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Abstract. Drought events are expected to become more frequent with climate change. To predict the effect of plant emissions on air quality and potential feedback effects on climate, the study of biogenic volatile organic compound emissions under stress is of great importance. Trees can often be subject to a combination of abiotic stresses, for example due to drought or ozone. Even though there is a large body of knowledge on individual stress factors, the effects of combined stressors are not much explored. This study aimed to investigate changes of biogenic volatile organic compound emissions and physiological parameters in Quercus robur L. during moderate to severe drought in combination with ozone stress. Results show that isoprene emissions decreased while monoterpene and sesquiterpene emissions increased during the progression of drought. We exposed plants with daily ozone concentrations of 100 ppb for 1 h for 7 d, which resulted in faster stomatal closure (e.g., a mean value of −31.3 % at an average stem water potential of −1 MPa), partially mitigating drought stress effects. Evidence of this was found in enhanced green leaf volatiles in trees without ozone fumigation, indicating cellular damage. In addition we observed an enhancement in (C8H8O3)H+ emissions likely corresponding to methyl-salicylate in trees with ozone treatment. Individual plant stress factors are not necessarily additive, and atmospheric models should implement stress feedback loops to study regional-scale effects.

Volatile organic compounds (VOCs) emitted by plants consist of a broad range of gasses which serve purposes such as protecting against herbivores, communicating with insects and neighboring plants, or increasing the tolerance to environmental stresses. Evidence is accumulating that the composition of VOC blends plays an important role in fulfilling these purposes. Constitutional emissions give insight into species-specific stress tolerance potentials and are an important first step in linking metabolism and function of co-occurring VOCs. Here, we investigate the blend composition and interrelations among co-emitted VOCs in unstressed seedlings of four broad-leaved tree species, Quercus robur, Fagus sylvatica, Betula pendula, and Carpinus betulus. VOCs of Q. robur and F. sylvatica mainly emitted isoprene and monoterpenes, respectively. B. pendula had relatively high sesquiterpene emission; however, it made up only 1.7% of its total emissions while the VOC spectrum was dominated by methanol (∼72%). C. betulus was emitting methanol and monoterpenes in similar amounts compared to other species, casting doubt on its frequent classification as a close-to-zero VOC emitter. Beside these major VOCs, a total of 22 VOCs could be identified, with emission rates and blend compositions varying drastically between species. A principal component analysis among species revealed co-release of multiple compounds. In particular, new links between pathways and catabolites were indicated, e.g., correlated emission rates of methanol, sesquiterpenes (mevalonate pathway), and green leaf volatiles (hexanal, hexenyl acetate, and hexenal; lipoxygenase pathway). Furthermore, acetone emissions correlated with eugenol from the Shikimate pathway, a relationship that has not been described before. Our results thus indicate that certain VOC emissions are highly interrelated, pointing toward the importance to improve our understanding of VOC blends rather than targeting dominant VOCs only.

Climate change poses one of the greatest threats to forest ecosystem integrity. An improved understanding of how trees respond to extreme climatic events is crucial to find new ways of managing forests in the face of global warming. In this work we look at the genetic mechanisms governing the production of the plant hormone abscisic acid (ABA), which safeguards plant’s water status by the means of two divergent modes across different conifer species. We find that conifers from evolutionary ancient families adopt a conservative water strategy during drought by accumulating high levels of ABA in their leaves, which we describe as Rising types, while more derived species accumulate the hormone in a transient manner and allow for greater water loss, accordingly to a Peaking type. Moreover, we provide evidence that these contrasting strategies may be controlled by divergent gene expression, including sequences involved in the biosynthetic and catabolic pathways of ABA, and especially nine- cis-epoxycarotenoid dioxygenases ( NCEDs). Our results help to clarify the genetic and physiological bases of iso/anisohydric responses. We believe that studying these and other related genes that regulate plant water status, such as those involved in ABA storage and mobilisation, may help foresters develop and grow more resistant trees.

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