The mobilization of defence mechanisms in the early stages of pea seed germination against Ascochyta pisi

Ascochyta pisi is a necrotrophic pathogenic fungus, which mainly survives between seasons through infected seeds. Defence responses of pea embryo axes to A. pisi were investigated in the heterotrophic phase of seed germination and during the transition from the heterotrophic to the autotrophic phase. Germinated pea seeds, both non-inoculated and inoculated with A. pisi, were cultured in perlite for 96 h. Polarographic studies performed on intact embryo axes of germinating pea seeds infected with A. pisi showed a high respiratory intensity in time from 48 to 96 h after inoculation. Forty-eight-hour embryo axes of germinating pea seeds exhibited the highest respiration rate, which in infected axes was maintained at the following time points after inoculation. Moreover, at 72 and 96 h after inoculation, respiratory intensity was by 64% and 73% higher than in the control. Electron paramagnetic resonance analysis revealed a higher concentration of semiquinone free radicals with g values of g (||) = 2.0031 ± 0.0004 and g (⊥) = 2.0048 ± 0.0004 in infected axes than in the control. Generation of superoxide anion radical was also higher in infected axes than in the control but stronger at 72 and 96 h after inoculation. Starting from 72 h after infection, the level of Mn(2+) ions in infected axes decreased in relation to the control. At the same time, the highest activity of superoxide dismutase (EC 1.15.1.1) and catalase (EC 1.11.1.6) was observed in 72-h infected axes. In turn, the activity of peroxidase (EC 1.11.1.7) up to 72 h after infection was lower than in the control. In 48-h infected embryo axes, a very high level of pterocarpan pisatin was observed. Infection of germinating pea seeds with A. pisi restricted mainly the growth of the epicotyl, but did not inhibit the increase in length and fresh weight of root embryo axes versus cultivation time. These results indicate that in pea during the stages of seed germination and early seedling growth, protective mechanisms are induced in embryo axes against A. pisi.