Silicon (Si) is a ubiquitous element in soil with well-known beneficial effects under certain conditions, in several plant species, if supplied in available form for uptake. It may alleviate damage in various stress situations and may also promote growth when no obvious stressors are applied. Effects of Si are often linked to mitigation of oxidative stress, in particular to the induction of antioxidant defense mechanisms. In the work presented, the impact of silicon provision on pro-oxidant systems was investigated in cucumber. Plants of the F1 cultivar hybrid ‘Joker’ were grown under in vitro conditions in the absence of any applied external stressor. Silicon provision decreased H₂O₂ content and lowered lipid peroxidation in the leaves of the treated plants. This was paralleled by declining polyamine oxidase (PAO) and diamine oxidase (DAO) activities. Several PAO as well as lipoxygenase (LOX) genes were coordinately downregulated in Si-treated plants. Unlike in similar systems studied earlier, the Si effect was not associated with an increased transcript level of gene coding for antioxidant enzymes. These results suggest an inhibitory effect of Si provision on pro-oxidant amine oxidases, which may decrease the level of reactive oxygen species by retarding their production. This extends the molecular mechanisms linked to silicon effects onto redox balance in plants.

Cucumber (Cucumis sativus L.) F1 cultivar hybrid ‘Dirigent’ was investigated for growth, photosynthetic pigment content and level of reactive oxygen species upon long term silicon Si treatment in a perlite based, semi-hydroponic growth system. Element analysis approved elevated Si content in the shoot of silicate treated plants. Increased shoot growth as well as higher chlorophyll and carotenoid contents of leaves indicated beneficial effects of silicate provision. Increased transcript level of all known silicon transporter genes was revealed in leaves and roots of Si treated plants by RT-PCR and RT-qPCR. Furthermore, decreased level of hydrogen peroxide and lipid peroxidation indicated lower oxidative stress level in silicated shoot tissues. Potentially contributing to decreased lipid peroxidation, coordinately downregulated transcription of all expressed lipoxygenase genes was observed in leaves. Results suggest that downregulation of lipoxygenases may contribute to the antioxidant effect of silicon provision in plants.