| Abstract: | The leaves and photosynthetic stems of Cistus ladanifer, a plant that colonizes arid lands, secrete an exudate that shows a large seasonal variation in its flavonoid content. The maximum secretion of flavonoids in the exudate is produced during summer, increasing approximately three-to fourfold with respect to the secretion measured in spring. Summer is the season in which the plant suffers the greatest stress from environmental physical variables such as UV irradiation, high temperatures, and hydric stress. Studies were conducted in plants from several locations, which were selected considering daily UV irradiation (open or shaded areas), annual precipitation, and annual average maximum and minimum temperatures. Additional studies to control UV irradiation, drought, and temperature separately were performed with C. ladanifer plants growing in a glasshouse and in a culture room. The UV irradiation was found to be the major inducer of the enhanced flavonoid secretion during summer, because no significant increase of flavonoid secretion during summer was observed when the C. ladanifer plants in the field were covered with a Plexiglas box (total UV absorption below 380 nm). These results support an ecophysiological role of the flavonoids in the exudate to protect the plant against the damaging effects of UV irradiation. The culture room experiments confirmed this point and also showed that the induction of flavonoid secretion by UV irradiation is synergistically augmented by drought. The glasshouse and culture room experiments showed drought and high temperatures (between 30° and 45°C) to correlate with the summer increase of the more methylated flavonoids (kaempferols and 7-methylated apigenins) in the exudate. Because these more methylated flavonoids have higher hydropathy than the less methylated, these results suggest that the secretion of more methylated flavonoids is part of the defense mechanism of the plant against the hydric stress of summer. |
