Dan Bruhn

Diurnal and seasonal variation in light and dark respiration in field-grown Eucalyptus pauciflora

Respiration from vegetation is a substantial part of the global carbon cycle and the responses of plant respiration to daily and seasonal fluctuations in temperature and light must be incorporated in models of terrestrial respiration to accurately predict these CO2 fluxes. We investigated how leaf respiration (R) responded to changes in leaf temperature (T(leaf)) and irradiance in field-grown saplings of an evergreen tree (Eucalyptus pauciflora Sieb. ex Spreng). Seasonal shifts in the thermal sensitivity of leaf R in the dark (R(dark)) and in the light (R(light)) were assessed by allowing T(leaf) to vary over the day in field-grown leaves over a year. The Q10 of R (i.e., the relative increase in R for a 10 °C increase in T(leaf)) was similar for R(light) and R(dark) and had a value of ∼ 2.5; there was little seasonal change in the Q10 of either R(light) or R(dark), indicating that we may be able to use similar functions to model short-term temperature responses of R in the dark and in the light. Overall, rates of R(light) were lower than those of R(dark), and the ratio of R(light)/R(dark) tended to increase with rising T(leaf), such that light suppression of R was reduced at high T(leaf) values, in contrast to earlier work with this species. Our results suggest we cannot assume that R(light)/R(dark) decreases with increasing T(leaf) on daily timescales, and highlights the need for a better mechanistic understanding of what regulates light suppression of R in leaves.

Methods in plant foliar volatile organic compounds research

Plants are a major atmospheric source of volatile organic compounds (VOCs). These secondary metabolic products protect plants from high-temperature stress, mediate in plant–plant and plant–insect communication, and affect our climate globally. The main challenges in plant foliar VOC research are accurate sampling, the inherent reactivity of some VOC compounds that makes them hard to detect directly, and their low concentrations. Plant VOC research relies on analytical techniques for trace gas analysis, usually based on gas chromatography and soft chemical ionization mass spectrometry. Until now, these techniques (especially the latter one) have been developed and used primarily by physicists and analytical scientists, who have used them in a wide range of scientific research areas (e.g., aroma, disease biomarkers, hazardous compound detection, atmospheric chemistry). The interdisciplinary nature of plant foliar VOC research has recently attracted the attention of biologists, bringing them into the field of applied environmental analytical sciences. In this paper, we review the sampling methods and available analytical techniques used in plant foliar VOC research to provide a comprehensive resource that will allow biologists moving into the field to choose the most appropriate approach for their studies.

Tracking the origins of the Kok effect, 70 years after its discovery

Introduction The 18 New Phytologist Workshop was dedicated to possible causes of the Kok effect, the typical break in the light response curve of net photosynthesis. Available data obtained since its discovery in 1948 show that the effect is not purely caused by a down-regulation of respiration, contrary to the commonly accepted view. However, estimates of leaf respiratory rates obtained in various ecosystems with techniques including the Kok method appear to be widely consistent across different studies, suggesting that Kok-derived values can be used as a surrogate for actual day respiration values. Gross CO2 assimilation of photosynthetic organs of plants is accompanied by concurrent efflux of CO2 by photorespiration and day respiration (i.e. nonphotorespiratory CO2 evolution in the light).While the rate of photorespiration can be predicted using the internal CO2 mole fraction and equations that describe gas exchange (taking into account the stoichiometry of CO2 liberation with respect to O2 fixation by ribulose-1,5-bisphosphate carboxylase/oxygenase), estimating day respiration is much more challenging because there is no equation that can predict its rate as a function of net photosynthesis, CO2 mole fraction or other environmental parameters. That is, in equations describing gas exchange (or isotopic mass balance), day respiration (Rd) has to be determined separately or simply assumed to model net carbon (C) exchange. At the leaf level, day respiration represents a C loss of c. 5% of gross-fixed CO2 but this proportion is highly variable, depending on species and conditions (see, e.g. Atkin et al., 1997). Estimates of day respiratory CO2 loss rely on specific techniques used tomeasure Rd: amongst them, the Kokmethod is certainly the most popular, because it is easy to implement in the laboratory or in the field using classical gas-exchange systems. This method takes advantage of the ‘Kok effect’, a phenomenon first described in the 1940s in unicellular algae (Kok, 1948, 1949). This effect is further described later, and in Fig. 1. The Kok effect is believed to be primarily caused by the inhibition of respiration by light and thus provides a direct way to estimate Rd. At the present date, c. 800 published works have used, or cited, the Kokmethod, representing c. 40% of articles that involve ameasurement of Rd or deal with day respiration. However, some persisting doubt remains about the validity of this method, simply because the Kok effect is inconstant and influenced by environmental conditions (such as O2 mole fraction) in ways that may not be consistent with day respiratory metabolism. Considering the wide range of applications, and the considerable number of articles that have been published, there is an urgent need to clarify the origin of the Kok effect and to evaluate its relevance to measure Rd. This was the objective of the 18 th New Phytologist Workshop that took place in July 2016 in Angers (France).

Solar UV Irradiation-Induced Production of Greenhouse Gases from Plant Surfaces: From Leaf to Earth

During the past few decades it has been documented that the ultra-violet (UV) component of natural sunlight alone or in combination with visible light can instantaneously stimulate aerobic plant production of a range of important trace gases: CH4, CO2, CO, short-chain hydrocarbons/ non-methane volatile organic compounds (NMVOC), NOx and N2O. This gas production, near or at the plant surface, is a new discovery and is normally not included in emission budgets (e.g. by the Intergovernmental Panel on Climate Change, IPCC) due to a lack of information with respect to validation and upscaling. For CH4 it is known that the light dose controls emission under ambient and artificial light conditions, but the atmospheric gas composition and other environmental factors can influence gas production as well. Several plant components, including pectin and leaf wax, have been suggested as a precursor for CH4 production, but underlying mechanisms are not fully known. For other gases such generating processes have not been established yet and mechanisms remain hypothetical. Field measurements of UV-induced emissions of the gases under natural light conditions are scarce. Therefore, realistic upscaling to the ecosystem level is uncertain for all gases. Nevertheless, based on empirical response curves, we propose the first global upscaling of UV-induced N2O and CO to illustrate emission ranges from a global perspective and as a contribution to an ongoing quantification process. When scaled to the global level, the UV-induced emission of CO by vegetation surfaces amounts to up to 22 Tg yr−1, which equals 11–44% of all the natural terrestrial plant sources accounted for so far. The total light-driven N2O emissions amount to 0.65–0.78 Tg yr−1, which equals 7–24% of the natural terrestrial source strength accounted for (range 3.3–9 Tg N yr−1). In this review, we summarize current knowledge, based on experimental work with sunlight and artificial light, and estimate potential emission ranges and uncertainties, placing the available data into perspective. We discuss the state of the art in proposed mechanisms, precursors and environmental relationships, we consider the relevance of measured emission rates, and we also suggest a range of future research topics. Furthermore we propose and describe methods and techniques that can be used for future research.

Temperature responses of photosynthesis and respiration in a sub-Antarctic megaherb from Heard Island

Understanding the response of sub-Antarctic plants to a warming climate requires an understanding of the relationship of carbon gain and loss to temperature. In a field study on Heard Island, we investigated the responses of photosynthesis and respiration of the sub-Antarctic megaherb Pringlea antiscorbutica R. Br. to temperature. This was done by instantaneously manipulating leaf temperature in a gas exchange cuvette on plants adapted to natural temperature variation along an altitudinal gradient. There was little altitudinal variation in the temperature response of photosynthesis. Photosynthesis was much less responsive to temperature than electron transport, suggesting that Rubisco activity was generally the rate-limiting process. The temperature response of leaf respiration rates was greater in cold-grown (high altitude) plants compared with warm-grown (low altitude) plants. This thermal acclimation would enable plants to maintain a positive carbon budget over a greater temperature range.