**5. Conclusions**

At the early stages of salt stress (1 h and 1-day-treatment), the plasma membrane NADPH oxidase was activated in the cucumber (*Cucumis sativus* var. Krak) seedlings. This activation correlated with an observed enhancement in H2O2 content, suggesting the involvement of NADPH oxidase in the generation of H2O2, which may function as a signaling molecule participating in stress response. The observed stimulation of NADPH oxidase may have been at least partially related to the induction of some *CsRboh* genes, as well as the increased activity of G6PDH, providing NADPH. Another factor which may contribute to positive RBOH regulation at the post-translational level, could be H2O2. This proposal, however, requires additional studies concerning direct or indirect H2O2 action.

After 6-day exposure to salt stress, the plasma membrane NADPH oxidase activity, and consequently the H2O2 level, were diminished. This protected the cells against the toxic effect of high H2O2 concentrations. Additionally, antioxidant enzymes, including SOD, APX, and CAT, were significantly activated to maintain low H2O2 content in cells, and to avoid ROS-dependent damages. However, to explain the precise mechanism of

NADH oxidase regulation, and its interrelation with the antioxidant system in cucumber seedlings subjected to salinity, further analysis is needed.

**Author Contributions:** Conceptualization, M.J.; methodology, K.K., M.R., A.W., and M.J.; formal analysis, K.K., M.R., A.W., and M.J; writing—original draft preparation, K.K.; writing—review and editing, M.R., A.W., and M.J; supervision, M.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented are available in this manuscript and in the repository at https://www.repozytorium.uni.wroc.pl/dlibra/publication/139358/edition/128721# (accessed on 10 September 2021).

**Acknowledgments:** We thank Beata Kuligowska for her excellent technical assistance.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

