helicoverpa zea

One critical aspect of an insect's ability to overwinter successfully is the effective management of its water resources. Maintenance of adequate water levels during winter is challenging because of the prevailing low relative humidity at that time of year and the short supply of environmental water that is not in the form of ice. These issues are further exacerbated for insects overwintering as pupae, comprising an immobile stage that is unable to move to new microhabitats if conditions deteriorate. The present study compares the water balance attributes of diapausing and nondiapausing pupae of the corn earworm Helicoverpa zea Boddie, aiming to identify the mechanisms used by diapausing pupae to maintain water balance during winter. Diapausing pupae are 10% larger than nondiapausing individuals. Water loss rates for nondiapausing pupae are low (0.21 mg h^?1) and are suppressed (0.01 mg h^?1) in diapausing pupae. Cuticular lipids, which serve to waterproof the cuticle and thus suppress cuticular water loss, are more than two‐fold more abundant on the surface of diapausing pupae, and oxygen consumption rates during diapause drop to almost one‐third the rate observed in nondiapausing pupae. Water gain can be accomplished only when atmospheric water content is near saturation or during contact with free water. At moderate relative humidities (20–40%), water loss rates are very low for diapausing pupae, suggesting that these moth pupae have robust mechanisms for combating water loss. The exceptional ability of H. zea to suppress water loss during diapause is probably a result of the combined effects of increased size, more abundant cuticular lipids and decreased metabolic rates.     

Interaction of hypochlorous acid and myeloperoxidase with phosphatidylserine in the presence of ammonium ions

The close association of the heme enzyme myeloperoxidase to phosphatidylserine epitopes on the surface of non-vital polymorphonuclear leukocytes (PMNs) and other apoptotic cells at inflammatory sites favours modifications of this phospholipid by myeloperoxidase products. As detected by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, ammonium ions inhibit in a concentration-dependent manner the hypochlorous acid-mediated formation of aldehyde and nitrile products from 1,2-dipalmitoyl-sn-glycero-3-phosphoserine (DPPS). Concomitantly, the formation of monochloramine (NH₂Cl) raises with increasing NH₄⁺ concentrations. A transchlorination from monochlorinated O-phospho-l-serine to NH₄⁺ with the formation of NH₂Cl occurs only when extraordinary high NH₄⁺ concentrations are applied. Due to the low rate of 0.044 M^− 1 s^− 1 for this process, a transhalogenation reaction from transient chlorinated intermediates of the serine moiety to NH₄⁺ can be ruled out as an important process contributing to the HOCl-mediated formation of NH₂Cl. A significant formation of NH₂Cl by myeloperoxidase interacting with DPPS in the presence of ammonium ions takes only place at acidic pH values around 5, a scenario that may occur in phagosomes of macrophages after the uptake of apoptotic PMNs.     

The ammonium content in the tissues of selected species of squid (Cephalopoda: Teuthoidea)

The ammonium ion (NH₄⁺) content in the mantles, fins, arms, haemolymph, and buoyancy fluid of 17 species of squids belonging to nine families was determined. Great individual variation of ammonium concentration was found in the buoyancy fluid of Liocranchia reinhardti(Steenstrup, 1856), i.e. 38–1108 mM, and in the vacuolized tissues of Histioteuthis macrohista N. Voss, 1969, i.e. 50–775 mM.     

Stimulation of ammonium ion excretion in the toad urinary bladder by an extract of human urine

It is well known that ammonium ion excretion is increased during metabolic acidosis in mammals. The purpose of this study was to determine whether we could isolate from human urine during metabolic acidosis a factor that would stimulate NH₄⁺ and/or H⁺ excretion in toad urinary bladder. Extracts of urine from six human subjects collected during NH₄Cl-induced acidosis were prepared. These extracts were tested for their effect on NH₄⁺ excretion in hemibladders mounted between plastic chambers. The extracts significantly increased NH₄⁺ excretion in the toad urinary bladder. We found no effect on H⁺ excretion by these extracts. This ammoniuretic activity was not present in the urine when the same individuals were in metabolic alkalosis. We conclude that during metabolic acidosis a humoral factor is present which stimulates the excretion of NH₄⁺. The factor could act as a permease in the bladder cell or as a stimulator of an NH₄⁺ transport system.     

Influence of ammonium on fine root development and rhizosphere pH of Douglas-fir seedlings in sand

Ammonium sulphate is a major component of the air pollutants deposited on forests in the Netherlands. Different amounts of NH₄ ⁺ were added to Douglas-fir seedlings grown in tall containers of sand, to study the influence of high concentrations of NH₄ ⁺ in the soil on the development of fine roots and the effects of nitrogen uptake on rhizosphere pH. At the end of this eight-month experiment part of the ammonium appeared to have nitrified into nitrate. High doses of ammonium negatively affected root length and root length per unit of dry matter (specific root length). Although Douglas fir shows a preferential ammonium uptake in nutrient solutions the increases in the pH of the rhizosphere in this experiment indicate that nitrogen was mostly taken up as nitrate. When the ammonium concentration in the soil is low, it cannot be taken up readily because of its low mobility in soil. Shoot growth was stimulated by high availability of nitrogen. The possible effects of high doses of ammonium on long-term forest vitality are discussed.     

Ammonium content and buoyancy in midwater cephalopods

The majority of squid families (Teuthoidea: Cephalopoda) exchange sodium for ammonium, creating a low-density fluid that imparts lift for neutral buoyancy. However, previous methods for measuring ammonium did not distinguish between NH₄⁺ and various other amine compounds. The present study, using single column ion chromatography, reassessed the cation concentrations in several midwater cephalopod species. High NH₄⁺ levels were confirmed for histioteuthid, cranchiid, and chiroteuthid and related squids. A strong relationship is reported between ammonium content and body mass in Histioteuthis heteropsis, suggesting a gradual accumulation of ammonium coincident with an ontogenetic migration to greater depths. The bathypelagic squids Bathyteuthis abyssicola and Bathyteuthis berryi, on the other hand, contained very little ammonium but rather contained large quantities of an as yet unidentified cation. The ecological significance of this compound is not yet known. Morphology in Bathyteuthid squids suggests that the unknown cation is contained intracellularly and so, unlike sequestered ammonia, does not diminish the space available for muscle tissue. Accordingly, protein measurements in B. berryi mantle muscle are on par with shallower-living muscular squids, and in situ submersible observations reveal strong locomotory abilities relative to other deep-water squids.     

Diapause of Cyclops vicinus (Uljanin) in Lake Søbygård: Indication of a Risk-spreading Strategy

Cyclops vicinus is reported to enter summer diapause triggered by day length in order to survive food scarcity and fish predation. Development ceases and the fourth or fifth copepodid stages persist in the sediment for several weeks. In Lake Søbygård, however, a small eutrophic lake in Denmark, C. vicinus is found in the sediment as well as in the open water throughout the year. We performed laboratory experiments to elucidate the life cycle strategy of this population. In order to find the factors that induce diapause we tested the influence of food composition and light regime on the proportion of copepods entering diapause. Further we examined the diapause pattern of the offspring from diapausing and non-diapausing parents to see whether the co-occurrence of diapausing and non-diapausing copepods results from genetic differentiation within the population. In all experiments some of the copepods developed directly into adults, while others remained at the fourth or fifth copepodid stage and displayed diapause features. The proportion of copepods that developed directly into adults was influenced by food supply. Fewer individuals entered diapause when they received a mixed diet of algae and ciliates or algae, ciliates and seston compared with a pure algal diet. The response to light was different than described in literature: about 30% of the copepods entered diapause under dark conditions, and the diapause frequency was not higher when copepods had been exposed to continuous light. The offspring from diapausing parents showed no higher tendency to enter diapause than the offspring of non-diapausing parents, indicating no genetic differentiation. We conclude that diapause in C. vicinus represents a risk-spreading strategy, modified by food. The simultaneous production of diapausing and non-diapausing offspring ensures survival under harsh conditions; the influence of the food supply on the proportion of individuals entering diapause may adjust the population to the actual environmental condition. This flexible life cycle strategy might contribute to the exclusive dominance of C. vicinus in Lake Søbygård.     

Altered cell wall properties are responsible for ammonium‐reduced aluminium accumulation in rice roots

The phytotoxicity of aluminium (Al) ions can be alleviated by ammonium (NH₄⁺) in rice and this effect has been attributed to the decreased Al accumulation in the roots. Here, the effects of different nitrogen forms on cell wall properties were compared in two rice cultivars differing in Al tolerance. An in vitro Al‐binding assay revealed that neither NH₄⁺ nor NO₃^? altered the Al‐binding capacity of cell walls, which were extracted from plants not previously exposed to N sources. However, cell walls extracted from NH₄⁺‐supplied roots displayed lower Al‐binding capacity than those from NO₃^?‐supplied roots when grown in non‐buffered solutions. Fourier‐transform infrared microspectroscopy analysis revealed that, compared with NO₃^?‐supplied roots, NH₄⁺‐supplied roots possessed fewer Al‐binding groups (‐OH and COO‐) and lower contents of pectin and hemicellulose. However, when grown in pH‐buffered solutions, these differences in the cell wall properties were not observed. Further analysis showed that the Al‐binding capacity and properties of cell walls were also altered by pHs alone. Taken together, our results indicate that the NH₄⁺‐reduced Al accumulation was attributed to the altered cell wall properties triggered by pH decrease due to NH₄⁺ uptake rather than direct competition for the cell wall binding sites between Al³⁺ and NH₄⁺.