Impact of heat stress on foliar biogenic volatile organic compound emission and gene expression in tomato (Solanum lycopersicum) seedlings

We present foliar biogenic volatile organic compound (BVOC) emission data from 24-h heat-stressed tomato (Solanum lycopersicum) seedlings including speciated monoterpenes and sesquiterpenes and high time-resolution measurements of methyl salicylate and total monoterpenes. The median total monoterpene and total sesquiterpene emission rates at 30°C were 18.5 and 0.172 pmol m–2 s–1, respectively, which falls within the negligible emission category of previous studies. However, initial heat exposure (39°C or 42°C) increased the emissions of approximately half of the targeted compounds beyond what was predicted by current BVOC emission temperature response algorithms. The enhanced emissions were not always sustained for the entire duration of the heat stress and some plants exhibited a delayed monoterpene response, where emissions peaked toward the end of the heat treatment. Methyl salicylate, a known stress marker, responded differently to the heat stress than most of the other compounds. Heat stress increased methyl salicylate emissions in some plants (at least initially), but in others, emissions declined or did not change significantly. There was no significant correlation between the magnitude of gene expression and emission induction of monoterpenes or methyl salicylate. Furthermore, the emitted monoterpenes did not exhibit any apparent light-dependent behavior, which suggests that these monoterpene stress emissions mostly originated from light-independent foliar storage pools and not from increased de novo production. In contrast, methyl salicylate emissions appear to have contributions from both de novo synthesis and stored pools, as they showed both enzyme-controlled (i.e., light-dependent) and light-independent behaviors. Analyses of the foliar essential oils demonstrate that most of the emitted BVOCs were also present in stored pools. The pool sizes were generally large enough to sustain unstressed emission levels for days to months, and even years for some compounds. However, heat-induced emission enhancement can diminish the pool sizes of some BVOCs, which could decrease subsequent emissions.