The fate of nitrogen (15N) released from different plant materials during decomposition under field conditions

Release of N, retention in soil, availability to a subsequent crop and total recovery of N derived from different¹⁵N-labelled plant materials decomposing in soil was investigated in two field experiments. In the first experiment five different plant species (white clover, red clover, subterranean clover, field bean and timothy) and in the second subterranean clover of different maturity (2,3 and 4 months old) were buried in mesh bags in the soil and allowed to decompose for 10 and 4 months, respectively. Most of the N released from the decaying plant materials was retained in the soil (27–46% of input). The subsequent crop (barley) took up 6–25% of input. The uptake correlated with the amount of N released from the decomposing material (r=0.936^*, I experiment). Similar amounts of subterranean clover N were taken up by barley regardless to whether the material was buried in soil in the previous autumn or just before sowing of the crop. At the end of the experiments, the total recovery of the introduced plant-derived N varied between 89 and 102%. The results present evidence that the ability of the soil to retain plant-derived N is strong in comparison with the ability of the subsequent crop and different loss mechanisms to remove it.     

Improvement of regeneration ability in Phleum pratense L. in vitro culture by dicamba

Calli were induced from mature caryopses of timothy grass (Phleum pratense L.) on MS medium (Murashige and Skoog 1962) supplemented with 500 mg·dm⁻³ casein hydrolysate and 5 mg·dm⁻³ 2,4-D (2,4-dicholorophenoxyacetic acid) or 2 mg·dm⁻³ dicamba (3,6-dichloro-o-anisic acid). Twelve-week-old calli were passaged on media with reduced levels of auxins (2 mg·dm⁻³ 2,4-D or 1 mg·dm⁻³ dicamba). Tissues induced on medium with 2,4-D were transferred on medium with 2,4-D and on medium with dicamba; parallely calli initiated on medium with dicamba were passaged on medium with 2,4-D or dicamba. Calli from various media sequences were used to establish cell suspension cultures in media containing 2 mg·dm⁻³ 2,4-D or 1 mg·dm⁻³ dicamba. An assessment of regeneration ability of calli was made on MS medium containing 0.2 mg·dm⁻³ kinetin. Callus tissue induced and/or subcultured on any of the media with 2,4-D did not regenerate plants while dicamba added to the media was the effective stimulator of regenerability. In the presence of 2,4-D calli and suspensions produced a jelly-like extracellular matrix. In cell suspension this phenomenon was observed 4–5 days after each passage. The measurements of electric potential of calli, growing on MS medium with kinetin were performed. Non-regenerating callus areas had an electric potential close to 0 mV while parts of tissue with meristematic centres were characterized by lower values of electric potential.     

Vernalization response of Phleum pratense and its relationships to stem lignification and floral transition

Background

Timothy is a long-day grass species well adapted for cultivation in northern latitudes. It produces elongating tillers not only in spring growth but also later in summer. As the quantity and quality of harvested biomass is dictated by canopy architecture and the proportion of stem-forming flowering tillers, the regulation of flowering is of great interest in forage grass production.

Methods

Canopy architecture, stem morphology and freezing tolerance of vernalized timothy were investigated in greenhouse and field experiments. The molecular control of development was examined by analysing the relationship between apex development and expression of timothy homologues of the floral inducer VRN1 and repressor VRN2.

Key Results

True stem formation and lignification of the sclerenchyma ring occur in both vernalized and regrowing stems irrespective of the developmental stage of the apex. The stems had, however, divergent morphology. Vernalization enhanced flowering, and the expression of the VRN1 homologue was elevated when the apex had passed into the reproductive stage. High VRN1 homologue expression was not associated with reduction in freezing tolerance and the expression coincided with increased levels of the floral repressor VRN2 homologue. Field experiments supported the observed linkage between the upregulation of the VRN1 homologue and the transition to the reproductive stage in vernalized tillers. The upregulation of putative VRN1 or VRN2 genes was restricted to vernalized tillers in the spring yield and, thus, not detected in non-vernalized tillers of the second yield; so-called regrowth.

Conclusions

The formation of a lignified sclerenchyma ring that efficiently reduces the digestibility of the stem was not related to apex development but rather to a requirement for mechanical support. The observed good freezing tolerance of reproductive timothy tillers could be one important adaptation mechanism ensuring high yields in northern conditions. Both VRN1 and VRN2 homologues required a vernalization signal for expression so the development of yield-forming tillers in regrowth was regulated independently of the studied genes.     

Arabinogalactans in a purified allergen preparation from pollen of timothy (Phleum pratense)

The purified allergen preparation representing a certain fraction of an aqueous timothy pollen extractcontained ca. 20% carbohydrate, mainly as arabinose (7%) and galactose (13%). The protein content was 63%. Fractionation on DEAE-Sephadex and Sephadex G-100 gave one neutral and two acidic fractions, all containing protein, arabinose and galactose. The structure of the carbohydrate moiety was investigated by methylation analysis, periodate oxidation and enzyme incubation. The acidic fraction contained (1→6)-linked galactose residues, some being substituted on O-3 with arabinose. The neutral fraction consisted of a more extensively branched arabinogalactan with longer side chains of (1→3)- and (1→5)-linked arabinose. The arabinose was present mainly as α-l-arabinofuranosyl residues. Alkaline degradation and subsequent fractionation indicated the presence of a covalent linkage between hydroxyproline and arabinose. Periodate oxidation or incubation with α-l-arabinofuranosidase did not affect the allergenic activity of the extract.     

Estimation of nitrogen fixation and transfer from alfalfa to associated grasses in mixed swards under field conditions

Fixation and transfer of nitrogen (N) from alfalfa (Medicago sativa L.) to different grass species including timothy (Phleum pratense L.) and bromegrass (Bromus inermis Leyss) were studied under field conditions, using the¹⁵N dilution technique.The percentage of alfalfa N derived from fixation (%NF) increased throughout the growing seasons and ranged from 62 to 83%. Nitrogen transfer (NT) from alfalfa to associated grasses was evident and contributed 26,46 and 38% of the total annual N yield of associated grasses or represented absolute amounts of 5, 20 and 19 kg N ha^–1 during the first, second and third year, respectively. The gradual and consistent percentage of NT that occurred before first harvest indicated that this transfer is a result of a direct excretion of N compounds from alfalfa root systems. Decomposition of root and nodule debris seems to contribute to the NT from alfalfa to associated grasses in the later cuts. All grass species benefitted similarly from alfalfa, although earlier maturing species with greater competitive ability were slightly more responsive.Contribution No. 1159 from the Plant Research Centre     

Dynamics of nitrogen remobilization in defoliated Phleum pratense and Festuca pratensis under short and long photoperiods

The impact of photoperiod on the rate and magnitude of N remobilization relative to uptake of inorganic N during the recovery of shoot growth after a severe defoliation was compared over 18 days in two temperate grass species, timothy (Phleum pratense L. cv. Bodin) and meadow fescue (Festuca pratensis Huds. cv. Salten). Plants were grown in flowing solution culture with N supplied as 20 mM NH₄NO₃ and pre-treated by extending the 11 h photosynthetically significant light period either by 1 h (short-day or SD plants) or 7 h (long-day or LD plants) of very low light intensity, during the 10 days prior to defoliation. Following a single severe defoliation, ¹⁵N-labelled NH₄⁺ or NH₄⁺+ NO₃^? was supplied over a 20-day recovery period under the same SD and LD conditions. Changes in the relative contributions of remobilized N and newly acquired mineral N to shoot regrowth were assessed by sequential harvests. Both absolute and relative rates of N remobilization from root and stubble fractions were higher in LD than SD plants of both species, with the enhancement more acute but of shorter duration in timothy than fescue. Remobilized N was the predominant source of N for shoot regrowth in all treatments between days 0 and 8 after cutting; on average more so for fescue than timothy, because the presence of NO₃^? reduced the proportional contribution of remobilized N to the regrowth of timothy but not of fescue. Net uptake of mineral N began to recover between days 4 and 6 after cutting, with NO₃^? uptake restarting 1 or 2 days earlier than NH₄⁺ uptake, even when NH₄⁺ was the only form of N supply. LD timothy plants supplied solely with NH₄⁺ were slowest to resume uptake of mineral N. Supplying NO₃^? in addition to NH₄⁺ after defoliation promoted shoot regrowth rate but not remobilization of N. Rates of regrowth (shoot dry weight production per plant) were not correlated with rates of N remobilization from stubble either over the short-term (days 0–8) or longer term (days 0–18), interpreted as evidence against a causal dependence of regrowth rate on N remobilization under these conditions. The results are discussed in relation to hypotheses for source/sink-driven rates of N remobilization and their interactions with mineral N uptake following defoliation.     

Comparison of three variations of the radioallergosorbent test-inhibition assay for measuring allergenic activity of grass pollen extracts

Three variations of the radioallergosorbent (RAST)-inhibition assay (RI) have been compared for measuring the allergenic activity of grass pollen extracts. The main difference between the three assays consisted in the solid supports to which the allergens were coupled. These supports were paper disks, microcrystalline cellulose, and Matrex. It could be demonstrated that all three variations of RI yielded almost identical results as expressed by their F value (F_o). Correlations between the three methods were highly significant (P < 0.001). Three independent valid assays showed an excellent reproducibility of each test system (P < 0.01). Three different preparations of each of the supports yielded highly reproducible F_o (0.76 ± 3% SD). Evaluation of the 50% inhibition values achieved with the different assays based on protein showed differences between the three supports. Provided the same allergen extract is coupled to the different solid supports and the results are related to a reference preparation, different laboratories using different forms of RI will be able to compare their results.