Effects of the exposure of roots of Alnus glutinosa to light on flavonoids and nodulation

Abstract

Exposure of the roots of Alnus glutinosa (L.) Gaertn. to white light stimulated within 5 days a substantial increase in the content of quercetin and kaempferol, two major flavonols in acid hydrolysates of both roots and root exudates. Both compounds were detected also in hydrolysates of root extracts of Myrica gale and Casuarina equisetifolia, the latter a species not nodulated by Frankia strains effective on Alnus. A 7-fold reduction in nodulation of seedlings 12 days after inoculation with Frankia preincubated for 24 h with kaempferol supported the possibility that flavonols might be involved in the regulation of nodulation. Nodulation of seedling roots that were inoculated with Frankia and then exposed to light was inhibited by 50% after 11 days compared with seedlings with darkened roots. This effect of light treatment was preceded by a 30-fold increase in quercetin and kaempferol. The inhibitory effects on nodulation of preincubation of Frankia with kaempferol persisted for 18 days after inoculation, but there was no significant effect on nodulation after prolonging exposure of the root system to light for 40 days. The data support indirectly the suggestion that the balance between stimulatory and inhibitory flavonoids in roots and root exudates may contribute to the regulation of nodulation of actinorhizal plants.Key words: Alnus glutinosa, flavonoids, Frankia, kaempferol, nodulation, roots.