Continuous ultraviolet-B (UV-B; 290-320 nm) irradiation can induce rapid expansion and toluidine blue O staining in epidermal cells surrounding the trichomes on the surface of cucumber (Cucumis sativus L.) cotyledons. To directly evaluate the roles of these trichomes against UV-B irradiation, the trichomes were removed without affecting the growth of cucumber seedlings and the effect of UV-B irradiation over 7 days on the cucumber seedlings without trichomes was analyzed. After UV-B exposure for 7 days, the leaf area of the cotyledons, fresh weight of the aerial parts, and chlorophyll content of the cotyledons in the cucumber seedlings without trichomes were significantly lower than those in cucumber seedlings with trichomes. The cucumber seedlings without trichomes showed neither expansion nor toluidine blue O staining in the epidermal cells surrounding the trichomes after UV-B exposure for 7 days. Thus, trichomes on the surface of cucumber cotyledons protect the cucumber seedlings from UV-Binduced senescence. These trichomes may be necessary for UV-B-induced rapid expansion and toluidine blue O staining, namely, accumulation of polyphenolic compounds in the epidermal cells surrounding the trichomes on the surface of cucumber cotyledons. The trichomes may act as sensors to mediate the defense responses against UV-B irradiation in cucumber seedlings.
Key words: cucumber, trichome, toluidine blue O, UV-B.
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