1CNRS, Centre d’Ecologie Fonctionnelle et Evolutive, Montpellier Cedex 05, France.
2Max Planck Institute for Chemistry, Biogeochemistry Department, 55020 Mainz, Germany.
Leaves of many plant species of Mediterranean-type ecosystems accumulate monoterpenes in secretory organs, which are volatile isoprenoids generally considered as carbon-based defensive compounds against herbivore and phytopathogen attack. Leaves of the evergreen oak Quercus ilex also produce and emit large amounts of monoterpenes, but without accumulating them. They are produced inside chloroplasts of the leaf assimilation parenchyma, within a light-dependent process linked to photosynthesis. No ecological function of this terpene formation is known. It has been hypothesised that it would protect against heat or photo damages by using excessive photosynthates or by stabilising the thylakoid membrane, thus helping to overcome environmental stresses during summer conditions. We studied the influence of light and temperature regimes on monoterpene emission capacity and photosynthesis as well as on their short term responses to light and temperature variations, and explored the extent to which changes in terpene production are related to leaf ontogeny, foliar structure and chemistry. We found that both factors, temperature and light strongly promote terpene production capacity, but that temperature dominates for acclimation. The capacity was always high under warm regimes even when carbon gain was low and sink demand for growth was high. The acclimation was found to be largely reversible and almost independent of leaf ontogeny and foliar chemistry. These results suggest that natural variation in plastidic monoterpene production capacity is an adaptation to specific climate conditions and do not support current resource allocation theories which predict increased production of plant secondary compounds when carbon is in excess of that required for growth.