Heatwave frequency and seedling death alter stress-specific emissions of volatile organic compounds in Aleppo pine

Benjamin Birami,Elizabeth Gaona-Colmán,Andrea Ghirardo,Rüdiger Grote,Nadine K Ruehr,Almut Arneth,Daniel Nadal-Sala,Ines Bamberger

doi:10.1007/s00442-021-04905-y

Abstract

Biogenic volatile organic compounds (BVOC) play important roles in plant stress responses and can serve as stress indicators. While the impacts of gradual environmental changes on BVOCs have been studied extensively, insights in emission responses to repeated stress and recovery are widely absent. Therefore, we studied the dynamics of shoot gas exchange and BVOC emissions in Pinus halepensis seedlings during an induced moderate drought, two four-day-long heatwaves, and the combination of drought and heatwaves. We found clear stress-specific responses of BVOC emissions. Reductions in acetone emissions with declining soil water content and transpiration stood out as a clear drought indicator. All other measured BVOC emissions responded exponentially to rising temperatures during heat stress (maximum of 43 °C), but monoterpenes and methyl salicylate showed a reduced temperature sensitivity during the second heatwave. We found that these decreases in monoterpene emissions between heatwaves were not reflected by similar declines in their internal storage pools. Because stress intensity was extremely severe, most of the seedlings in the heat-drought treatment died at the end of the second heatwave (dark respiration ceased). Interestingly, BVOC emissions (methanol, monoterpenes, methyl salicylate, and acetaldehyde) differed between dying and surviving seedlings, already well before indications of a reduced vitality became visible in gas exchange dynamics. In summary, we could clearly show that the dynamics of BVOC emissions are sensitive to stress type, stress frequency, and stress severity. Moreover, we found indications that stress-induced seedling mortality was preceded by altered methanol, monoterpene, and acetaldehyde emission dynamics.

Highlights

Climate change is expected to cause higher temperatures and a higher variability of precipitation, and to produce more frequent and more intense extreme events such as heatwaves and drought spells (Baldwin et al 2019; Kornhuber et al 2019)
The drought treatment alone did at first not result in a distinct Biogenic volatile organic compounds (BVOC) response (Fig. 2a), only with the progression of the experiment and increasing soil drought (Fig. 2b)
We found a strong stimulation of BVOC emissions during two consecutive heatwaves

Summary

Introduction

Climate change is expected to cause higher temperatures and a higher variability of precipitation, and to produce more frequent and more intense extreme events such as heatwaves and drought spells (Baldwin et al 2019; Kornhuber et al 2019) This is likely to intensify forest degradation as has been already observed in many areas worldwide (Anderegg et al 2019; Brodribb et al 2020; Hartmann et al 2018a). The water potential of the conductive xylem can drop below a species-specific critical threshold (Anderegg et al 2019; Ruehr et al 2019), followed by embolism impairing water transport At this point, the probability of droughtinduced mortality increases (Hammond et al 2019) because living tissue becomes dehydrated (Körner 2019). If the stress is not lethal, the organism requires carbon for repair and/or recovery processes, which is why individuals might still die sometime after the stress ceased, if sufficient reserves are not available (Ruehr et al 2019)

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Conclusion