Stickstoff-Assimilation und Proteinsyntheseraten bei Fichten in Relation zu den neuartigen Waldschaeden in NRW : Luftverunreinigungen und Waldschaeden

1991

S. 25.1-25.41

18 Abb., 3 Tab., 24 Lit. Ang.

Unselbständiges Werk

200059439

Früherer Titel: früher u.d.T. : Forschungsberichte zum Forschungsprogramm des Landes Nordrhein-Westfalen "Luftverunreinigungen und Waldschaeden"

Spruce needles (Picea abies L. Karst.) contain all enzymes, necessary for assimilatory nitrate reduction: nitrate reductase (EC 1.6.6.2), nitrite reductase (EC 1.7.7.1)) and glutamine synthetase (EC 1.3.1.2) (plastidic isoform, exclusively). Judged from the enzyme activities, the capacity for NO3- Reduktion at least equals that of the mykorrhized fine roots. In addition, nitrate reductase activity was demonstated in trunk xylem parenchyma. Inducibility of nitrate reductase by nitrate is restricted to the root enzyme, unless derooted trees were incubated with NO3-. Even at low, physiological levels nitrogen dioxide stimulates the needle enzyme 3-4 fold. Airborne nitrogen oxides seem to be the major source, if not the only one, for nitrate reduction in spruce needles, since xylem sap contains only traces of nitrate but carries a heavy load of organic nitrogen instead. Protein synthesis rates in spruce needles, measured as 14C-leucine incorporation into the total leaf protein fraction are increased by 30% and 55%, respectively in the light and in the dark in adult trees, which are affected by the specific symptoms of new type forest decline ("montane Vergilbung"). Since protein content is not increased, this enhancement might indicate a compensation for a likewise enhanced rate of protein degradation, due to stress influences. Further included may be a higher rate of synthesis of enzymes/proteins involved in repair-, detoxification- or protection- mechanisms. Light stimulation of protein synthesis has been shown to correlate well with light-enhanced ATP-levels. While overall-respiration rates do not exhibit any correlation to the degree of damage of affected trees, the cyanide-resistant, i.e. SHAM-sensitive respiration is markedly higher in sick trees, showing needle discoloration and needle loss. It is suggested tentatively that accelerated senescence metabolism might cause the enhancement of this energetically wasteful dissimilatory pathway. Another interpretation could be that an increased demand for anaplerotic metabolites may initiate an overflow of reducing power which is discharged by the alternative terminal oxidase. A quantitative test has been established for measuring the in vivo rate of TTC-reduction in spruce needles. This tetrazolium salt is reduced predominantly by the redox chain ascorbate/glutathione in the chloroplasts, since the cytochrome respiration chain - also able to form reduced formazane from TTC - exhibits circannual changes which are antagonistic to the TTC-reductive potential. The ascorbate/glutathione redox chain is indicative for detoxification of reactive oxygen species (e.g. the superoxide anion) via superoxide dismutase (SOD). Hence, an increase in the rate of formazane formation, as observed in a fumigation and a fertilization experiment, is open for interpretation in terms of photooxidative stress.