Diterpenes and antioxidative protection in drought-stressed Salvia officinalis plants

Previous studies have shown that diterpene carnosic acid plays a role as an antioxidant in rosemary, a drought-tolerant species. However, this mechanism has not been tested so far in other species. The endogenous levels of diterpene carnosic acid and α-tocopherol, which are lipophilic antioxidants found in chloroplasts, were measured in a drought-recovery cycle in leaves of sage (Salvia officinalis L. subs. officinalis), a drought-susceptible species, growing in Mediterranean field conditions. The relative leaf water content of the sage plants fell markedly when exposed to drought during the summer. As the drought progressed, the carnosic acid amounts in the leaves decreased, giving rise to increased levels of its oxidation products, rosmanol and isorosmanol, thereby revealing the antioxidant activity of carnosic acid in drought-stressed leaves. At the same time, α-tocopherol levels decreased progressively with drought. Midday depression of carnosic acid and α-tocopherol was observed throughout the study, and the highest levels of carnosic acid oxidation products (i.e. rosmanol and isorosmanol) were observed at midday when high light was simultaneous to water deficit. This report suggests that the same mechanism of antioxidative protection by carnosic acid applies in rosemary and sage, and shows that it is the complete set of antioxidants (and not a single mechanism) that is responsible for avoiding drought-induced damage in plants.