Abstract
Both plant physiology and atmospheric chemistry are substantially altered by the emission of volatile isoprenoids (VI), such as isoprene and monoterpenes, from plant leaves. Yet, since gaining scientific attention in the 1950’s, empirical research on leaf VI has been largely confined to laboratory experiments and atmospheric observations. Here, we introduce a new field instrument designed to bridge the scales from leaf to atmosphere, by enabling precision VI detection in real time from plants in their natural ecological setting. With a field campaign in the Brazilian Amazon, we reveal an unexpected distribution of leaf emission capacities (EC) across the vertical axis of the forest canopy, with EC peaking in the mid-canopy instead of the sun-exposed canopy surface, and moderately high emissions occurring in understory specialist species. Compared to the simple interpretation that VI protect leaves from heat stress at the hot canopy surface, our results encourage a more nuanced view of the adaptive role of VI in plants. We infer that forest emissions to the atmosphere depend on the dynamic microenvironments imposed by canopy structure, and not simply on canopy surface conditions. We provide a new emissions inventory from 52 tropical tree species, revealing moderate consistency in EC within taxonomic groups. We highlight priorities in leaf volatiles research that require field-portable detection systems. Our self-contained, portable instrument provides real-time detection and live measurement feedback with precision and detection limits better than 0.5 nmolVI m–2leaf s–1. We call the instrument ‘PORCO’ based on the gas detection method: photoionization of organic compounds. We provide a thorough validation of PORCO and demonstrate its capacity to detect ecologically driven variation in leaf emission rates and thus accelerate a nascent field of science: the ecology and ecophysiology of plant volatiles.
Highlights
Forest-atmosphere interactions are shaped by the exchange of carbon, water, and energy, and by the biological production of organic trace gasses (Laothawornkitkul et al, 2009; Unger, 2014; Rieksta et al, 2020; Tani and Mochizuki, 2021)
Due to the low detection limit achieved with PORCO, we report a higher proportion of species with light-dependent volatile isoprenoids (VI) emissions (67% above detection limit; 56% > 1 nmol m−2 s−1) compared to the inventory at the same site by Harley et al (2004) (46% of 26 species, after correcting their reporting of E. uncinatum from non-emitter to emitter)
PORCO adaptations enable unprecedented in situ detection of leaf volatile emissions with the precision and detection limits required to resolve physiological and ecological drivers of VI emissions from terrestrial vegetation
Summary
Forest-atmosphere interactions are shaped by the exchange of carbon, water, and energy, and by the biological production of organic trace gasses (Laothawornkitkul et al, 2009; Unger, 2014; Rieksta et al, 2020; Tani and Mochizuki, 2021). Models of VI emissions rely on inventories of species measurements from the field to estimate regional fractions of emitting vegetation (Guenther, 2013) This presents a challenge for modeling areas that are under-sampled due to remoteness or high species diversity. Leaves are placed in flowthrough chambers such as commercial photosynthesis cuvettes or less quantitative enclosures, and gas samples are collected from the cuvette exhaust in bags or adsorption cartridges for later analysis by mass spectrometry (Tholl et al, 2006; Niinemets et al, 2010a; Alves et al, 2014; Jardine et al, 2020b; Rieksta et al, 2020) The advantages of this method include control of the leaf environment using a sophisticated leaf cuvette, precision quantification of emissions, and the ability to distinguish VI species. Leaves are enclosed in custom acrylic cuvettes with custom light-emitting-diode (LED) panels providing photosynthetically active radiation (PAR: 90% red, 660 nm; 10% blue, 460 nm) to the leaf to drive photosynthesis
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