Accumulation Of 32P Research Articles

Relative comparison of tissue specific bioaccumulation and radiation dose estimation in marine and freshwater bivalve molluscs following exposure to phosphorus-32

With respect to environmental protection, understanding radionuclide bioconcentration is necessary to relate exposure to radiation dose and hence to biological responses. Few studies are available on tissue specific accumulation of short-lived radionuclides in aquatic invertebrates. Short-lived radionuclides such as 32Phosphorus (32P), although occurring in small quantities in the environment, are capable of concentrating in the biota, especially if they are chronically exposed. In this study, we firstly compared tissue specific bioaccumulation and release (depuration) of 32P in adult marine (Mytilus galloprovincialis, MG) and freshwater bivalve molluscs (Dreissena polymorpha, DP). Secondly, using the Environmental Risk from Ionising Contaminants Assessment and Management (ERICA) tool, we calculated tissue specific doses following determination of radionuclide concentration. Marine and freshwater bivalves were exposed for 10 days to varying 32P concentrations to acquire desired whole body average dose rates of 0.10, 1.0 and 10 mGy d−1. Dose rates encompass a screening dose rate value of 10 μGy h−1 (0.24 mGy d−1), in accordance with the ERICA tool. This study is the first to relate tissue specific uptake and release (via excretion) of 32P from two anatomically similar bivalve species. Results showed highly tissue specific accumulation of this radionuclide and similarity of accumulation pattern between the two species. Our data, which highlights preferential 32P accumulation in specific tissues such as digestive gland, demonstrates that in some cases, tissue-specific dose rates may be required to fully evaluate the potential effects of radiation exposure on non-human biota. Differential sensitivity between biological tissues could result in detrimental biological responses at levels presumed to be acceptable when adopting a ‘whole-body’ approach.

Ca2+ Signaling Occurs via Second Messenger Release from Intraorganelle Synthesis Sites

SummaryCyclic ADP-ribose is an important Ca2+-mobilizing cytosolic messenger synthesized from β-NAD+ by ADP-ribosyl cyclases (ARCs). However, the focus upon ectocellular mammalian ARCs (CD38 and CD157) has led to confusion as to how extracellular enzymes generate intracellular messengers in response to stimuli. We have cloned and characterized three ARCs in the sea urchin egg and found that endogenous ARCβ and ARCγ are intracellular and located within the lumen of acidic, exocytotic vesicles, where they are optimally active. Intraorganelle ARCs are shielded from cytosolic substrate and targets by the organelle membrane, but this barrier is circumvented by nucleotide transport. We show that a β-NAD+ transporter provides ARC substrate that is converted luminally to cADPR, which, in turn, is shuttled out to the cytosol via a separate cADPR transporter. Moreover, nucleotide transport is integral to ARC activity physiologically because three transport inhibitors all inhibited the fertilization-induced Ca2+ wave that is dependent upon cADPR. This represents a novel signaling mechanism whereby an extracellular stimulus increases the concentration of a second messenger by promoting messenger transport from intraorganelle synthesis sites to the cytosol.

Contributions of pro‐oxidant and anti‐oxidant conditions to the actions of 24,25‐dihydroxyvitamin D3 and 1,25‐dihydroxyvitamin D3 on phosphate uptake in intestinal cells

The steroid hormone 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] rapidly stimulates the uptake of phosphate in isolated chick intestinal cells, while the steroid 24,25-dihydroxyvitamin D3 [24,25(OH)2D3] inhibits the rapid stimulation by 1,25(OH)2D3. Earlier work in this laboratory has indicated that a cellular binding protein for 24,25(OH)2D3 is the enzyme catalase. Since binding resulted in decreased catalase activity and increased H2O2 production, studies were undertaken to determine if pro-oxidant conditions mimicked the inhibitory actions of 24,25(OH)2D3, and anti-oxidant conditions prevented the inhibitory actions of 24,25(OH)2D3. An antibody against the 24,25(OH)2D3 binding protein was found to neutralize the inhibitory effect of the steroid on 1,25(OH)2D3-mediated 32P uptake. Incubation of cells in the presence of 50 nM catalase was also found to alleviate inhibition. In another series of experiments, isolated intestinal epithelial cells were incubated as controls or with 1,25(OH)2D3, each in the presence of the catalase inhibitor 3-amino-1,2,4-triazole, or with 1,25(OH)2D3 alone. Cells exposed to hormone alone again showed an increased accumulation of 32P, while cells treated with catalase inhibitor and hormone had uptake levels that were indistinguishable from controls. We tested whether inactivation of protein kinase C (PKC), the signaling pathway for 32P uptake, occurred. Incubation of cells with phorbol-13-myristate (PMA) increased 32P uptake, while cells pretreated with 50 microM H2O2 prior to PMA did not exhibit increased uptake. Likewise, PMA significantly increased PKC activity while cells exposed to H2O2 prior to PMA did not. It is concluded that catalase has a central role in mediating rapid responses to steroid hormones.

Evaluation of the biological transfer of 32P, 137Cs and 65Zn by fish in the Yenisei River

The biological transfer of three radionuclides (32P, 137Cs, 65Zn) by fish in the Yenisei River (Central Siberia, Russia) was evaluated using a radioecological model. The modelling is based on the general ECOMOD methodology, where radionuclide behavior in an aquatic organism is linked with the processes of growth and metabolism, also with the concentrations of stable analogous elements in the organism, its food and the environment. The model was applied to explain the peculiarities of 32P, 137Cs and 65Zn accumulation in different ecological groups of fish, including non-migratory and migratory fish, non-predatory and predatory fish species. The highest activity concentrations in non-migratory fish from the Yenisei River were found for 32P. The accumulation of 32P by fish was shown to depend on the fish size (age, weight); however, it did not depend on the trophic status of fish. The modelling approach was developed to evaluate the biological transfer of radionuclides by the migratory fish, which spend the most part of life in the Yenisei delta, inlet or bay, and go upstream the Yenisei River for spawning. The results of the ECOMOD model calculations are in good agreement with available measurement data.

Evaluation of the biological transfer of 32P, 137Cs and 65Zn by fish in the Yenisei River

The biological transfer of three radionuclides ( 32P, 137Cs, 65Zn) by fish in the Yenisei River (Central Siberia, Russia) was evaluated using a radioecological model. The modelling is based on the general ECOMOD methodology, where radionuclide behavior in an aquatic organism is linked with the processes of growth and metabolism, also with the concentrations of stable analogous elements in the organism, its food and the environment. The model was applied to explain the peculiarities of 32P, 137Cs and 65Zn accumulation in different ecological groups of fish, including non-migratory and migratory fish, non-predatory and predatory fish species. The highest activity concentrations in non-migratory fish from the Yenisei River were found for 32P. The accumulation of 32P by fish was shown to depend on the fish size (age, weight); however, it did not depend on the trophic status of fish. The modelling approach was developed to evaluate the biological transfer of radionuclides by the migratory fish, which spend the most part of life in the Yenisei delta, inlet or bay, and go upstream the Yenisei River for spawning. The results of the ECOMOD model calculations are in good agreement with available measurement data.

Phosphorus uptake by bean nodules

Phosphate (P) uptake experiments were conducted to determine the routes of P entry into nitrogen-fixing common bean root nodules and the availability of this P to the bacteroids within the nodules. Nodulated common bean plants were cultured for 30 days in a hydroponic system that contained either a complete nutrient solution (+P) or a nutrient solution where P was not added (−P). The plant roots were then exposed to [32P]-phosphate-labeled +P nutrient solution and P accumulation in bacteroids and nodules was measured. For labeling, plants were suspended in the nutrient solution so that the root system was either fully or partially submerged. Because bean nodules are heavily clustered in the root crown, most of the nodules in the partial submergence treatments were not in contact with the nutrient solution and we could study nodules that had not been directly exposed to 32P. Regardless of P treatment of the host plant, P accumulation in bacteroids was significantly greater and more rapid when the nodule surface was in contact with the labeled nutrient solution, suggesting that nodules can import P from the solution. P entry through the nodule surface was verified in experiments where 32P-labeled nutrient solution was applied to the nodule surface only. In full submergence treatments, 32P accumulation in bacteroids of −P plants was significantly greater than in bacteroids of +P plants. As measured by total leaf label, uptake and translocation of P to the shoot was greater in −P plants and still greater in partial submergence treatments where most of the nodules were not in contact with the labeled nutrient solution. Results of these experiments suggest that, in addition to P acquisition via the vascular system, P flow to symbiotic tissues and bacteroids can originate from the soil solution immediately surrounding the nodule. This was particularly evident in nodules of plants that had been limited for P.

The uptake of radioactive phosphorus by brown trout ( Salmo trutta L.) from water and food

Brown trout were exposed to 32P in their tank water (7.4 Bq l −1) and the uptake to muscle followed over 6 weeks. A steady-state concentration factor (C ss) of 1.7 and a biological half-time for clearance ( t b1 2 ) of 45 days were calculated from the results. The low C ss indicates that uptake from water is not a major route of 32P accumulation in these fish. Brown trout were given a single 32P-spiked meal, and the uptake and clearance in muscle, liver and blood followed over 6 weeks. Assimilation of the isotope by these organs was low, the maximum activity in whole muscle reaching only 0.2–0.4% of that in the meal with lower values in the other two organs. There was no appreciable clearance of 32P from muscle during the experiment. The slow clearance of 32P after uptake from water and lack of any clearance after uptake from food indicates that the C ss for this isotope following uptake by either route is likely to depend on radioactive decay rather than intake rate and physiological clearance.

Seed source variation in 32P uptake in inus taeda L. seedlings: A possible regulation by efflux

Short-term 32P uptake experiments were conducted with intact seedlings of loblolly pine (Pinus taeda L.) to examine possible seed source variation in net accumulation of 32P in roots and shoots, and in rates of unidirectional influx. Seed source had a highly significant effect on biomass and P concentrations of shoots and roots. Seedlings from two seed sources representing fast-growing populations (a broadly-adapted and wet-site seed source) accumulated over 60% more total seedling P than smaller seedlings from a drought-hardy seed source, reflecting higher biomass and root P concentrations. Rates of unidirectional 32P influx in seedlings from the drought-hardy seed source were more than twice the rates of the seedlings from the broadly-adapted seed source. However, after 24 h in labeled uptake solution, net accumulation of 32P was similar, suggesting that rates of unidirectional efflux from roots of the drought-hardy seed source were also high. Although there were no significant differences in biomass and tissue P concentrations between the two fast-growing seed sources, rates of unidirectional influx in seedlings from the broadly-adapted seed source were 42% lower than rates in seedlings from the wet-site source. Yet, after 24 h in labeled uptake solution, net accumulation of 32P in seedlings from the broadly-adapted seed source was 50% higher. Unidirectional efflux out of the root may regulate net uptake of P as much, if not more, than influx in loblolly pine seedlings-at least under high-P growth conditions. The results in this study do not support previous studies with herbaceous plants suggesting that fast-growing species typically exhibit higher rates of nutrient uptake than slow-growing species.

Maintenance of 32P uptake and transport in Pinus serotina seedlings under hypoxic growth conditions

In the present study, we examined the effects of long‐ and short‐term hypoxia on net uptake and transport of phosphorus to shoots of pond pine (Pinus serotina Michx.), a moderately flood‐tolerant southern pine, and the influence aerenchyma formation might have in maintenance of P uptake and transport. Seedlings were grown under aerobic (250 μM O2) or hypoxic (≤50 μM O2) solution conditions for 5.3 weeks in continuously flowing solution culture containing 100 μM P. Intact seedlings were then labeled with 32P for up to 24 h to determine how short‐ and long‐term hypoxic solution conditions affected rates of unidirectional influx and the accumulation of 32P in roots and shoots. Seedlings in the long‐term hypoxic treatment were grown for 5.3 weeks in hypoxic solution and also labeled in hypoxic uptake solution. The short‐term hypoxic treatments included a 24‐h hypoxic pretreatment followed by time in labeled hypoxic uptake solution for seedlings grown under aerobic or hypoxic conditions; in the latter case, diffusion of atmospheric O2 entry into stem and root collar lenticels was blocked, thus removing any influence that aerenchyma formation might have had on enhancing O2 concentrations of root tissue. Although unidirectional influx rates of 32P in roots of seedlings grown under long‐term hypoxic conditions were 1.4 times those of aerobically grown seedlings, accumulation of 32P in roots was similar after 24 h in labeled uptake solution. These results suggest that 32P efflux was also higher under hypoxic conditions. Higher shoot/root fresh weight ratios and lower shoot P concentrations in seedlings grown under hypoxic solution conditions suggest that the “shoot P demand” per unit root should be high. Yet accumulation of 32P in shoots was reduced by 50% after 24 h in hypoxic uptake solution. Both short‐term hypoxic treatments decreased accumulation of 32P in roots by more than 50%. Short‐term hypoxia decreased shoot accumulation in seedlings grown under aerobic and hypoxic conditions by 84 and 50%. respectively. Short‐ and long‐term hypoxic conditions increased the percentage of root 32P in the nucleic acid and chelated‐P pools, resulting in a significantly smaller percentage of 32P in the soluble inorganic phosphate (pi) pool, the pool available for transport to the shoot. However, a reduction in pool size or in labeling of the pool available for transport cannot fully account for the large reduction in accumulation of 32P in shoots, particularly in the short‐term hypoxic treatment of aerobically grown seedlings. Our results suggest that both influx and transport of 32P to shoots of pond pine seedlings are O2‐dependent processes, and that the transport of 32P to shoots may be more sensitive to hypoxic solution conditions than influx at the cortical and epidermal plasmalemma, with aerenchyma formation supporting a substantial amount of both 32P uptake and transport.

32P uptake and transport to shoots in Pinuus serotina seedlings under aerobic and hypoxic growth conditions

We examined the effects o long‐term hypoxic growth conditions on net uptake and transport of P to shoots of pond Pine (Pinus serotina Michx.), a moderately flood‐tolerant southern pine. Seedlings were grown under aerobic orhypoxic solution conditions for 4–5 weeks in continuously flowing solution culture containing 100 μM P. Short – and long‐term 32P. experiments were then concluded with intact seedlings to determine rates of 32P influx, efflux and net transport to the shoot. Shoot fresh weight/root fresh weight ratios were significantly higher under hypoxic gorwth conditions, reflecting the larger reduction in root growth than shoot growth, despite extensive aerechyma formation in roots. Estimates for the unidirectional influx of 32P in aerobic and hypoxic seedlings were 1.43 and 3.20 μmol P (gFW root)−1 h−1, respectively. However, 32P accumulation between the two treatments became similar within 8 h, suggesting that efflux was also higer in seedlings from the hypoxic treatment. Indeed in a separate experiment, hypoxic growth conditions increased efflux by over 60%. Transport of 32P to shoots was significantly reduced under hypoxic growth conditions, despite higher root P concentrations and lower shoot P concentrations. After 48 h, 32P accumulation in roots was similar between the two treatments. Yet total accumulation of seedling 32P decrcased by 31% under the hypoxic treatment, largely because of reduced transport of 32p to the shoot. The lower accumulation of 32 by shoots of seedlings in the hypoxic treatment may be the result of a direct inhibition on the transport process in O2‐defident tissues, but could also reflect a slower turnover or labeling of the ool available for transport. Indeed, the percentage of total 32P in. roots present in the soluble P. (or transportable form of P) was about 33% lower in seedlings from the hypoxic treatment, probably reflecting increased assimilation into organic compounds as well as chelation with iron. Our results suggest that P transport to the shoots of acclimated seedlings may be more sensitive to hypoxic solution conditions than influx at the root Plasmalemma.

Distribution of intraperitoneally injected microspheres labeled with the α-emitter astatine ( 211At) compared with phosphorus ( 32P) and yttrium ( 90Y) colloids in mice

The α-emitter 211At was bound to polymer microspheres with a diameter of 1.8 μm. The distributions in mice of intraperitoneally injected 211At microspheres, 90Y silicate colloid, and 32P chromic phosphate colloid were compared. The microspheres with 211At spread rapidly in the peritoneal cavity and remained mainly on the intraperitoneal surfaces. Intraperitoneal injection of 90Y colloid resulted in high levels in intraperitoneal fat and the diaphragm, but 1 day after injection 8.5% of the injected dose per gram was found in blood and after 6 days 2.5% was observed in bone. The highest accumulation of 32P was found in liver and spleen. The injection of additional nonradioactive chromic phosphate colloid resulted in an even higher accumulation of 32P in spleen and liver. The same phenomenon was not observed with 211At microspheres. It is suggested that it is not only the particle size which is important in the distribution of intraperitoneally injected colloid, but the amount of colloid, the type of colloid, the addition or presence of other substances such as ascites, and the animal species might also influence the distribution. In conclusion, the intraperitoneal distribution of 211At-labeled microspheres in mice was favorable compared with 90Y and 32P colloid. These data must be viewed cautiously since the distribution might be different in other animal species or humans.

The Effects of Phosphorus and Magnesium on Sclerotium Formation in Rhizoctonia solani Kuehn.

None or very few sclerotia of Rhizoctonia solani (AG-1, IA) were formed on Hopkins agar medium without KH2PO4, though hyphae developed in some extent. While, the number and weight of sclerotia were decreased when MgSO4 was absent. As KH2PO4 in the basal medium was replaced with other phosphates different in cation, sclerotia were well formed. The optimal concentration of phosphate as KH2PO4 for the sclerotium formation, in number and weight, and hyphal development was in the region of 100ppm. The addition of magnesium resulted in the increase of the number and weight of sclerotia proportionally with the amount of the magnesium compound. The results indicate that phosphorus ion is indispensable to the sclerotium formation, whereas magnesium ion is promotive. The absence of phosphorus at the maturing stage resulted in the formation of few sclerotia, even though phosphorus was presented at the hyphal and initial phases. While defficiency of phosphorus in the early stage of sclerotium development showed no effect on the number and weight of sclerotia formed, if the phosphorus was supplied in subsequent periods. The branching internodes of hyphae grown on the phos-phorus free medium were longer than those of on the medium containing phosphorus. This decrease of the hyphal branching will lead to the decrease of interweaving hyphae and will result in the scarce sclerotium formation. A large accumulation of 32P to sclerotia was strikingly observed in this experiment.

Bioaccumulation factor for 32P measured in bluegill, Lepomis macrochirus, and catfish, Ictalurus punctatus.

The ratio of the bioaccumulation factors for 32P and phosphorus was determined for edible tissue in two species of freshwater fish by measuring the specific activity (32P activity per milligram phosphorus) in muscle relative to feed. The 32P tracer was added to the feed at a uniform level throughout the study. Feeding was at two levels: ad libitum and at a lower but constant intake per body weight. In the main experiment, bluegill were maintained in a large flow-through tank and sacrificed at approximately weekly intervals for 51 d of 32P accumulation and 28 d of depuration to compare the specific activity with values predicted with a calculational model. In experiments performed in smaller aquaria, the specific activity in bluegill and catfish muscle was compared at two feeding levels and two temperatures. In addition, unfed fish were exposed to 32P in water at a known specific activity to determine the extent of phosphorus uptake directly from water. The pattern of specific activity increase and decrease in fish muscle during the accumulation/depuration experiment was consistent with a one-compartment model, so that specific activity ratios at steady state could be predicted from measurements during relatively brief exposures. On this basis, the ratio of the bioaccumulation factors of 32P and phosphorus in fish feeding ad libitum was 0.081 for bluegill and 0.17 for catfish. Hence, at a mean phosphorus bioaccumulation factor of 70,000, the factors for 32P are 6,000 and 12,000, respectively. The ratios were less at lower phosphorus intakes associated with lower feeding rates; moreover, the lesser value for bluegill occurred at a much lower phosphorus intake than by catfish. The bioaccumulation factor ratio was lower by an order of magnitude at a water temperature of 11 degrees C than at 16-27 degrees C, and was lower by two orders of magnitude when phosphorus uptake was from water by unfed fish.

Effect of aluminum on 32P uptake and translocation by red spruce seedlings

32P uptake and translocation was simultaneously studied in 11-week-old red spruce (Picearubens Sarg.) seedlings that had been pretreated for 24 h in hydroponic culture with or without 10 mg Al L−1. Aluminum in solution enhanced the accumulation of 32P in roots by more than two orders of magnitude over uptake in control seedlings, while the translocation of 32P to apical buds and mature primary needles was reduced to 20.0 and 9.1% of control levels, respectively. Kinetic analysis of 32P uptake indicated that P absorption is mediated by an uptake system exhibiting Michaelis–Menten behavior from solutions ranging from 0.01 to 5.00 mM KH2PO4; uptake at 10.0 mM KH2PO4 was inconsistent with this model. Changes in the patterns of 32P uptake and translocation in seedlings treated with Al suggest that Al in solution altered accumulation processes.

5-Methyltryptamine decreases net accumulation of 32P into the polyphosphoinositides from [gamma-32P]ATP in a cell-free system from blowfly salivary glands. Activation of breakdown of the newly synthesized [32P]polyphosphoinositides.

Incubation of blowfly salivary gland homogenates with 30 microM [gamma-32P]ATP resulted in a rapid, Mg2+-dependent, synthesis of [32P]polyphosphoinositides and [32P]phosphatidic acid. 5-Methyltryptamine, in the presence of 10 microM guanosine 5'-(3-O-thio)trisphosphate, reduced the net accumulation of 32P label into phosphatidylinositol-4,5-P2 and phosphatidylinositol-4-P by 35 and 20%, respectively. 5-Methyltryptamine did not affect synthesis of [32P]phosphatidic acid. Phosphorylation of polyphosphoinositides was not affected by 5-methyltryptamine. In membranes labeled in vitro with [gamma-32P]ATP, 5-methyltryptamine stimulated a rapid breakdown of the [32P]polyphosphoinositides. These results indicate that in blowfly salivary gland homogenates, hormone stimulates breakdown of the newly synthesized polyphosphoinositides. In the presence of hormone, the rate of polyphosphoinositide synthesis does not compensate for the rate of polyphosphoinositide degradation.

Distribution of radioactivity after intra-peritoneal administration of32P colloids

The data presented by Mr. Young are interesting supplements to our previous report on the intraperitoneal distribution of 32P-labelled colloids (Boye et al, 1984). There are some differences in the procedures which may make direct comparisons between the two studies difficult. We frequently find that scintigrams taken 24 h after instillation of 32P show a homogeneous distribution of activity while the distribution after 7 days gives evidence of 32P accumulation in certain areas.

Interactions of various cations on centripetal passage of phosphorus across intact roots

Abstract The interacting effects of NB4, Mg, and K on P nutrition in intact bean plants was studied. The NH4 and Mg treatments increased lateral passage of 32P across the root to the xylem (Centripetal Passage (CP)), while K had no effect. The changes in CP were accurately reflected in accumulation of 32P in the plant top. Centripetal Passage was specifically identified as the site where NH4 and Mg interacted with P. Evidence was given to suggest that these interactions occurred during the initial step of the CP process‐transport across the plasmalemma ‐ rather than in the later stage of symplast transport and trans‐membrane efflux.

Pathway and regulation of phosphate translocation to the pods of soybean explants

We investigated the degree to which developing fruit compete directly with leaves for mineral nutrients, e.g. phosphate coming up from the roots. When soybean (Glycine max (L.) Merrill cv. Anoka) explants cut at mid‐late podfill were given a 15‐min pulse of 32Pi via the cut stem and then transferred to distilled water, 75% of the 32P accumulated in the leaves and 21% in stem and petiole during the first hour. The amount of 32P entering the seeds was low (1%) initially, but thereafter increased to 30% in 48 h. An accumulation of 32P in the seed coats preceded its entry into the embryos. Disruption (with hot steam) of the phloem between the leaf and the pods after pulse labelling indicated that more than 80% of the 32Pi pulse moved to the leaf before redistribution to the pods. Increasing “sink” size by adjusting the pod load from 1 to 2–3 did not increase the 32P accumulated by the pods proportionally. Conversely, excision of the seeds after pulse labelling did not prevent translocation of 32P out of the leaves. These results suggest that the rate of transport of phosphate to the pods at mid‐late podfill is controlled primarily by factors in the leaves. The results are consistent with the observation that the relative size of the sink (pod load) does not regulate leaf senescence.

Uptake and transfer of carbon and phosphorus by eelgrass ( Zostera marina L.) and its epiphytes

The uptake of carbon and phosphorus by eelgrass ( Zostera marina L.) and its epiphytes under laboratory conditions was examined by the use of 14C and 32P in partitioned chamber experiments. Both carbon and phosphorus were taken up by eelgrass roots and subsequently transferred through the plants to epiphytes on the grass blades. The data suggested that only a small portion of the carbon fixed in photosynthesis is supplied through the roots and rhizomes. There was very little net transfer of phosphorus through the plants during the 12-h experiments; the most active movement of phosphorus was from the water into the roots where most of the material remained. Phosphorus uptake was dependent on the concentration of dissolved inorganic phosphorus in the medium. An increase in phosphorus concentration of the medium resulted in increased uptake rates; however, the proportion of accumulation of 32P in the roots, leaves, and epiphytes remained similar. Experimental design should be carefully considered when comparing the results of various phosphorus uptake studies. The data indicated that a close relationship exists between eelgrass and its epiphytic community; from 15 to 100% of the phosphorus released by the leaves was taken up by the epiphytes.

Comparison of Root Activity in Vegetative and Reproductive Soybean Plants1

AbstractAttempts to delineate the factor(s) which limit soybean (Glycine max (L.) Merrill) yields suggest that one of the reasons may be the decline in new root growth, longevity, and/or metabolic activity of the root system as the plant changes from the vegetative to the reproductive stage of growth. The objective of this research was to investigate the absorption and desorption characteristics of intact soybean roots of plants in the vegetative and reproductive stages of growth. Compartmental analysis was used to determine whether the movement of 45Ca and 32P within the roots of intact soybean plants is different in the vegetative and reproductive stages. Soybeans were grown under greenhouse conditions in nutrient solution. Half the plants were induced to flower at 7 weeks of age, by reducing the day length from 16 to 12 hours. At 12, 14, 15, and 16 weeks of age both vegetative and reproductive plants were transferred to a controlled environment chamber, and 45Ca and 32P were used to characterize nutrient translocation to the tops and nutrient efflux from the root tissue to the nutrient solution. At 16 weeks of age the cation exchange capacity, mitotic activity, and oxygen consumption of the roots of the greenhouse‐grown plants were determined. The initial rate of 45Ca and 32P efflux from the cytoplasmic compartment and the initial 45Ca and 32P content of the cytoplasmic compartment of the roots of reproductive plants were twice those of vegetative plants. Oxygen consumption and mitotic activity were greater in the roots of vegetative plants. Cation exchange capacity was greater in the roots of reproductive than in vegetative plants. There was little evidence that the accumulation of 32P and 45Ca in the tops, the rate constants of 32P and 45Ca from the cytoplasmic or vacuolar compartments, or the initial rate of efflux of 32P or 45Ca from the vacuolar compartments differed significantly in vegetative and reproductive plants. The implications of the results are discussed with regard to nutrition of reproductive soybeans.