Effect of exogenous flavonoids on nodulation of pea (Pisum sativum L.).

Selected flavonoids that are known as inducers and a suppressor of nodulation (nod) genes of the symbiotic bacterium Rhizobium leguminosarum bv. viciae were tested for their effect on symbiosis formation with garden pea as the host. A solid substrate was omitted from the hydroponic growing system in order to prevent losses of flavonoids due to adsorption and degradation. The presumed interaction of the tested flavonoids with nod genes has been verified for the genetic background of strain 128C30. A stimulatory effect of a nod gene inducer naringenin on symbiotic nodule number formed per plant 14 d after inoculation was detected at concentrations of 0.1 and 1 micro g ml(-1) nutrient solution. At 10 micro g ml(-1), the highest concentration tested, naringenin was already inhibitory. By contrast, nodulation was negatively affected by a nod gene suppressor, quercetin, at concentrations above 1 micro g ml(-1), as well as by another tested nod gene inducer, hesperetin. The deleterious effect of hesperetin might be due to its toxicity or to the toxicity of its degradation product(s) as indicated by the inhibition of root growth. Both the stimulatory effect of naringenin and the inhibitory effect of quercetin on nodule number were more pronounced at earlier stages of nodule development as revealed with specific staining of initial nodules. The lessening of the flavonoid impact during nodule development was ascribed to the plant autoregulatory mechanisms. Feedback regulation of nodule metabolism might also be responsible for the fact that the naringenin-conditioned increase in nodule number was not accompanied by any increase in nitrogenase activity. By contrast, the inhibitory action of quercetin and hesperetin on nodule number was associated with decreases in total nitrogenase activity. Naringenin also stimulated root hair curling (RHC) as one of the earliest nodulation responses at concentrations of 1 and 10 microg ml(-1), however, the same effect was exerted by the nod gene suppressor, quercetin, suggesting that feedback regulatory mechanisms control RHC in the range of nodulation-inhibiting high flavonoid concentrations. The comparison of the effect of the tested flavonoids in planta with nod gene activity response showed a two orders of magnitude shift to higher concentrations. This shift is explained by the absorption and degradation of flavonoids by both the symbionts during 3 d intervals between hydroponic solution changes. The losses were 99, 96.4, and 90% of the initial concentration of 10 micro g ml(-1) for naringenin, hesperetin, and quercetin, respectively.

Comparative response of Pisum sativum nodulated with indigenous soil Rhizobium populations and/or co-inoculated with a Rhizobium leguminosarum strain. I. Acetylene-reducing, dihydrogen- and carbon dioxide-evolving activities.

No significant differences in the acetylene-reducing activity and evolution of H2 and CO2 from nodulated roots of Pisum sativum inoculated with soil Rhizobium populations from two soils with different acidities (Ruzyne soil 7.6; Lukavec soil 4.9) were observed. Rhizobium population from Lukavec soil formed nodules, exhibiting a higher H2 evolution. Co-inoculation with the Hup+ strain 128C30 (7 x 10(7) cells per seedling) eliminated, to some extent, the effect of soil populations on physiological activity.

Symbiotic dinitrogen fixation as affected by short-term application of nitrate to nodulated Pisum sativum L.

Effect of nitrate on the nitrogenase (C2H2-reduction) activity, growth of nodule tissue accumulation of nitrate and nitrate reductase activity in 4-weeks-old nodulated peas (Pisum sativum L.) was investigated. A relatively slow decrease of the total nitrogenase activity (mumol C2H4 per root per h), as compared with plants cultivated without nitrate, was due to both retardation of further growth of the nodule tissue and to a decrease of their specific nitrogenase activity (mumol C2H4 per gf.wt. per h). However, an absolute and pronounced decrease of both nitrogenase activities occurred only 4 or 7 d after the application of nitrate. The addition of nitrate led to its rapid accumulation in the nodule and leaf tissue with a simultaneous induction of the nitrate reductase activity. The nitrogenase activity was not completely inhibited even after a 7-d cultivation with 280 m NO3- -N in the nutrient medium and after accumulation of up to 180 ppm NO3- -Nf.wt. in the nodule tissue. The results obtained indicate that the "photosynthate deprivation" reflects competition between assimilation of nitrate and fixation of dinitrogen.

Strain-specific antigens in Rhizobium leguminosarum.

Fifty strains of Rhizobium leguminosarum, isolated from five species of host plant (Pisum sativum, P. arvense, Vicia sativa, V. faba, and a Lathyrus sp.) were examined for the presence of strain-specific somatic antigens by immune-diffusions against 13 antisera. Thirty eight strains (76 per cent) were found to belong to the same sero-group and were serologically indistinguishable from each other, but four of these strains also exhibited non-reciprocal cross reactivity with other antisera. In contrast to this, five Australian strains, isolated from P. sativum, showed a high degree of strain specificity.

Acetylene reduction (dinitrogen fixation) and nitrification in soil as affected by the structural aggregate size.

The effect of size of structural aggregates on the intensity of nitrification and nitrogenase (nitrogen: acetylene oxidoreductase) activity was investigated in three soils. In two of them the nitrogenase activity was limited by addition of glucose. Aggregates of a larger diameter (2-4 mm) exhibited a considerably higher nitrogenase activity than those with a diameter smaller than 2 mm. This effect was even more pronounced when the soil samples were repeatedly intensively aerated. On the contrary, smaller aggregates (0.5-2 mm) exhibited more intensive nitrification.