Abiotic Stress Responses in Raphanus sativus L.: Insights into the Role of Endopolyploidization in Stress Adaptation

Endoreduplication is a process in which cells repeatedly undergo the S and G phases of the cell cycle without dividing, thereby increasing their ploidy level. Cells with different ploidy levels subsequently form tissues and organs, resulting in a condition called endopolyploidy. This phenomenon plays an important role in plant growth, development, and homeostasis and may also help compensate for adverse environmental conditions, representing one of the mechanisms of stress adaptation. Two radish cultivars, ‘Lada’ and ‘Ester’, were grown hydroponically and treated with 10 or 20 μM cadmium and UV-B irradiation. The level of endopolyploidization and its role in adaptation to abiotic stress were studied by flow cytometry. Stress intensity was evaluated by measuring fresh weight, assimilatory pigment content, and antioxidant enzyme activities. The experimental data confirmed that cadmium exposure represents a critical limiting factor for radish growth, as manifested by a significant reduction in biomass and decreased content of photosynthetic pigments. Induced abiotic stress did not significantly alter endopolyploidization in the petioles and leaves of either cultivar. In the storage hypocotyl of ‘Ester’, the degree of endopolyploidy decreased under 20 μM cadmium treatment, whereas the storage hypocotyl of ‘Lada’ remained relatively stable. The cultivars also differed in their stress response strategies. ‘Ester’ showed higher sensitivity to cadmium stress, accompanied by elevated SOD, GPOX, and APX activity, while ‘Lada’ exhibited higher CAT activity. UV-B irradiation alone caused significant changes only in ‘Ester’, where APX, GPOX, and CAT activities increased. However, the combined effect of cadmium and UV-B was more pronounced, particularly in the chlorophyll a/b ratio and antioxidant enzyme activities in both cultivars. The results indicate that UV-B acts more as a modulator of the physiological response to cadmium than as a primary stress factor. These findings suggest that the maintenance of endopolyploidy may be associated with greater tolerance to the combined action of cadmium and UV-B stress.