Vertical farming is considered to be a key enabler for transforming agrifood systems, especially in or nearby urbanized areas. Vertical farming systems (VFS) are advanced indoor cropping systems that allow for highly intensified and standardized plant production. The close control of environmental parameters makes crop production stable and repeatable, ensuring year-round uniform product quality and quantity irrespective of location. However, due to continuous changes in plant physiology and development, as well as frequent changes in electricity prices, the optimum conditions for crop production and its associated costs can change within days or even minutes. This makes it beneficial to dynamically adjust setpoints for light (intensity, spectrum, pattern, and daylength), CO2, temperature, humidity, air flow, and water and nutrient availability. In this review, we highlight the beneficial effects that dynamic growth conditions can have on key plant processes, including improvements in photosynthetic gas exchange, transpiration, organ growth, development, light interception, flowering, and product quality. Our novel findings based on modeling and experimentation demonstrate that a dynamic daily light intensity pattern that responds to frequent changes in electricity prices can save costs without reducing biomass. Further, we argue that a smart, dynamic VFS climate management requires feedback mechanisms: several mobile and immobile sensors could work in combination to continuously monitor the crop, generating data that feeds into crop growth models, which, in turn, generate climate setpoints. In addition, we posit that breeding for the VFS environment is at a very early stage and highlight traits for breeding for this specialized environment. We envision a continuous feedback loop between dynamic crop management, crop monitoring, and trait selection for genotypes that are specialized for these conditions.

Leaf-level gas exchange data support the mechanistic understanding of plant fluxes of carbon and water. These fluxes inform our understanding of ecosystem function, are an important constraint on parameterization of terrestrial biosphere models, are necessary to understand the response of plants to global environmental change, and are integral to efforts to improve crop production. Collection of these data using gas analyzers can be both technically challenging and time consuming, and individual studies generally focus on a small range of species, restricted time periods, or limited geographic regions. The high value of these data is exemplified by the many publications that reuse and synthesize gas exchange data, however the lack of metadata and data reporting conventions make full and efficient use of these data difficult. Here we propose a reporting format for leaf-level gas exchange data and metadata to provide guidance to data contributors on how to store data in repositories to maximize their discoverability, facilitate their efficient reuse, and add value to individual datasets. For data users, the reporting format will better allow data repositories to optimize data search and extraction, and more readily integrate similar data into harmonized synthesis products. The reporting format specifies data table variable naming and unit conventions, as well as metadata characterizing experimental conditions and protocols. For common data types that were the focus of this initial version of the reporting format, i.e., survey measurements, dark respiration, carbon dioxide and light response curves, and parameters derived from those measurements, we took a further step of defining required additional data and metadata that would maximize the potential reuse of those data types. To aid data contributors and the development of data ingest tools by data repositories we provided a translation table comparing the outputs of common gas exchange instruments. Extensive consultation with data collectors, data users, instrument manufacturers, and data scientists was undertaken in order to ensure that the reporting format met community needs. The reporting format presented here is intended to form a foundation for future development that will incorporate additional data types and variables as gas exchange systems and measurement approaches advance in the future. The reporting format is published in the U.S. Department of Energy's ESS-DIVE data repository, with documentation and future development efforts being maintained in a version control system.

Scaling current cereal production to a growing global population will be a challenge. Wheat supplies approximately one-fifth of the calories and protein for human diets. Vertical farming is a possible promising option for increasing future wheat production. Here we show that wheat grown on a single hectare of land in a 10-layer indoor vertical facility could produce from 700 ± 40 t/ha (measured) to a maximum of 1,940 ± 230 t/ha (estimated) of grain annually under optimized temperature, intensive artificial light, high CO2 levels, and a maximum attainable harvest index. Such yields would be 220 to 600 times the current world average annual wheat yield of 3.2 t/ha. Independent of climate, season, and region, indoor wheat farming could be environmentally superior, as less land area is needed along with reuse of most water, minimal use of pesticides and herbicides, and no nutrient losses. Although it is unlikely that indoor wheat farming will be economically competitive with current market prices in the near future, it could play an essential role in hedging against future climate or other unexpected disruptions to the food system. Nevertheless, maximum production potential remains to be confirmed experimentally, and further technological innovations are needed to reduce capital and energy costs in such facilities.

Summary The Kok effect is a well‐known phenomenon in which the quantum yield of photosynthesis changes abruptly at low light. This effect has often been interpreted as a shift in leaf respiratory metabolism and thus used widely to measure day respiration. However, there is still no formal evidence that the Kok effect has a respiratory origin. Here, both gas exchange and isotopic labeling were carried out on sunflower leaves, using glucose that was 13C‐enriched at specific C‐atom positions. Position‐specific decarboxylation measurements and NMR analysis of metabolites were used to trace the fate of C‐atoms in metabolism. Decarboxylation rates were significant at low light (including above the Kok break point) and increased with decreasing irradiance below 100 µmol photons m−2 s−1. The variation in several metabolite pools such as malate, fumarate or citrate, and flux calculations suggest the involvement of several decarboxylating pathways in the Kok effect, including the malic enzyme. Our results show that day respiratory CO2 evolution plays an important role in the Kok effect. However, the increase in the apparent quantum yield of photosynthesis below the Kok break point is also probably related to malate metabolism, which participates in maintaining photosynthetic linear electron flow.

Abstract We describe the baseline coupled model configuration and simulation characteristics of GFDL's Earth System Model Version 4.1 (ESM4.1), which builds on component and coupled model developments at GFDL over 2013–2018 for coupled carbon‐chemistry‐climate simulation contributing to the sixth phase of the Coupled Model Intercomparison Project. In contrast with GFDL's CM4.0 development effort that focuses on ocean resolution for physical climate, ESM4.1 focuses on comprehensiveness of Earth system interactions. ESM4.1 features doubled horizontal resolution of both atmosphere (2° to 1°) and ocean (1° to 0.5°) relative to GFDL's previous‐generation coupled ESM2‐carbon and CM3‐chemistry models. ESM4.1 brings together key representational advances in CM4.0 dynamics and physics along with those in aerosols and their precursor emissions, land ecosystem vegetation and canopy competition, and multiday fire; ocean ecological and biogeochemical interactions, comprehensive land‐atmosphere‐ocean cycling of CO2, dust and iron, and interactive ocean‐atmosphere nitrogen cycling are described in detail across this volume of JAMES and presented here in terms of the overall coupling and resulting fidelity. ESM4.1 provides much improved fidelity in CO2 and chemistry over ESM2 and CM3, captures most of CM4.0's baseline simulations characteristics, and notably improves on CM4.0 in (1) Southern Ocean mode and intermediate water ventilation, (2) Southern Ocean aerosols, and (3) reduced spurious ocean heat uptake. ESM4.1 has reduced transient and equilibrium climate sensitivity compared to CM4.0. Fidelity concerns include (1) moderate degradation in sea surface temperature biases, (2) degradation in aerosols in some regions, and (3) strong centennial scale climate modulation by Southern Ocean convection.

Trees typically experience large diurnal depressions in water potential, which may impede carbon export from leaves during the day because the xylem is the source of water for the phloem. As water potential becomes more negative, higher phloem osmotic concentrations are needed to draw water in from the xylem. Generating this high concentration of sugar in the phloem is particularly an issue for the ∼50% of trees that exhibit passive loading. These ideas motivate the hypothesis that carbon export in woody plants occurs predominantly at night, with sugars that accumulate during the day assisting in mesophyll turgor maintenance or being converted to starch. To test this, diurnal and seasonal patterns of leaf nonstructural carbohydrates, photosynthesis, solute, and water potential were measured, and carbon export was estimated in leaves of five mature (>20 m tall) red oak (Quercus rubra) trees, a species characterized as a passive loader. Export occurred throughout the day at equal or higher rates than at night despite a decrease in water potential to -1.8 MPa at midday. Suc and starch accumulated over the course of the day, with Suc contributing ∼50% of the 0.4 MPa diurnal osmotic adjustment. As a result of this diurnal osmotic adjustment, estimates of midday turgor were always >0.7 MPa. These findings illustrate the robustness of phloem functioning despite diurnal fluctuations in leaf water potential and the role of nonstructural carbohydrates in leaf turgor maintenance.

Abstract We describe the Geophysical Fluid Dynamics Laboratory's CM4.0 physical climate model, with emphasis on those aspects that may be of particular importance to users of this model and its simulations. The model is built with the AM4.0/LM4.0 atmosphere/land model and OM4.0 ocean model. Topics include the rationale for key choices made in the model formulation, the stability as well as drift of the preindustrial control simulation, and comparison of key aspects of the historical simulations with observations from recent decades. Notable achievements include the relatively small biases in seasonal spatial patterns of top‐of‐atmosphere fluxes, surface temperature, and precipitation; reduced double Intertropical Convergence Zone bias; dramatically improved representation of ocean boundary currents; a high‐quality simulation of climatological Arctic sea ice extent and its recent decline; and excellent simulation of the El Niño‐Southern Oscillation spectrum and structure. Areas of concern include inadequate deep convection in the Nordic Seas; an inaccurate Antarctic sea ice simulation; precipitation and wind composites still affected by the equatorial cold tongue bias; muted variability in the Atlantic Meridional Overturning Circulation; strong 100 year quasiperiodicity in Southern Ocean ventilation; and a lack of historical warming before 1990 and too rapid warming thereafter due to high climate sensitivity and strong aerosol forcing, in contrast to the observational record. Overall, CM4.0 scores very well in its fidelity against observations compared to the Coupled Model Intercomparison Project Phase 5 generation in terms of both mean state and modes of variability and should prove a valuable new addition for analysis across a broad array of applications.

Foliar uptake of water from the surface of leaves is common when rainfall is scarce and non-meteoric water such as dew or fog is more abundant. However, many species in more mesic environments have hydrophobic leaves that do not allow the plant to uptake water. Unlike foliar uptake, all species can benefit from dew- or fog-induced transpiration suppression, but despite its ubiquity, transpiration suppression has so far never been quantified. Here, we investigate the effect of dew-induced transpiration suppression on the water balance and the isotope composition of leaves via a series of experiments. Characteristically, hydrophobic leaves of a tropical plant, Colocasia esculenta, are misted with isotopically enriched water to reproduce dew deposition. This species does not uptake water from the surface of its leaves. We measure leaf water isotopes and water potential and find that misted leaves exhibit a higher water potential and a more depleted water isotope composition than dry leaves, suggesting a ∼ 30% decrease in transpiration rate compared to control leaves. We propose three possible mechanisms governing the interaction of water droplets with leaf energy balance: increase in albedo from the presence of dew droplets, decrease in leaf temperature from the evaporation of dew, and local decrease in vapor pressure deficit. Comparing previous studies on foliar uptake to our results, we conclude that transpiration suppression has an effect of similar amplitude, yet opposite sign to foliar uptake on leaf water isotopes.

A fundamental challenge in plant physiology is independently determining the rates of gross O-2 production by photosynthesis and O-2 consumption by respiration, photorespiration, and other processes. Previous studies on isolated chloroplasts or leaves have separately constrained net and gross O-2 production (NOP and GOP, respectively) by labeling ambient O-2 with O-18 while leaf water was unlabeled. Here, we describe a method to accurately measure GOP and NOP of whole detached leaves in a cuvette as a routine gas-exchange measurement. The petiole is immersed in water enriched to a delta O-18 of similar to 9,000 parts per thousand, and leaf water is labeled through the transpiration stream. Photosynthesis transfers O-18 from H2O to O-2. GOP is calculated from the increase in delta O-18 of O-2 as air passes through the cuvette. NOP is determined from the increase in O-2/N-2. Both terms are measured by isotope ratio mass spectrometry. CO2 assimilation and other standard gas-exchange parameters also were measured. Reproducible measurements are made on a single leaf for more than 15 h. We used this method to measure the light response curve of NOP and GOP in French bean (Phaseolus vulgaris) at 21% and 2% O-2. We then used these data to examine the O-2/CO2 ratio of net photosynthesis, the light response curve of mesophyll conductance, and the apparent inhibition of respiration in the light (Kok effect) at both oxygen levels. The results are discussed in the context of evaluating the technique as a tool to study and understand leaf physiological traits.

A fundamental challenge in plant physiology is independently determining the rates of gross O2 production by photosynthesis and O2 consumption by respiration, photorespiration, and other processes. Previous studies on isolated chloroplasts or leaves have separately constrained net and gross O2 production (NOP and GOP, respectively) by labeling ambient O2 with 18O while leaf water was unlabeled. Here, we describe a method to accurately measure GOP and NOP of whole detached leaves in a cuvette as a routine gas-exchange measurement. The petiole is immersed in water enriched to a δ18O of ∼9,000‰, and leaf water is labeled through the transpiration stream. Photosynthesis transfers 18O from H2O to O2 GOP is calculated from the increase in δ18O of O2 as air passes through the cuvette. NOP is determined from the increase in O2/N2 Both terms are measured by isotope ratio mass spectrometry. CO2 assimilation and other standard gas-exchange parameters also were measured. Reproducible measurements are made on a single leaf for more than 15 h. We used this method to measure the light response curve of NOP and GOP in French bean (Phaseolus vulgaris) at 21% and 2% O2 We then used these data to examine the O2/CO2 ratio of net photosynthesis, the light response curve of mesophyll conductance, and the apparent inhibition of respiration in the light (Kok effect) at both oxygen levels. The results are discussed in the context of evaluating the technique as a tool to study and understand leaf physiological traits.

Phosphorus (P) limitation is known to have substantial impacts on leaf metabolism. However, uncertainty remains around whether P deficiency alters scaling functions linking leaf metabolism to associated traits. We investigated the effect of P deficiency on leaf gas exchange and related leaf traits in 17 contrasting Eucalyptus species that exhibit inherent differences in leaf traits. Saplings were grown under controlled-environment conditions in a glasshouse, where they were subjected to minus and plus P treatments for 15 weeks. P deficiency decreased P concentrations and increased leaf mass per area (LMA) of newly-developed leaves. Rates of photosynthesis (A) and respiration (R) were also reduced in P-deficient plants compared with P-fertilised plants. By contrast, P deficiency had little effect on the temperature sensitivity of R. Irrespective of P treatment, on a log-log basis A and R scaled positively with increasing leaf nitrogen concentration [N] and negatively with increasing LMA. Although P deficiency had limited impact on A-R-LMA relationships, rates of CO2 exchange per unit N were consistently lower in P-deficient plants. Our results highlight the importance of P supply for leaf carbon metabolism and show how P deficiencies (i.e. when excluding confounding genotypic and environmental effects) can have a direct effect on commonly used leaf trait scaling relationships.

Editor: Thank you for your interests in HESS. We received review comments from two qualified reviewers. Both acknowledged the importance of the topic, but pointed out places that require clarifications and justifications. For example, reviewer #2 mentioned “I suggest authors take advantage of various satellite products to define what scale of heterogeneity needs to be incorporated in each ESM grid cell to better represent land surface states and fluxes.” This would need a more direct response. In addition, you mentioned that “As such it is meant to be primarily a technical paper” in your response. Please make sure to select the appropriate manuscript type during revision submission (e.g., technical notes)

Dept. of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544, USA Dept. of Geosciences, Princeton University, Princeton, NJ 08544, USA Dept. of Biology, University of New Mexico, Albuquerque, NM 87131, USA Dept. of Geological, Environmental, and Marine Sciences, Rider University, Lawrenceville, NJ 08648, USA EarthRes Group, Inc., Pipersville, PA 18947, USA Dept. of Geography, UC Santa Barbara, Santa Barbara, CA 93106, USA Bren School of Environmental Science and Management, UC Santa Barbara, Santa Barbara, CA 93106, USA

The 18th 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 to measure Rd: amongst them, the Kok method 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 Kok method, representing c. 40% of articles that involve a measurement 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 18th New Phytologist Workshop that took place in July 2016 in Angers (France). The ‘Kok effect’ refers to the change in quantum yield of net photosynthesis (Φ) at low light levels: at very low light levels (typically 0–20 μmol m−2 s−1 of incident photosynthetically active radiation, iPAR), the quantum yield (denoted as Φ1) is larger than that observed at higher light levels (Φ2). In practice, when a light response curve of net photosynthesis is performed, there is a change in the slope and a break point (examples are shown in Fig. 1). In general, Φ1 is c. 0.1 under standard conditions (25°C, 21% O2 and 380 μmol mol−1) while Φ2 is c. 0.06 (Fig. 2). These values are rather similar when net photosynthesis is measured as CO2 fixation or O2 release (but data on the assimilatory quotient at low light, presumably close to 1, are scarce). Thus, the relative change in quantum yield above the break point is about (0.1–0.06)/0.1 = 40% at ambient CO2 (380 μmol mol−1). Note that computing a true value of quantum yield requires a correction for leaf absorbance so as to convert incident radiation into absorbed light. The extrapolated intercept associated with the second portion of the response curve gives an estimate of Rd (illustrated in Fig. 1a), which is typically lower than Rn, the rate of respiration in darkness (night respiration). In other words, in this region of irradiance, the response curve of net assimilation is modelled as A = Φ2·iPAR·α – Rd while at very low light, it is modelled as A = Φ1·iPAR·α – Rn, where α is leaf light absorbance. Problems associated with the Kok method itself should be recognized. First, in practice, carrying out a light response curve at very low light can be difficult due to the small difference between inlet and outlet air in open gas exchange systems (since A is low) and leaks, even very modest, can be an issue. Second, observing the two linear portions of the light response curve (and thus calculating Rd, Φ1 and Φ2) can be rather difficult when the number of data points is limited. A good graphical resolution is also necessary to see the break point (e.g. compare the resolution of Fig. 1a and b). Consequently, there is often some uncertainty in the choice of data points to draw linear regressions. Including or excluding points in the presumed neighbourhood of the break point can change Φ-values and Rd significantly. For example, in Fig. 1(a), excluding and including the third point gives Φ1 values of 0.099 and 0.085, respectively, and gives Rd values of 0.47 and 0.35 μmol m−2 s−1, respectively. A recommendation to solve this problem is to have a sufficient number of measurements: typically, at least three in the 0–10 μmol m−2 s−1 region. The Kok method is used widely to estimate Rd, including in wide-spectrum studies carried out in different species or ecosystems under various conditions. For example, the Kok method has been implemented recently in an unpublished world-wide survey presented at the Workshop by Owen Atkin, Mary Heskel and others, in arctic species (Heskel et al., 2014), in tropical tree canopies (Weerasinghe et al., 2014), in trees in different seasons (Way et al., 2015) and at varying CO2 (Crous et al., 2012; Kroner & Way, 2016), or in different species along a vegetation chronosequence (Atkin et al., 2013). The usefulness of Kok-derived estimates of leaf respiration in the light for ecosystem C budget studies has been extensively discussed (Heskel et al., 2013). Interestingly, the Kok effect has been shown to scale up to the ecosystem, that is, with a break in the response curve of net ecosystem uptake of CO2 to measured irradiance (Bruhn et al., 2011). At the Workshop, it has been recognized that in general, Kok-derived estimates of Rd are lower than Rn by 20–40%, consistent with the well-accepted inhibition of leaf respiratory metabolism by light. Comparisons with Rd values obtained using other techniques (such as the Laisk method, which takes advantage of response curves to CO2 mole fraction) have also been shown to be rather satisfactory despite some variability (see, e.g. Villar et al., 1994). Further data presented during the Workshop also showed a relatively good agreement between Kok-, Laisk- and isotope-derived Rd values in spinach, cocklebur and Magnolia leaves (Barbour et al., 2017). The widely-accepted (historical) origin of the effect is the inhibition of respiratory metabolism by light (linear decrease of Rd with light) and in fact, mechanisms for the down-regulation of respiratory decarboxylation reactions by light have been described (reviewed in Tcherkez et al., 2012). In addition, the pentose phosphate pathway (PPP), which also liberates CO2, has been shown to be inhibited by light, even at very low light levels (Singh et al., 1993; Farr et al., 1994). A metabolic steady-state model has also suggested that at low light, the enhancement of the PPP can potentially explain the Kok effect (Buckley & Adams, 2011). However, a purely respiratory (catabolic) origin of the Kok effect is highly unlikely. In fact, it strongly depends on gaseous conditions whereas Rd is not expected to be very sensitive to CO2 and O2 mole fraction. The Kok effect disappears at low oxygen (Fig. 1b,c; Cornic & Jarvis, 1972; Ishii & Murata, 1978; Sharp et al., 1984), suggesting that photorespiration could be involved. The Kok effect also depends on CO2: the relative difference between Φ1 and Φ2 decreases, but does not disappear, at high CO2 mole fraction (Fig. 2), suggesting again that photorespiration could explain part of the effect. It should nevertheless be noted that the Kok effect disappears at extremely high CO2 (≥ 1%) (Björkman & Demmig, 1987; Evans, 1987) but this observation might not be very conclusive due to side effects of extremely high CO2 on C metabolism (including cellular acidification and inhibition of respiration). Potentially, a photorespiratory origin could be due to: (1) a different photorespiratory metabolism at low light (such as a change in O2/CO2 stoichiometry) thereby making the ‘scaling factor’ (cc − Γ*)/(cc + 2Γ*) erroneous in Eqn 1. Recently, slight changes in photorespiratory stoichiometry have been found at high O2 or low CO2 but significant changes at very low photorespiration rates seem unlikely (Abadie et al., 2016); or (2) changes in cc along a light curve. Usually, the classical correction used to adjust A values to what they would be if intercellular CO2 (ci) were constant (Kirschbaum & Farquhar, 1987) is minimal and does not suppress the Kok effect. Still, the second hypothesis appears very likely, through the influence of internal conductance so that cc/Γ* (rather than ci/Γ*) increases considerably at low light (see the companion article Farquhar & Busch, 2017). It is nevertheless improbable that an effect on cc only can explain the Kok effect in totality. In fact, the effect persists at high CO2 (Fig. 2). Furthermore, it has been originally described in unicellular algae with a carbon concentrating mechanism (CCM) (Kok, 1948) and has also been found in other CCM-containing algae (Peltier & Sarrey, 1988). Also, the break in the light response curve, when it happens to be visible (as in Fig. 1c), would not be easy to explain since there is no clear reason for a discontinuous effect of internal conductance on cc/Γ*. It should also be noted that the effect of gaseous conditions might not be inconsistent with metabolism: under the steady-state hypothesis, the balance of reductive power predicts that PPP activity should depend on CO2 mole fraction (Buckley & Adams, 2011). Unfortunately, there is presently no published data (of either metabolomics or fluxomics) obtained at very low light along a Kok curve. Therefore, fluxes in catabolic pathways responsible for CO2 generation at very low light are not very well known. Recent unpublished data obtained using isotopic (13C) labelling and presented during the Workshop by Gauthier and co-workers have nevertheless suggested that at very low light, decarboxylation by the pyruvate dehydrogenase is up-regulated. Finally, other mechanisms associated with electron transport cannot be excluded. First, at very low light, there is an abrupt decrease in the cyclic electron flux around PSI that disappears under 2% O2, thereby suggesting that γ can change, may be due to the Mehler reaction (Laisk et al., 2005; Kou et al., 2013). Second, both the light partition to PSII (a) and the photochemical yield of PSII (ΦPSII) have been found to increase at low light (Oberhuber et al., 1993; Yin et al., 2014). A summary of possible explanations of the Kok effect is shown in Table 1. It is clear that the origin of this effect is not unique, and it is likely a combination of several processes that lead to an increase in the quantum yield of photochemistry, and cause gaseous (decrease in photorespiration due to the increase in cc/Γ*) and metabolic changes at very low light. In an effort to disentangle the mechanism of the Kok effect, more experiments should be done at very low light to ascertain catabolic pathways involved, examine electron transport parameters and the CO2/O2 assimilatory quotient, and use species where the Kok effect does not occur like C4 plants (Cornic & Jarvis, 1972; Ishii et al., 1979) and perhaps, C3/C4 intermediates. There is little doubt that the rate of day respiration Rd is lower than Rn because it has been shown using several methods (for a review, see Tcherkez & Ribas-Carbó, 2012). However, it seems clear that the Kok effect is not purely respiratory and thus, the values of Rd or Rd/Rn obtained with the Kok method have to be considered as proxies.

Contents 986 I. 987 II. 987 III. 988 IV. 991 V. 992 VI. 995 VII. 997 VIII. 998 References 998 SUMMARY: It has been 75 yr since leaf respiratory metabolism in the light (day respiration) was identified as a low-flux metabolic pathway that accompanies photosynthesis. In principle, it provides carbon backbones for nitrogen assimilation and evolves CO2 and thus impacts on plant carbon and nitrogen balances. However, for a long time, uncertainties have remained as to whether techniques used to measure day respiratory efflux were valid and whether day respiration responded to environmental gaseous conditions. In the past few years, significant advances have been made using carbon isotopes, 'omics' analyses and surveys of respiration rates in mesocosms or ecosystems. There is substantial evidence that day respiration should be viewed as a highly dynamic metabolic pathway that interacts with photosynthesis and photorespiration and responds to atmospheric CO2 mole fraction. The view of leaf day respiration as a constant and/or negligible parameter of net carbon exchange is now outdated and it should now be regarded as a central actor of plant carbon-use efficiency.

As global climatic changes increase plant susceptibility to large-scale disturbances such as drought and pathogens, understory responses to these disturbances will become increasingly important to long-term forest dynamics. To better understand understory responses to canopy disturbance, we measured changes in the growth and physiology of the dominant understory shrub, American witch-hazel (Hamamelis virginiana L.), in response to girdling of canopy oaks in a temperate hardwood forest of the northeastern United States. Changes in the growth and physiology of H. virginiana may be important to the regeneration of northeastern temperate forests, as this common shrub largely shapes the microenvironment for seedlings on the forest floor where it occurs. Canopy disturbance by girdling resulted in significant increases in light and soil nitrogen availability. In response to these environmental changes, basal-area growth of H. virginiana increased by an average 334%. This growth increase corresponded to significant increases in foliar nitrogen, respiration, and leaf chlorophyll and carotenoid concentrations. These findings indicate improved environmental conditions and increased growth for this understory shrub following the loss of dominant canopy trees. This study suggests that following large-scale canopy disturbance, H. virginiana and shrubs like it may play an important role in competing for soil N and shading seedlings of regenerating canopy species.

Spatial patterns of leaf water isotopes are challenging to predict because of the intricate link between vein and lamina water. Many models have attempted to predict these patterns, but to date, most have focused onmonocots with parallel veins. These provide a simple system to study, but do not represent the majority of plant species. Here, a new protocol is developed using a Picarro induction module coupled to a cavity ringdown spectrometer to obtain maps of the leaf water isotopes (O-18 and H-2). The technique is applied to Colocasia esculenta leaves. The results are compared with isotope ratio mass spectrometry. In C. esculenta, a large enrichment in the radial direction is observed, but not in the longitudinal direction. The string-of-lakes model fails to predict the observed patterns, while the Farquhar-Gan model is more successful, especially when enrichment is accounted for along the radial direction. Our results show that reticulate-veined leaves experience a larger enrichment along the axis of the secondary veins than along the midrib. We hypothesize that this is due to the lower major/minor vein ratio that leads to longer pathways between major veins and sites of evaporation.