EFFECT OF WATER-STRESS ON MONOTERPENE EMISSIONS FROM YOUNG POTTED HOLM OAK (QUERCUS-ILEX L) TREES

We investigated the effects of a short period of water stress on monot erpene emissions from Quercus ilex, a common oak species of the Medite rranean vegetation and a strong emitter of monoterpenes. The experimen t was carried out on two young saplings with a branch enclosure system under semi-controlled conditions. Under unstressed conditions, small qualitative (cis-beta-ocimene, trans-beta-ocimene, beta-caryophyllene and 1,8-cineol) and large quantitative (as much as 40% for the main co mpounds emitted) differences were observed between the two apparently similar trees. Nevertheless these differences did not affect the short - and long-term responses to temperature and water stress. Daily cours es of emissions and gas exchanges were similar before and after the st ress. During the most severe stress, emissions were reduced by a facto r of two orders of magnitude and the log-linear relationship between e missions and temperature no longer existed. Photosynthesis and transpi ration rates decreased as soon as the soil started to dry. whereas mon oterpene emissions slightly increased for few days and then dropped wh en the daily CO2 balance approached zero. We concluded that under wate r stress monoterpene emissions were highly limited by moneterpene synt hesis resulting from a lack of carbon substrate and/or ATP. After rewa tering, both emissions and gas exchanges recovered immediately, but to a level lower than the pre-stress level. These results have many impl ications for monoterpene emission modelling in the Mediterranean area, since the dry period generally extends from May to August. If our res ults are confirmed by field experiments, water stress could lead to a large overestimation of the emissions under summer conditions, when th e algorithms based on light and temperature would give high emission r ates.