**Vladislav V. Yemelyanov 1,2,\*, Tamara V. Chirkova 2, Maria F. Shishova <sup>2</sup> and Sylvia M. Lindberg <sup>3</sup>**


Received: 7 August 2020; Accepted: 15 September 2020; Published: 16 September 2020

**Abstract:** Both ion fluxes and changes of cytosolic pH take an active part in the signal transduction of different environmental stimuli. Here we studied the anoxia-induced alteration of cytosolic K<sup>+</sup> concentration, [K+]cyt, and cytosolic pH, pHcyt, in rice and wheat, plants with different tolerances to hypoxia. The [K+]cyt and pHcyt were measured by fluorescence microscopy in single leaf mesophyll protoplasts loaded with the fluorescent potassium-binding dye PBFI-AM and the pH-sensitive probe BCECF-AM, respectively. Anoxic treatment caused an efflux of K<sup>+</sup> from protoplasts of both plants after a lag-period of 300–450 s. The [K<sup>+</sup>]cyt decrease was blocked by tetraethylammonium (1 mM, 30 min pre-treatment) suggesting the involvement of plasma membrane voltage-gated K<sup>+</sup> channels. The protoplasts of rice (a hypoxia-tolerant plant) reacted upon anoxia with a higher amplitude of the [K<sup>+</sup>]cyt drop. There was a simultaneous anoxia-dependent cytosolic acidification of protoplasts of both plants. The decrease of pHcyt was slower in wheat (a hypoxia-sensitive plant) while in rice protoplasts it was rapid and partially reversible. Ion fluxes between the roots of intact seedlings and nutrient solutions were monitored by ion-selective electrodes and revealed significant anoxia-induced acidification and potassium leakage that were inhibited by tetraethylammonium. The K<sup>+</sup> efflux from rice was more distinct and reversible upon reoxygenation when compared with wheat seedlings.

**Keywords:** anoxic signaling; potassium; pH; acidification; fluorescence microscopy; *Triticum aestivum*; *Oryza sativa*
