Editorial

Abstract

Periods of soil dryness are typical for the climate of Southern Europe. Since drought induced restriction in carbon acquisition may reduce carbon reemission through terpenoids, drought stress can be a factor influencing the overall emission of terpenoids in the Mediterranean area. In this area orange plantations are common. In order to investigate the relationship between the water status of Citrus sinensis and terpenoid emissions, monoterpene and sesquiterpene emission rates were followed during a drought treatment and subsequent recovery. A dynamic enclosure technique was used to determine emissions of terpenoids from a branch of a young orange tree in a greenhouse. Terpenoid emissions from Citrus sinensis consisted mainly of the sesquiterpene beta-caryophyllene and the monoterpene trans-β-ocimene. Taken together these compounds accounted for 82% of the total terpenoid carbon emission from orange leaves. Other terpenoids were emitted in traces. Under severe drought stress the emission of both compounds was reduced to less than 6% of the prestress level. Mild drought stress induced a decrease in the trans-β-ocimene emission rate whereas the β-caryophyllene emission showed no response to slight drought stress. During the first phase of the experiment, before severe drought stress occured, the β-caryophyllene emission rate was closely related to temperature. The emission increased by the 5.6 fold with a 10°C increase in temperature. At standard conditions (30°C, PPFD 1069 μmol photons m−2 s−1trans-β-ocimene was emitted from orange leaves at a rate of 0.33 μg g−1 and β-caryophyllene at a rate of 0.41 μg g−1 h−1 given on a total dry weight basis, rates corresponding to 10.5 and 13.1 ng m−2 s−1, respectively, calculated per total projected leaf area. With flowers the total terpenoid carbon emission was 7.8 fold the emission from the same branch without flowers. The data indicate, that severe drought stress and flowering should be considered in terpenoid emission inventories to avoid error in the estimation of total terpenoid emissions from vegetation in model calculations.