Role of Endogenous Plant Growth Regulators in Seed Dormancy of Avena fatua: II. Gibberellins

Gibberellin A₁ (GA₁) was identified by combined gas chromatographymass spectrometry as the major biologically active gibberellin (GA) in seeds of wild oat (Avena fatua L.) regardless of the depth of dormany or stage of imbibition. Both unimbibed dormant and nondromant seeds contained similar amounts of GA₁ as estimated by the d5-maize bioassay. During imbibition, the level of GA₁ declined in both dormant and non-dormant seeds, although the decline was more rapid in dormant seeds. Only in imbibing nondormant seeds did the GA biosynthesis inhibitor, 2-chloroethyltrimethyl ammonium chloride (CCC), cause a reduction in the level of GA₁ from that observed in control seeds. These results are interpreted as an indication that while afterripening does not cause a direct change in the levels of GAs during dry storage, it does induce a greater capacity for GA biosynthesis during imbibition.

Nondormant seeds imbibed in the presence of 50 millimolar CCC germinated equally as well as untreated seeds. When wild oat plants were fed CCC throughout the entire life cycle, viable seeds were produced that lacked detectable GA-like substances. These seeds afterripened at a slightly slower rate than the controls. Moreover, completely afterripened (nondormant) seeds from plants fed CCC continuously contained no detectable GA-like substances, and when these seeds germinated, dwarf seedlings were produced, indicating GA biosynthesis was inhibited during and after germination. In total, these results suggest that the increased capacity for GA biosynthesis observed in imbibing nondormant seeds is not a necessary prerequisite for germination. It is therefore possible that GA biosynthesis in imbibing nondormant seeds is one of many coordinated biochemical events that occur during germination rather than an initiator of the processes leading to germination.

     

Alterations of fast-reacting sulfhydryl groups of rat brain microsomes by ethanol.

Brain microsomes isolated from rats chronically imbibing 10% ethanol contained 12–16% more 5,5′-dithiobis-(2-nitrobenzoic acid) (DTNB) fast-reacting sulfhydryl (SH) groups than microsomes from control animals. (³H)N-ethylmaleimide was also shown to react with more SH groups in the microsomes of ethanol imbibing rats than the controls. No changes were found in the total SH groups or in the disc gel electrophoresis protein banding profiles between the two fractions. However, the acute exposure of microsomes from ethanol-naive animals to ethanol resulted in a dose-dependent decrease in DTNB-reactive SH groups. These findings were interpreted as arising from time-dependdent conformational changes in the membrane due to the presence of ethanol or compensatory response to such changes.     

Prolonged maintenance of water balance by adult females of the American spider beetle, Mezium affine Boieldieu, in the absence of food and water resources

Moisture requirements were evaluated for female adults of spider beetles Mezium affine Boieldieu and Gibbium aequinoctiale Boieldieu to determine how they are differentially adapted for life in a dry environment. Features showing extreme desiccation resistance of M. affine were an impermeable cuticle wherein activation energies (43 kJ/mol) were suppressed, daily water losses as little as 0.3%/day with an associated group effect, a low 64% water content and an impressive ability to survive nearly 3 months with no food and water. Behaviorally, the extended period of water stress and fasting was marked by long intervals of physical inactivity (quiescence), as though dead. These characteristics emphasizing water retention rather than gain are shared by G. aequinoctiale and reflect a typical xerophilic water balance profile. Water uptake was restricted to imbibing liquid, as evidenced by uptake of dye-stained droplets of free water and a critical equilibrium activity of 1.00a_v, where the inability to absorb water vapor from the air fails to equilibrate declining water levels (gain≠loss) except at saturation. Four-fold reduction in survival time within dry air and accelerated water loss rates with high activation energies for female adults of the closely related winged Prostephanus truncatus (Say) suggest that the enhanced water conservation of spider beetles is due, in part, to fusion of their elytra supplemented by entering into quiescence.     

Changes in the expression of four heat shock proteins during the aging process in Brachionus calyciflorus (rotifera)

Heat shock proteins (HSPs) are molecular chaperones and have an important role in the refolding and degradation of misfolded proteins, and these functions are related to aging. Rotifer is a useful model organism in aging research, owing to small body size (0.1–1 mm), short lifespan (6–14 days), and senescence phenotypes that can be measured relatively easily. Therefore, we used rotifer as a model to determine the role of four typical hsp genes on the aging process in order to provide a better understanding of rotifer aging. We cloned cDNA encoding hsp genes (hsp40, hsp60, hsp70, and hsp90) from the rotifer Brachionus calyciflorus Pallas, analyzed their molecular characteristics, determined its modulatory response under different temperatures and H₂O₂ concentrations and investigated the changes in expression of these genes during the aging process. We found that Bchsp70 mRNA expression significantly decreased with aging. In addition, we also studied the effects of dietary restriction (DR) and vitamin E on rotifer lifespan and reproduction and analyzed the changes in expression of these four Bchsp genes in rotifers treated with DR and vitamin E. The results showed that DR extended the lifespan of rotifers and reduced their fecundity, whereas vitamin E had no significant effect on rotifer lifespan or reproduction. Real-time PCR indicated that DR increased the expression of these four Bchsps. However, vitamin E only improved the expression of Bchsp60, and reduced the expression of Bchsp40, Bchsp70, and Bchsp90. DR pretreatment also increased rotifer survival rate under paraquat-induced oxidative stress. These results indicated that hsp genes had an important role in the anti-aging process.     

Response of Lymantria dispar L. (Lepidoptera: Lymantriidae) to Bacillus thuringiensis subsp. kurstaki at different ingested doses and temperatures

We examined mortality and feeding inhibition response of Lymantria dispar L. (Lepidoptera: Lymantriidae) larvae to ingested doses of Bacillus thuringiensis subsp. kurstaki as a function of dose, instar and temperature. We developed generalized (logistic) linear mixed models and a mixture survival model, commonly used in medical statistics, to analyze the complex data set. We conducted bioassays of Foray 48B with larvae from the NJSS laboratory stock, using droplet imbibing or force-feeding to ensure dose ingestion. The dose causing mortality in 50% of the test population (LD₅₀) under standard test conditions (22 °C) ranged from 0.019 International Units (IU)/larva for first instar larvae (L₁) to 1.6 IU/larva for L₄. Temperature affected larval mortality in two ways. Mortality occurred sooner and progressed more rapidly with increasing temperature (13-25 °C) at each dose level and instar, while the maximum level of mortality attained by each instar decreased with increasing rearing temperature. The mechanisms underlying this effect are being investigated. Larvae that survived exposure to B. thuringiensis resumed feeding after a period that was dependent on instar, dose, and temperature. The equations describing observed mortality and feeding recovery responses were used to construct a simulation model, which was able to predict both processes, and which forms the basis for a process-oriented model that can be used as a decision support tool in aerial sprays.     

Faster Metabolite 1H Transverse Relaxation in the Elder Human Brain

¹H magnetic resonance spectroscopy (MRS) is unique among imaging modalities because signals from several metabolites are measured during a single examination period. Each metabolite reflects a distinct intracellular process. Furthermore transverse (T₂) relaxation times probe the viability of the cell microenvironment, e.g., the viscosity of the cellular fluids, the microscopic susceptibility distribution within the cells, and the iron content. In this study, T₂s of brain metabolites were measured in the occipital lobe of eighteen young and fourteen elderly subjects at a field strength of 4 tesla. The T₂s of N-acetylaspartate, total creatine, and total choline were 23%, 16% and 10% shorter in elderly than in young subjects. The findings of this study suggest that noninvasive detection of T₂ provides useful biological information on changes in the cellular microenvironment that take place during aging.     

Aging-dependent modification of lipid composition and lipid structural order parameter of hepatic mitochondria

The effect of aging on the lipid composition of hepatic mitochondria has been determined using a rigorously defined group of Fischer 344 rats with known survivorship data. The age groups studied included mature adults as controls (8.5-month, 100% survivorship); an intermediate aged group (17.5-month, 90% survivorship); and an aged group (29-month, 20% survivorship). Lipid extracts of mitochondria were prepared using chloroform-methanol (2:1, by volume) and total phospholipid-P_i, cholesterol (free and esterfied), and phospholipid composition were determined. In the aged animals, total phospholipid-P_i decreased significantly (P = 0.019) whereas cholesterol increased (P = 0.048) with a progressive aging-dependent increase in the molar ratio of cholesterol/phospholipid. The lower total phospholipid content of hepatic mitochondria from the aged 29-month animals was due primarily to decreases in the major phospholipids with the most notable decrease being in cardiolipin (approximately 39%). Steady-state fluorescence polarization using 1,6-diphenyl-1,3,5-hexatriene as the probe was used to estimate the lipid structural order parameter of hepatic mitochondria. There was a highly significant (P = 0.01) aging-dependent increase in the lipid structural order parameter which correlated well with the increased molar ratio of cholesterol/phospholipid in the hepatic mitochondria isolated from the aged animals. The data suggest alterations in mitochondrial membrane lipid-protein interactions in aging and are consistent with the hypothesis of impairment of membrane function in the aging process.     

Aging-related decrease in hepatic cytochrome oxidase of the Fischer 344 rat

The effect of aging on the hepatic mitochondrial population has been determined using a rigorously defined group of Fischer 344 rats with known survivorship data. The age groups studied included mature adult controls (8.5 months; 100% survivorship), an intermediate aged group (17.5 months; 90% survivorship), and an aged group (29 months; 20% survivorship). Cytochrome oxidase activity and content were determined in homogenates and mitochondrial fractions. The mitochondrial fractions were characterized by determination of respiratory activity, and monoamine oxidase activity as well as evaluation of the polypeptide composition by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and two-dimensional electrophoresis. The yield of protein in the isolated mitochondrial fraction as well as the mitochondrial specific content decreased significantly as a function of aging. Mitochondrial specific content was determined from the specific activities of cytochrome oxidase in the homogenate (per gram liver) and in the isolated mitochondrial fraction (per mg protein). Specific activity of hepatic cytochrome oxidase decreased approximately 15% (P = 0.035) in homogenates from the 17.5-month animals with a further, highly significant (P = 0.0002) decrease (29%) in the 29-month animals. In contrast, there was no statistically significant difference among the age groups in the cytochrome oxidase specific activity in the isolated hepatic mitochondrial fractions. However, the percentage of the total homogenate cytochrome oxidase activity recovered in the isolated mitochondrial fraction decreased significantly in the 29-month animals (P = 0.0063 vs the 8.5-month controls; P = 0.022 vs the 17.5-month group). Cytochrome aa₃ content of total liver homogenates from aged animals decreased (P = 0.00064) which is in agreement with the decline in cytochrome oxidase specific activity in this age group. In the mitochondrial fraction from the aged animals, cytochrome aa₃ content was essentially unchanged which is consistent with the lack of aging-related change in mitochondrial cytochrome oxidase specific activity. In freshly isolated mitochondrial fractions, no aging-related alterations were observed in respiratory control and $\text{ADP}\text{O}$ ratios. The addition of exogenous NADH and cytochrome c did not change significantly the respiratory rate of hepatic mitochondria from control or aged animals. These results demonstrate the integrity of freshly isolated mitochondrial preparations from both control and aged Fischer 344 rats. In addition, there was no aging-related alteration in either monoamine oxidase specific activity or polypeptide composition. The similarities observed in the specific activities of cytochrome oxidase and monoamine oxidase, as well as in the cytochrome aa₃ content and polypeptide composition of the isolated mitochondrial fraction, suggest a generalized decrease in hepatic mitochondrial content as a function of aging rather than a selective loss of mitochondrial components.     

生物炭及其老化对农田NH3挥发及N2O排放的影响

生物炭具有减缓农田NH₃挥发和N₂O排放的重要潜力,但在施入环境后常常存在“老化”现象,这为其缓解全球变暖的长期有效性带来了不确定性。为了探明生物炭的长期效应,人工加速模拟了自然界中水分、温度、氧气、土壤矿物质及微生物多种老化因素,结合多元表征手段对比不同老化方式对生物炭性质的影响,利用主成分分析法建立新的生物炭性质综合指标来反映老化强度。再通过大田控制试验,采用原位通气法和静态箱—气相色谱法监测夏玉米生长周期内老化前后生物炭施用对农田NH₃挥发和N₂O排放的影响,为生物炭的可持续应用提供科学依据。结果表明,老化过程增加了原生物炭(BC)的氧含量、比表面积(S_BET)、总孔容(V_t)及含氧官能团数量,降低了灰分、碱性、碳含量、平均孔径及其芳香性,各老化作用强度排序为：氧化老化生物炭(OBC)>矿化老化生物炭(KBC)>微生物老化生物炭(MBC)>干湿循环老化生物炭(WBC)>冻融循环老化生物炭(FBC)>BC。生物炭的添加减少...     

Abscission: the phytogerontological effects of ethylene

The role of ethylene in the aging of bean (Phaseolus vulgaris L. cv. Red Kidney) petiole abscission zone explants was examined. The data indicate that ethylene does accelerate aging in addition to inducing changes in break strength. Application of ethylene during the aging stage (stage 1) promoted abscission when followed by a second ethylene treatment during the cell separating stage (stage 2). The half-maximal effective concentration of ethylene to induce aging was around 0.3 microliter per liter; 10 microliters per liter was a saturating dose. CO₂ reversal of ethylene action during stage 1 was incomplete and gave ambiguous results. CO₂ (10%) reversed the effect of 10 microliters per liter ethylene but not 1 microliter per liter ethylene. The possibility that ethylene not only accelerated aging but was also a requirement for it was tested, and experimental evidence in favor of this idea was obtained. It was concluded that ethylene plays a dual role in the abscission of bean petiole explants: a phytogerontological effect and a cellulase-inducing effect.     

Foraging profits and thoracic temperature of honey bees (Apis mellifera)

Summary Body temperature and duration of foraging activities were affected by the concentration of sucrose solution imbibed. When experienced foragers of Apis mellifera arrived at a gravity feeder from the hive, thoracic temperature (T_TH) was independent of sucrose concentration (X = 36.3 °C). While imbibing 40% and 60% (g solute per g of solution) solutions bees maintained T_TH at approximately the same high level as upon arrival, but those imbibing 10%, 20%, and 30% solutions regulated T_TH lower (X = 33.5 °C). All bees departed the feeder for the hive at the same T_TH (X = 36.1 °C). Bees that imbibed 40% and 60% solutions sometimes immediately took flight after imbibition and averaged less than 15 s to takeoff. Time to takeoff was 2–3 times longer for bees that had imbibed 10% and 20% solutions because warmup preceded takeoff. The rate of energy expenditure at T_TH=36.3°C (at 40% and 60% solutions) was 20% greater than that at 33.3°C (at 10%, 20%, and 30% solution). Bees that fed on the highly concentrated solutions regulated T_TH so that rate of net energy gain was enhanced, but bees that fed on less concentrated solutions could have increased rate of net gain by maintaining a higher T_TH which would have reduced time required for takeoff. The latter bees lowered rate of expenditure of their limited energetic costs and thereby lowered short-term net profits in favor of improved long-term contribution to the colony.Abbreviations T _A ambient temperature - T _TH thoracic temperature     

Age-related renal disease in female Dahl salt-sensitive rats is attenuated with 17²-estradiol supplementation by modulating nitric oxide synthase expression

Background: The incidence of chronic renal disease in women increases with aging, especially after menopause, suggesting that loss of sex hormones may contribute to the development and progression of renal disease. However, the mechanisms by which sex hormones, particularly estrogens, contribute to the disease process are unclear.Objective: The present study examined the effects of ovariectomy (OVX) with or without 17²-estradiol (E₂) supplementation (OVX+E₂) on the expression of inducible (iNOS) and endothelial (eNOS) nitric oxide synthase in the kidney.Methods: The study was performed in young (4 months [4M]) and aged (12 months [12M]) female Dahl salt-sensitive rats fed a low-sodium (0.1% NaCl) diet. At 3 months of age, the animals were either subjected to sham surgery, OVX, or OVX with implantation of an E₂ silastic pellet. The treatments were administered for either 1 or 9 months, rendering the animals 4 months of age or 12 months of age at the time of sacrifice, respectively. Renal expression of NOS isoforms was measured by Western blotting and immunohistochemistry.Results: OVX in the aged rats was associated with 35% and 25% decreases in medullary iNOS (mean [SEM] relative optical density [ROD]: 4M OVX, 1.81 [0.14] vs 12M OVX, 1.17 [0.16]; P < 0.05) and eNOS (mean ROD: 4M OVX, 1.91 [0.09] vs 12M OVX, 1.43 [0.15]; P < 0.05) protein expression, respectively, and a 25-fold increase in the abundance of CD68-positive cells, indicating macrophage infiltration (mean cells/mm²: 4M OVX, 1.18 [0.09] vs 12M OVX, 30.0 [0.74]; P < 0.001). E₂ supplementation either partially or completely attenuated these changes in iNOS (mean ROD: 4M OVX+E₂, 2.26 [0.08] vs 12M OVX+E₂, 1.70 [0.09]; P < 0.05), eNOS (mean ROD: 4M OVX+E₂, 2.03 [0.07] vs 12M OVX+E₂, 1.77 [0.11]; P = NS) and CD68 (mean cells/mm²: 4M OVX+E₂, 1.46 [0.07] vs 12M OVX+E₂, 6.87 [1.6]; P < 0.01) associated with OVX in the aging kidney.Conclusions: These data suggest that ovarian E₂ loss with aging may contribute to the development of age-related renal disease through downregulation of iNOS and eNOS protein abundance and increased renal inflammation in this animal model. Furthermore, E2 supplementation may be protective in the aging kidney by attenuating these changes.     

Ethanol and acetaldehyde in imbibing soybean seeds in relation to deterioration

Deterioration as evidenced by decline in germination or seedling growth of soybean (cv. Essex) seeds during accelerated aging treatments at 41 C and 100% relative humidity is accompanied by increased levels of acetaldehyde and ethanol in imbibing embryonic axes and seeds. These increases become more pronounced with duration of the aging treatment. A similar inverse relationship between levels of acetaldehyde and ethanol and deterioration was observed when seeds were “naturally” aged for several years. During imbibition of low-vigor, accelerated-aged seeds at 25 C, acetaldehyde and ethanol increased from near trace amounts in dry tissues to maximum levels at 4 hours. Increases in acetaldehyde and ethanol during imbibition were less in high- than in low-vigor seeds. Increases were also less pronounced in low-vigor seeds when water uptake injury was avoided by osmotically decreasing water uptake rate with 30% polyethylene glycol. Embryonic axes from deteriorated seeds were characterized by low rates of O₂ uptake and high respiratory quotients relative to the unaged controls. Anaerobic conditions and respiratory inhibitors, such as sodium azide, increased acetaldehyde and ethanol in unaged seeds to levels similar to those in accelerated-aged seeds after 2 hours imbibition. It is suggested that, during aging, an imbalance between tricarboxylic and glycolytic activities, present during early imbibition to some degree even in vigorous unaged seeds, becomes more pronounced and leads to accumulation of ethanol and acetaldehyde.     

Molecular mobility studies on the amorphous state of disaccharides. I—thermally stimulated currents and differential scanning calorimetry

The relaxational processes in amorphous solid gentiobiose and cellobiose are studied by thermally stimulated depolarization currents (TSDC) in the temperature region from 108 K up to 423 K. The slow molecular mobility was characterized in the crystal and in the glassy state. The features of different motional components of the secondary relaxation have been monitored as a function of time as the glass structurally relaxes on aging. It is concluded that some modes of motion of this mobility are aging independent, while others are affected by aging. The value of the steepness index or fragility (T_g-normalized temperature dependence of the relaxation time) was obtained by differential scanning calorimetry (DSC) from the analysis of the scanning rate dependency of T_g.     

Effect of laser irradiation on microviscosity of aqueous medium in imbibing maize seeds as studied with a spin probe method

A spin probe method was used to study microviscosity of aqueous medium in embryos and endosperm of maize (Zea mays L.) seeds after their irradiation with a Lvov-1 Elektronika laser device. Based on parameters of electron spin resonance (ESR) spectra of nitroxyl radicals (probes) absorbed by imbibing seeds during water uptake, correlation times τ_C were determined for the rotational diffusion of probes in embryos and endosperm of seeds. The τ_C values were found smaller in embryos of irradiated seeds than in untreated seeds; the dependence of τ_C on the duration of seed imbibition was determined. It is concluded that laser irradiation of seeds decreases the microviscosity of aqueous medium in embryo cells and elevates the mobility of the probes. Effect of laser irradiation on τ_C in seed endosperm was less pronounced but also led to the increase in probe mobility.     

Confounding of alternate respiration by lipoxygenase activity

The initial burst of respiratory activity (Q_o2) of imbibing soybean (Glycine max [L.] Merr. var. Wayne) seed tissue is cyanide-insensitive, and sensitive to salicylhydroxamate: presumptive evidence for the presence of alternate respiration. The initial O₂ consumption is also highly sensitive to propyl gallate. Soybean lipoxygenase exhibits similar characteristics of insensitivity to cyanide and sensitivity to salicylhydroxamate and to propyl gallate. The initial burst of respiration is enhanced by the addition of linoleic acid, a lipoxygenase substrate. These results indicate that the conventional tests for alternate respiration in plant tissues can be confounded by lipoxygenase; they also suggest that propyl gallate can be used to assess the possible participation of lipoxygenase in the O₂ uptake by plant tissues.     

Combined In Silico,In Vivo,and In Vitro Studies Shed Insights into the Acute Inflammatory Response in Middle-Aged Mice

We combined in silico, in vivo, and in vitro studies to gain insights into age-dependent changes in acute inflammation in response to bacterial endotoxin (LPS). Time-course cytokine, chemokine, and NO₂ ^−/NO₃ ⁻ data from “middle-aged” (6–8 months old) C57BL/6 mice were used to re-parameterize a mechanistic mathematical model of acute inflammation originally calibrated for “young” (2–3 months old) mice. These studies suggested that macrophages from middle-aged mice are more susceptible to cell death, as well as producing higher levels of pro-inflammatory cytokines, vs. macrophages from young mice. In support of the in silico-derived hypotheses, resident peritoneal cells from endotoxemic middle-aged mice exhibited reduced viability and produced elevated levels of TNF-α, IL-6, IL-10, and KC/CXCL1 as compared to cells from young mice. Our studies demonstrate the utility of a combined in silico, in vivo, and in vitro approach to the study of acute inflammation in shock states, and suggest hypotheses with regard to the changes in the cytokine milieu that accompany aging.     

Mitochondrial alterations and apoptosis in smooth muscle from aged rats

We studied changes in mitochondrial morphology and function in the smooth muscle of rat colon. Under confocal microscopy, tissues loaded with potentiometric dye displayed rapid and spontaneous depolarization. Cyclosporin A (CsA), inhibitor of the permeability transition pore (PTP), caused an increase in mitochondrial membrane potential (ΔΨ_m) in tissues from adult young animals. In aged rats these changes were not observed. This suggests that physiological activation of PTP in aged rats is reduced. Electron microscopy showed alterations of the mitochondrial ultrastructure in tissues from aged rats involving a decreased definition of the cristae and fragmentation of the mitochondrial membranes. We also detected an increase in apoptotic cells in the smooth muscle from aged animals. Our results show that the aging process changes PTP activity, the ability to maintain ΔΨ_m and mitochondrial morphology. It is suggested that these can be associated with mitochondrial damage and cell death.     

Biochemical Studies on Development of Mitochondria in Pea Cotyledons during the Early Stage of Germination: Effects of Antibiotics on the Development

l-Leucine-U-¹⁴C was incorporated into mitochondrial protein in pea (Pisum sativum var. Alaska) cotyledons during the imbibing stages. Incorporation was almost completely inhibited by cycloheximide but not by chloramphenicol. Both antibiotics did not affect increases in mitochondrial activities and components of the cotyledons during imbibition. Therefore, mitochondrial development seems to be achieved by a transfer of protein pre-existing in the cytoplasm into the mitochondria rather than by de novo synthesis of mitochondrial protein. Cycloheximide stimulated an increase in bile saltsoluble protein of mitochondria in imbibing pea cotyledons. The recovery of cytochrome oxidase activity after sucrose density gradient centrifugation was enhanced, and the morphological properties of mitochondria were altered by cycloheximide.     

Aging of TiO2 Nanoparticles Transiently Increases Their Toxicity to the Pelagic Microcrustacean Daphnia magna

During their aquatic life cycle, nanoparticles are subject to environmentally driven surface modifications (e.g. agglomeration or coating) associated with aging. Although the ecotoxicological potential of nanoparticles might be affected by these processes, only limited information about the potential impact of aging is available. In this context, the present study investigated acute (96 h) and chronic (21 d) implications of systematically aged titanium dioxide nanoparticles (nTiO₂; ~90 nm) on the standard test species Daphnia magna by following the respective test guidelines. The nTiO₂ were aged for 0, 1, 3 and 6 d in media with varying ionic strengths (Milli-Q water: approx. 0.00 mmol/L and ASTM: 9.25 mmol/L) in the presence or absence of natural organic matter (NOM). Irrespective of the other parameters, aging in Milli-Q did not change the acute toxicity relative to an unaged control. In contrast, 6 d aged nTiO₂ in ASTM without NOM caused a fourfold decreased acute toxicity. Relative to the 0 d aged particles, nTiO₂ aged for 1 and 3 d in ASTM with NOM, which is the most environmentally-relevant setup used here, significantly increased acute toxicity (by approximately 30%), while a toxicity reduction (60%) was observed for 6 d aged nTiO₂. Comparable patterns were observed during the chronic experiments. A likely explanation for this phenomenon is that the aging of nTiO₂ increases the particle size at the start of the experiment or the time of the water exchange from <100 nm to approximately 500 nm, which is the optimal size range to be taken up by filter feeding D. magna. If subjected to further agglomeration, larger nTiO₂ particles, however, cannot be retained by the daphnids’ filter apparatus ultimately reducing their ecotoxicological potential. This non-linear pattern of increasing and decreasing nTiO₂ related toxicity over the aging duration, highlights the knowledge gap regarding the underlying mechanisms and processes. This understanding seems, however, fundamental to predict the risks of nanoparticles in the field.