Drought effect on urban plane tree ecophysiology and its isoprene emissions

Urban trees emit a wide range of biogenic Volatile Organic Compounds (bVOC). Some of these bVOC, like isoprene can react with atmospheric oxidants to form secondary compounds, such as ozone (O₃) and secondary organic aerosols (SOA), which have impacts on air quality and climate. In addition, isoprene emissions are strongly influenced by environmental factors and urban sites are known as stressful environment, characterized for example by water scarcity. However, little is known on the contribution of urban trees to air quality, notably during drought periods. In a semi-controlled experiment, fourteen young plane trees (Platanus x hispanica, known as a strong isoprene emitter) were grown in containers, in an urban site (at Vitry-sur-Seine, near Paris), since 2020. In June 2022, half the trees were subjected to drought by total rainfall exclusion and by withholding watering. A comprehensive characterization of tree response to drought, including plant morphology (leaf density and area), water status (i.e., leaf water potential, δ¹³C isotopic composition) and physiology (stomatal conductance, net photosynthesis, leaf pigment contents, stress molecular markers, chlorophyll fluorescence) analyses, was undertaken along with the characterization of bVOC emissions by an original leaf scale method (portable GC-MS coupled to a leaf chamber). All together, these parameters provided relevant information on the relation between bVOC emissions and plant morphology, its water use efficiency and photosynthetic energy conversion.

Shortly after the onset of drought, the isoprene emissions of the plane trees remained unchanged even though typical responses to drought stress were observed, such as partial stomatal closure leading to a decrease in carbon assimilation. With the progression of drought stress, progressive leaf shedding occurred. When almost completely defoliated, the trees emitted lower amounts of isoprene emissions likely due to disruption of the photosynthetic energy conversion process. Despite the moderate decrease in absolute isoprene emissions rates (as expressed per dry leaf mass) induced by the drought treatment on plane trees with nearly zero gas exchange, total emissions were strongly affected because defoliation significantly reduced the total leaf area. We emphasize that this phenomenon should be taken into account in atmospheric models especially in species highly subjected to drought induced defoliation. Here, a simple parameterisation of this effect on plane tree-bVOC emissions is proposed.