Biogenic isoprenoid emissions under drought stress: different responses for isoprene and terpenes

Abstract

Emissions of volatile organic compounds (VOCs) by biogenic sources depend on different environmental conditions. Besides temperature and photosynthetic active radiation (PAR), the available soil water can be a major factor controlling the emission flux. This factor is expected to become more important under future climate conditions, including prolonged drying–wetting cycles. In this paper we use results of available studies on different tree types to set up a parameterization describing the influence of soil water availability (SWA) on different isoprenoid emission rates. Investigating SWA effects on isoprene (C5H8), monoterpene (C10H16) and sesquiterpene (C15H24) emissions separately, it is obvious that different plant processes seem to control the individual emission fluxes, providing a measure to which plants can react to stresses and interact. The SWA impact on isoprene emissions is well described by a biological growth type curve, while the sum of monoterpenes displays a hydraulic conductivity pattern reflecting the plant's stomata opening. However, emissions of individual monoterpene structures behave differently to the total sum, i.e., the emissions of some increase, whereas others decline at decreasing SWA. In addition to a rather similar behavior to that of monoterpene emissions, total sesquiterpene fluxes of species adapted to drought stress tend to reveal a rise close to the wilting point, protecting against oxidative damages. Considering further VOCs as well, the total sum of VOCs tends to increase at the start of severe drought conditions until resources decline. In contrast to declining soil water availability, OH and ozone reactivity are enhanced. Based on these observations, a set of plant protection mechanisms are displayed for fighting drought stress and imply notable feedbacks on atmospheric processes such as ozone, aerosol particles and cloud properties. With increasing lengths of drought periods, declining storage pools and plant structure effects yield different emission mixtures and strengths. This drought feedback effect is definitely worth consideration in climate feedback descriptions and for accurate climate predictions.