Continuous Long-term UV-B Irradiation Reduces Division and Expansion of Epidermal Cells in True Leaves, but Accelerates Developmental Stages Such as True Leaf Unfolding and Male Flower Bud Production in Cucumber (Cucumis sativus L.) Seedlings

Abstract

We examined the effects of 25 d of continuous ultraviolet-B (UV-B) irradiation (0.57 W m⁻²) on cucumber (Cucumis sativus L.) seedlings. The leaf areas of all unfolded true leaves and the plant height were reduced considerably by 25 d of UV-B irradiation. In each true leaf, both the epidermal cell area and the total epidermal cell number were reduced after UV-B treatment. The expression of five cell cycle-related genes (Cs Cyclin A [CsCycA], CsCycB, CsCycD3;1, CsCycD3;2, and Cs Cyclin-dependent kinase A [CsCDKA]) in the shoot apical meristem (SAM) was reduced within 48 h of UV-B exposure. These results indicate that continuous UV-B irradiation inhibits the cell cycle in the SAM and thus reduces cell division and cell expansion in true leaves. On the other hand, the number of unfolded true leaves increased after exposure to UV-B for 15–25 d. Furthermore, 60% of the plants produced male flower buds after exposure to UV-B for 25 d. Therefore, continuous long-term UV-B irradiation affects the SAM of cucumber seedlings, resulting in reduced plant growth, but does accelerate various developmental stages.