Mirror image molecules expose state of rainforest stress

Monoterpenes (C ₁₀ H ₁₆ ) serve as a vital medium for plant communication and defence against biotic (e.g., herbivory) and abiotic (e.g., heat and drought) stresses[1-6]. Some monoterpenes, such as alpha-pinene, are chiral, existing as a (-) and (+) mirror image pair of enantiomers [7, 8]. Despite having the same atmospheric reactivity, enantiomers are produced by different enzymes and emitted through separate leaf emission mechanisms[7-9]. Severe abiotic stress has been shown to change enantiomer emissions, implying chiral ratios could indicate stress severity[8, 10]. Here we present enantiomerically separated alpha-pinene and methyl salicylate data from the Amazon rainforest over time-of-day, season, and the 2023-24 El Niño - which caused the worst recorded drought in Amazonia[11]. Remarkably, correlations of (-)- and (+)-alpha-pinene from the rainforest shifted with increasing stress, aligning with weakening CO ₂ uptake by vegetation and transition between de novo and storage pool emissions. Low- and high-stress zones, along with a recovery zone, were clearly defined through (-)-alpha-pinene and (+)-alpha-pinene correlations, revealing a metric to assess the ecosystem response to stress. This chiral ratio reveals diel, seasonal, El Niño, and edge effect stresses, offering a method to gauge the severity of drought events that are becoming more frequent and intense due to climate change.