A New Field Instrument for Leaf Volatiles Reveals an Unexpected Vertical Profile of Isoprenoid Emission Capacities in a Tropical Forest

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.

An Overview of the Isoprenoid Emissions From Tropical Plant Species

“…LI-6400 (LI-COR, Inc., Lincoln, NE, United States; Keller and Lerdau, 1999 ; Lerdau and Throop, 1999 ; Geron et al, 2002 , 2006 ; Alves et al, 2014 ; Jardine et al, 2016 ) and ADC (LCpro, ADC BioScientific Ltd., Hoddesdon, United Kingdom; Geron et al, 2006 ; Llusià et al, 2010 , 2014 ) gas-exchange systems have also been widely used for sampling VOC emissions. In addition, Taylor et al (2021) introduced a new field instrument called “PORCO” based on photoionization of organic compounds for measuring emissions in the tropics.…”

Emissions of isoprenoids from dominant tree species in subtropical China

“…However, to date, measurements of BVOC emissions from tropical trees are quite limited, and most of them were conducted in South America (the Amazon region), South Africa and India (Mu et al, 2022). In addition, due to limitations in measurement methods or experimental facilities, most field measurements for tropical trees only reported emissions of isoprene (Keller and Lerdau, 1999;Lerdau and Throop, 1999;Geron et al, 2002;Singh and Varshney, 2006;Singh et al, 2007Singh et al, , 2008, total MTs (Singh et al, 2011(Singh et al, , 2014 and total SQTs, and lacked emissions of individual MT and SQT compounds (Guenther et al, 1996;Klinger et al, 1998Klinger et al, , 2002Otter et al, 2002;Harley et al, 2003Harley et al, , 2004Varshney and Singh, 2003;Padhy and Varshney, 2005a,b;Jardine et al, 2020;Taylor et al, 2021). Due to their different reactivities in atmospheric oxidation processes (Atkinson and Arey, 2003;Di Carlo et al, 2004;Edwards et al, 2017), it is necessary to further characterize the emissions of MT or SQT monomers from tropical plants.…”

Intra- and inter-annual changes in isoprene emission from central Amazonia

“…Fluxes were then calculated by correlating fluctuations of the vertical wind vector to the fluctuations of isoprene concentrations with the software package EddyPro® (LI-COR Inc., Lincoln, U.S.A.). A method for despiking and raw data statistical screening was employed (Vickers and Mahrt, 1997). Half-hourly averaged fluxes were flagged according to a method of data quality control (Mauder and Foken, 2004), and only data with the highest quality (flags 0 and 1) was used for further analyses.…”

“…Isoprene emission data were obtained from published data and new measurements for the study species. New measurements were conducted at the ATTO research site (described in section 2.6), and additional measurements were obtained using the PORCO method (Taylor et al, 2021), a customized photoionization detection system, on trees in tropical botanical gardens. Briefly, all PORCO measurements were made in situ on uncut 'sun' branches by enclosing one-to-few leaves inside rigid leaf cuvettes, acclimating them to darkness, and then exposing the leaves to photosynthetically active radiation controlled at 1000 μmol m -2 s -1 , and temperatures near 30˚C, for 3.5 minutes of measurement time.…”

“…Emission rates exceeding 1 nmol m -2 s -1 were considered positively indicative of isoprene emissions. See the full method validation and a discussion of the rarity of detection of other compounds as false positives for isoprene in Taylor et al (2021). Botanic gardens used for tree measurements were: A. Duque private collection, Retiro, Antioquia, Colombia; Fairchild Tropical Botanical Garden, Miami, FL, USA; Jardín Botánico de Cartagena "Guillermo Piñeres", Turbaco, Bolívar, Colombia; Jardín Botánico "Joaquín Antonio Uribe" de Medellín, Antioquia, Colombia; Montgomery Botanical Garden, Miami, FL, USA; Universidad Nacional de Medellín-Sede Medellín arboretum, Antioquia, Colombia.…”