Do volatile organic compound emissions of Tunisian cork oak populations originating from contrasting climatic conditions differ in their responses to summer drought?

The intrinsic variability of volatile organic compound emissions and photosynthetic parameters in response to two drying cycles was investigated on Quercus suber L. saplings originating from three Tunisian populations native to contrasting climates. Emissions mainly included monoterpenes plus traces of sesquiterpenes and methylsalicylate that steadily decreased during the experiment unrelated to treatments. Instead, monoterpene emissions increased by 30% during a first moderate drought and remained enhanced after rewatering, while CO2-H2O gas exchange was reduced by 30%. A second severe drought decreased monoterpene emissions to about two third of controls, strongly inhibited gas exchange and photochemical efficiency, and caused a partial loss of chlorophyll. Rewatering led to only partial recovery of emis- sions and photosynthetic parameters of stressed plants whose biomasses and leaf nitrogen and chlorophyll contents were lower than in control plants. There was no clear difference among populations in volatile organic compound emissions or photosynthetic parameters. However, the population inhabiting the most arid region had intrinsically smaller leaves with lower specific leaf masses and higher drought-related leaf losses than populations native to less arid climates. These results indicate an adaptive shift from stress-tolerating towards stress-avoiding ecotypes under arid climates involving mainly mor- phological adaptations without apparent differentiation in volatile organic compound production, probably because of its high phenological plasticity. Resume´ : La variabilitedes emissions foliaires de composes organiques volatils et des parametres photosynthetiques ont eteetudies au cours de deux cycles de dessechement sur des plants de Quercus suber L. provenant de trois populations tu- nisiennes adaptees a des differentes conditions climatiques. Les composes organiques volatils emis etaient principalement des monoterpenes, plus, independamment du traitement, des traces de sesquiterpenes et methyle-salicylate. Un premier des- sechement moderereduisant les echanges gazeux en CO2-H2O de 30%, a augmentede 30% les emissions de composes organiques volatils qui sont restees elevees apresr ehydratation. Un second dessechement plus severe a reduit les emissions d'environ 30% accompagned'une forte inhibition des echanges gazeux et de l'efficience photochimique et d'une perte partielle de chlorophylle. Apresr ehydratation les emissions et les parametres photosynthetiques ont partiellement recupere´. Les biomasses aeriennes et souterraines finales ainsi que l'azote foliaire et la teneur en chlorophylle des plantes stressees etaient inferieurs a ceux des temoins. Les emissions de composes organiques volatils et les parametres photosynthetiques n'etaient pas nettement differents entre populations. Cependant, la population provenant du site le plus aride presentait des feuilles plus petites de poids specifique plus faible, et des pertes de feuilles plus importantes apres le stress que les popula- tions adaptees a des sites moins arides. Ces resultats pourraient indiquer l'evolution adaptative d'une strategie de tolerance au stress vers une strategie d'evitement du stress sous climat aride impliquant surtout des adaptations morphologiques sans difference apparente dans la production de composes organiques volatils, probablement a cause d'une grande plasticite´ phenologique.