Analytical and ultrastructural examination of environmental effects on flavonoids and essential oils secretion in glandular trichomes of Ocimum basilicum

Two experiments were carried out, one in MAICh and another one in Reading University where plants of Ocimum basilicum were exposed to supplementary UV-B radiation for one hour, before dawn for 15 days. The essential oil and flavonoid contents were analysed as well as an ultrastructural study of the glandular hairs on the adaxial surface of the leaves. In both experiments the essential oil content did not show qualitative differences, but on the other hand, the young leaves presented a higher concentration than the old ones in both treatments, and the UV-B treated plants showed higher levels of essential oil than the control plants. These results were strongly correlated with the anatomical observations of the adaxial surface of the leaves. Two main flavonoids were identified, salvigenin and nevadensin and, as with the essential oils, it was found that the young leaves had a higher concentration of both compounds than the old ones. On the other hand, several previous studies report that the concentration of flavonoids was highly increased with supplementary UV-B radiation, while in the present work, the first observations showed similar levels in UV-treated and control plants. The anatomical study carried out with a SEM, revealed that the peltate glandular hairs changed their shape after the third day under UV-B exposure, and the typical tetra-celled head of the gland took a round shape, indicating that the glandular cavity was filled with essential oils. This observation was made in both, young and old leaves and it suggested to be related with the increase on the essential oil production, perhaps as a defence mechanism against external factor, such as UV-B radiation. On the other hand, no appreciable differences were found in the capitate and in the simple multicellular hair that also bear the surface of the leaves, fact that can be explain because those hairs are not much involved in the secretion of essential oils. The percentage of broken glands was higher in the UV-B treated plants and therefore, the released essential oils were more abundant in these plants. The UV-B radiation had also an effect on the epicuticular wax of the leaves, and it could be seen that the plants under supplementary radiation were loosing the wax as the days were passing. Furthermore, after analysing the composition of chloroform extract used for few seconds to remome the epicuticular wax of the adaxial surface of the leaves, it was found a similar amount of flavonoid as in the UV-B treated plants. Thus it seems likely that the UV-treated leaves do in fact produce higher levels of flavonoidsbut that the epicuticular wax in which at least some of them are bound is lost from the plants following UV-treatment. Overall, it is clear that UV-B treatment stimulates the production of both essential oil and surface flavonoids in Ocimum basilicum but that in both cases and for different reasons, measurements of the concentrations of each in the plant underestimate the effects of irradiation on their production