Activity-dependent nocturnal decrease in leaf respiration

New insights into the leaf economic spectrum could benefit terrestrial models

This article is a Commentary on Asao et al. (2025), 246: 1505–1519.

Importance of Timing of Dark Acclimation for Estimating Light Inhibition of Leaf Respiratory CO2 Efflux

The degree of inhibition of leaf respiration by light is often studied, but the methods used and the results obtained are variable. We suggest that in the future daytime leaf respiration is measured 3 min after dark acclimation to avoid under‐estimating the degree of light inhibition of leaf respiration. This will most likely speed up future surveys and perhaps also result in less inter‐study variation in the calculated degree of light inhibition of leaf respiration.

Leaf nonstructural carbohydrate residence time, not concentration, correlates with leaf functional traits following the leaf economic spectrum in woody plants

Nonstructural carbohydrate (NSC) concentrations might reflect the strategies described in the leaf economic spectrum (LES) due to their dependence on photosynthesis and respiration. We examined if NSC concentrations correlate with leaf structure, chemistry, and physiology traits for 114 species from 19 sites and 5 biomes around the globe. Total leaf NSC concentrations varied greatly from 16 to 199 mg g-1 dry mass and were mostly independent of leaf gas exchange and the LES traits. By contrast, leaf NSC residence time was shorter in species with higher rates of photosynthesis, following the fast-slow strategies in the LES. An average leaf held an amount of NSCs that could sustain one night of leaf respiration and could be replenished in just a few hours of photosynthesis under saturating light, indicating that most daily carbon gain is exported. Our results suggest that NSC export is clearly linked to the economics of return on resource investment.

Importance of the leaf respiratory quotient

Rates of leaf respiratory CO2-release (RCO2) are important for terrestrial biosphere models that estimate carbon exchange between plants and the atmosphere. Hitherto, models of RCO2 have primarily been based on considerations of respiratory energy demand (particularly ATP) for maintenance and growth purposes. Respiratory ATP synthesis is closely tied to the rate of respiratory O2-uptake (RO2), with relative engagement of the alternative oxidase influencing the ATP:O ratio. However, the extent to which respiratory ATP synthesis is coupled to leaf RCO2 depends on the respiratory quotient (RQ, mol CO2 efflux per unit mol O2 uptake), with models predicting leaf RCO2 assuming that the RQ is at unity. Here, we show systematic inter-specific, temporal and temperature-dependent variation in leaf RQ, with values of RQ ranging from 0.51 to 2.2, challenging model assumptions on the RQ. We discuss possible mechanisms underlying the variation in leaf RQ, potential ways forward in terms of new measurement protocols, and perspectives for modelled RCO2. Our analyses highlight a range of outstanding research questions that need to be answered before we can mechanistically model leaf RCO2 at various scales.

Simple and Accurate Representation of Cumulative Nighttime Leaf Respiratory CO2 Efflux

Leaf respiratory carbon loss decreases independent of temperature as the night progresses. Detailed nighttime measurements needed to quantify cumulative respiratory carbon loss at night are challenging under both lab and field conditions. We provide a simple yet accurate approach to represent variation in nighttime temperature-independent leaf respiratory CO2 efflux in environments with both stable and fluctuating temperatures, which requires no detailed measurements throughout the night. We demonstrate that the inter- and intraspecific variation in the cumulative leaf respiratory CO2 efflux at constant temperature, at any length of night, scales linearly with the inter- and intraspecific variation in initial measurement of leaf respiratory CO2 efflux at the same temperature at the beginning of the night. This approach informs large-scale predictions of cumulative leaf respiratory CO2 efflux, which is needed to understand plant carbon economy in global change studies as well as in global modeling and eddy covariance monitoring of the land-atmosphere exchange of CO2.

Instantaneous Q10 of night-time leaf respiratory CO2 efflux - measurement and analytical protocol considerations

The temperature sensitivity (e.g. Q10) of night-time leaf respiratory CO2 efflux (RCO2) is a fundamental aspect of leaf physiology. The Q10 typically exhibits a dependence on measurement temperature, and it is speculated that this is due to temperature-dependent shifts in the relative control of leaf RCO2. Two decades ago, a review hypothesized that this mechanistically caused change in values of Q10 is predictable across plant taxa and biomes. Here, we discuss the most appropriate measuring protocol among existing data and for future data collection, to form the foundation of a future mechanistic understanding of Q10 of leaf RCO2 at different temperature ranges. We do this primarily via a review of existing literature on Q10 of night-time RCO2 and only supplement this to a lesser degree with our own original data. Based on mechanistic considerations, we encourage that instantaneous Q10 of leaf RCO2 to represent night-time should be measured: only at night-time; only in response to short-term narrow temperature variation (e.g. max. 10°C) to represent a given midpoint temperature at a time; in response to as many temperatures as possible within the chosen temperature range; and on still attached leaves.

Variation in leaf dark respiration among C3 and C4 grasses is associated with use of different substrates

Measurements of respiratory properties have often been made at a single time point either during daytime using dark-adapted leaves or during nighttime. The influence of the day-night cycle on respiratory metabolism has received less attention but is crucial to understand photosynthesis and photorespiration. Here, we examined how CO2- and O2-based rates of leaf dark respiration (Rdark) differed between midday (after 30-min dark adaptation) and midnight in 8 C3 and C4 grasses. We used these data to calculate the respiratory quotient (RQ; ratio of CO2 release to O2 uptake), and assessed relationships between Rdark and leaf metabolome. Rdark was higher at midday than midnight, especially in C4 species. The day-night difference in Rdark was more evident when expressed on a CO2 than O2 basis, with the RQ being higher at midday than midnight in all species, except in rice (Oryza sativa). Metabolomic analyses showed little correlation of Rdark or RQ with leaf carbohydrates (sucrose, glucose, fructose, or starch) but strong multivariate relationships with other metabolites. The results suggest that rates of Rdark and differences in RQ were determined by several concurrent CO2-producing and O2-consuming metabolic pathways, not only the tricarboxylic acid cycle (organic acids utilization) but also the pentose phosphate pathway, galactose metabolism, and secondary metabolism. As such, Rdark was time-, type- (C3/C4) and species-dependent, due to the use of different substrates.

Measured leaf dark respiratory CO2 -release is not controlled by stomatal conductance

Leaf dark respiratory CO2 -release (RD ) is, according to some literature, dependent on the rate of leaf transpiration. If this is true, then at a given vapor pressure deficit, the leaf stomatal conductance (gs ) will be expected to be a controlling factor of measured RD at any given time. We artificially lowered leaf gs by applying abscisic acid (ABA). Although leaf RD generally covaried temporally with gs , artificially lowering gs by applying ABA does not affect the measured leaf RD . These results indicate that observed diel fluctuations in gs are not directly influencing the measured leaf RD , thereby simplifying both future studies and the interpretation of past studies of the underlying environmental- and physiological drivers of temporal variation in leaf RD .