INTERACTION OF UV-RADIATION AND IAA DURING GROWTH OF SEEDLINGS AND HYPOCOTYL SEGMENTS OF SUNFLOWER

Stem growth of sunflower seedlings (Helianthus annuus, c.v. Polstar L.) was increasingly reduced by UV-B radiation with shorter wavelengths (Schott cut-off filters: WG 360-280) at a constant low white light irradiance. The reduction in stem elongation measured with linear voltage transformers was observed after 10 h of enhanced UV-B irradiation (WG 305) and already after 5 h, when the seedlings were additionally irradiated with lateral shortwave UV-B (WG 305 hi.). Stem elongation of UV-B irradiated sunflower seedlings was not irreversibly reduced as demonstrated by changing UV-B irradiation conditions. UV-B irradiation (WG 305) had no negative influence on stem elongation when the hypocotyls were covered by a non-UV-B transmitting plastic film, indicating that the hypocotyl is most susceptible for the inhibiting effect of UV-B radiation on stem elongation. Elongation growth of isolated hypocotyl segments (HSEG-test) at simultaneous UV-B irradiation was inhibited both in water and IAA solution, dependent on wavelength in the UV-B range. In IAA solutions UV-B preirradiated for 1-48h under a WG 305 filter, elongation growth of segments decreased with preirradiation time. It was assumed that a destruction of IAA and/or a formation of growth inhibiting IAA photoproducts in the IAA solution are reasons for the observed growth reduction. Therefore, IAA-photooxidation kinetics were studied, the IAA photoproducts separated by HPLC and identified as 3-Hydroxymethyloxindole, Indole-3-aldehyde, 3-Methyleneoxindole (3-M), 3-Methyloxindole and Indole. The application of these compounds in the HSEG-test showed that only 3-M inhibited elongation growth. Furthermore, the in vivo IAA concentration of UV-B irradiated sunflower seedlings under WG 305 was reduced by 51% compared with that of seedlings grown under WG 360. On the basis of these results the <<IAA destruction>> seems to be a potent mechanism for the growth inhibition of UV-B irradiated sunflower seedlings grown at low white light irradiances.