Uptake of 32P labelled phosphate by clover and ryegrass growing in mixed swards with different nitrogen treatments

Lolium perenne cv. S.23 and Trifolium repens cv. Olwen were sown together in 1975, fertilised then and in 1976, and finally given nitrogen doses of either 50, 100, 200 or 400 kg/ha (as N) combined with 0.64 times as much potassium (as K₂O) in 1977. As nitrogen increased, grass yield increased, but clover decreased. Grass roots absorbed more ³²P than clover roots, and nitrogen increased this difference. Grass roots bore more mycorrhiza than clover roots. The difference in ³²P uptake between grass and clover was less in June and July than in August. Clover roots took up most phosphate from the upper layers of soil, while grass absorbed ³²P rather uniformly down to 25 cm. It was concluded that optimum fertiliser placement for clover growth was a surface dressing in the early season.     

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 (p_i) 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.     

Varietal differences in uptake of 32P labelled phosphate in clover plus ryegrass swards and monocultures

Lolium perenne cv. S.23, L. multiflorum cv. RvP, and Trifolium repens cvs S.184 and Olwen, were grown in mixed sward and monoculture during 1979. Whereas in mixtures grass roots absorbed more ³²P than clover roots, in monoculture clover generally absorbed more ³²P than grass roots. This showed that grass was a very strong competitor for uptake in mixed swards. Clover and grass monocultures absorbed most ³²P from 10 or 15 cm depth in the soil, while grass in mixtures absorbed most ³²P at 22.5 cm depth. Comparing varieties, in monocultures in June, Olwen was most active in absorbing ³²P at 15 cm. In August, Olwen absorbed more at 15 cm and 22.5 cm than S.184 or the grass varieties. Differences in absorption depth between varieties were less in mixtures than in monocultures. S.23 absorbed more ³²P in the late season than RvP, both in monoculture and in mixtures. Thus Olwen differed from S. 184 in depth and timing of uptake, whilst S.23 differed from RvP in time of uptake. Such varietal differences could be exploited by manipulation of depth and timing of fertiliser application to increase the precision of sward management.     

Temporal changes in root growth and 15N uptake and water relations of two tussock grass species recovering from water stress

Physiological responses of Agropyron desertorum and Pseudoroegneria spicata , two common cold desert perennial tussock grass species of the North American Great Basin, were evaluated during and after a period of imposed drought in a pot study. The timing and the pattern of response of leaf water potential (Ψ₁), stomatal conductance (g_s), and root growth were strikingly similar in both species during and after drought. The severity of stress influenced the magnitude of Ψ₁ and g_s, but had little effect on the timing of these responses. Although drought inhibited total root length in prestressed plants, within 4 days after relief of drought both species showed similar increases in root growth which exceeded those of the control. Despite similarities in their root growth responses to increased soil water availability, the two grasses differed in their capacity to restore N uptake following drought. By 14 days after rewatering, N uptake in the prestressed Agropyron had recovered to levels of control plants, although both root biomass and root lenght were much less than those of the controls. This is attributed to elevated root uptake kinetics. Restoration of N uptake by prestressed Pseudoregneria was much less effective during the same period.     

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 (g_FW 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.     

Formation of creatine phosphate from creatine and 32P-labelled ATP by isolated rabbit heart mitochondria

With ATP [γ-³²P] we have demonstrated directly that mitochondrial creatine phosphokinase catalyzes the formation of large amounts of creatine phosphate with mitochondria generated ATP as substrate rather than added extramitochondrial ATP.     

Effect of genetically modified Pseudomonas fluorescens strains on the uptake of nitrogen by pea from 15N enriched organic residues

The effects of an antibiotic-producing Pseudomonas fluorescens strain (F113) carrying the marker gene cassette lac ZY and a marked, non-producing strain (F113G22) on the uptake of nitrogen from ¹⁵N enriched organic residues incorporated into a sandy soil were investigated in microcosm studies. Strain F113 produces the antibiotic 2,4-diacetylphloroglucinol (DAPG), while its modified derivative strain F113G22 has DAPG production deleted by Tn 5 mutagenesis. Uptake of nitrogen by pea ( Pisum sativum ) was estimated using isotope-ratio mass spectrometry. In addition, plant growth and microbial activity in soil were monitored. Both strains F113 and F113G22 enhanced the uptake of nitrogen from mineralized organic residues, even though the antibiotic producing strain F113 significantly reduced microbial activity in soil. It is suggested that the effect on nitrogen uptake was due to increased mineralization of organic residues by the introduced organisms, making greater quantities of inorganic nitrogen available for plant uptake. Unlike studies assessing impact in terms of perturbation to indigenous microbial communities, this study provides direct evidence of a change in ecosystem function as a result of the introduction of strains of a genetically marked bacterium, irrespective of whether its natural antibiotic-producing capacity has been genetically deleted.     

Short term 22Na+ and 42K+ uptake in intact, mid-vegetative plants

Spergularia marina (L.) Griseb. is. a rapidly growing, annual, coastal halophyte. Because of its small size, it is suitable for isotope studies of ion transport well beyond the seedling stage. The purpose of this report is to establish the similarities and differences between ²²Na⁺ and ⁴²K⁺ uptake in S. marina and in more commonly used mesophytic crop species. Vegetative plants were used 18 days after transfer to solution culture. Plants were grown either on Na⁺-free medium or on 0.2 × sea water. ²²Na⁺ uptake was linear with time for several hours. The rate was relatively insensitive to external concentration between 1 and 180 mol Na⁺ m?³, particularly in Na⁺-free plants. Transport to the shoot accounted for 40 to 70% of the total uptake, dependent on salinity but largely independent of time. ⁴²K⁺ uptake decreased with increasing salinity in Na⁺-free plants and increased in 0.2 × sea water plants. Both uptake and transport to the shoot were non-linear with time, upward concavity suggesting recovery from a manipulative and/or osmotic injury. Steady state root contents were compared with predicted contents based on cortical cell electrical potentials using the Nernst equation. Reasonable agreement was found in all cases except Na⁺ content of 0.2 × sea water plants, in which active efflux was indicated. Uptake studies conducted in the presence of chemical modifiers (dicyclohexylcarbodiimide, dinitrophenol and fusicoccin) showed responses of ⁴²K⁺ uptake as expected from studies on agronomic species, and implied the presence of a similar active uptake here despite the appearance of equilibrium. Active Na⁺ uptake was suggested at low Na⁺ levels. We conclude that S. marina is a promising experimental system combining the rapid nutrient acquisition strategy of agionomically important annuals with a high degree of salt tolerance.     

The incorporation of 32P into spectrin aggregates following incubation of erythrocytes in 32P-labelled inorganic phosphate

³²P was incorporated into spectrin by incubation of fresh erythrocytes with ³²P_i and glucose. The dimer and tetramer aggregates revealed only covalently-bound incorporation of phosphorus, while a higher aggregate of spectrin revealed both covalent and non-covalent incorporation. The specific activity of the covalently-bound phosphorus in all oligomers was identical, suggesting that the state of association is independent of phosphorylation. The non-covalent incorporation was shown to be due to the association of ATP with this higher aggregate. The nucleotide appears not to be bound directly to spectrin but rather to component 5 (erythrocyte actin) which is also found to be associated with this highly aggregated spectrin structure.     

The simultaneous use of 14CO2 and 15N2 labelling techniques to study the carbon and nitrogen economy of legumes grown under natural conditions

A method based on simultaneous short-term exposure to ¹⁴CO₂ and ¹⁵N₂ is described for studying nitrogen fixation and distribution in legumes relative to carbon assimilation and use. Equipment designed to accomodate experiments under natural conditions with very little disturbance of the N₂ fixing association is used. It permits continuous measurement and regulation of variables such as air temperature, humidity and CO₂ concentration as well as soil aeration. Measurements of distribution and use of assimilates, respiration of nodulated roots, quantitative N₂ fixation and the distribution and fate of fixed N as a function of time lead to a precise estimation of C and N budgets for each labelling period. When experiments are done at several phenological stages they give a new insight into the complex C and N interrelations in legume symbiosis.
A series of trials throughout the growth period of Glycine max (L.) Merr. cv. Hodgson demonstrated the sensitivity of the method. The development of the plants from vegetative to reproductive stages was accompanied by a complete change in the distribution patterns of current assimilates and products of nitrogen fixation. Maximum sink strength moved from the leaves to the pods and seeds which ended up receiving 70% of the incoming C and 35% of the fixed N. The fact that up to 85% of fixed N in the plants was in the reproductive organs at maturity can be accounted for by remobilisation from vegetative parts.
The respiration of nodulated roots utilized 33% of carbon translocated to below-ground plant parts before nitrogen fixation started, but as much as 50% during the period of optimal fixation. The advantages and limitations of the isotopic method described are critically discussed as a prelude to future investigations.     

Improvement of the 15N dilution method for estimation of absorption of NOx by plants supplied with 15N-labelled fertilizer

To develop further the methods for estimation of NOx absorption by plants supplied with ¹⁵N-labelled fertilizer, we proposed a new calculation method, total N fixed method (TNF), and compared with the ¹⁵N dilution method and the classical mass balance method (MB).
Hydroponically grown soybean plants were supplied with ¹⁵N-labelled nitrate and exposed to 200–250 nl l⁻¹ NO₂ for 7 d. The proportions of the N derived from NO₂ to total N in exposed plants were estimated by the three methods.
The reported rates of NO₂ absorption by several plant species, estimated by the ¹⁵N dilution method, were recalculated using the TNF method. The results of the two methods were compared and showed that: (1) The ¹⁵N dilution method overestimated the content of NO₂-N in exposed plants compared with the MB method whilst the TNF method produced estimations of NO₂-N closer to those by the MB method when the plants were supplied with 5 m M nitrate. (2) The differences in estimations between the MB method and either the ¹⁵N dilution method or the TNF method increased with decreasing supply of ¹⁵N-labelled nitrate to roots.