ABSTRACT
We describ an open leaf gas exchange system coupled to a tunable diode laser (TDL) spectroscopy system enabling measurement of the leaf respiratory CO2 flux and its associated carbon isotope ...composition (δ13CRl) every 3 min. The precision of δ13CRl measurement is comparable to that of traditional mass spectrometry techniques. δ13CRl from castor bean (Ricinus communis L.) leaves tended to be positively related to the ratio of CO2 produced to O2 consumed respiratory quotient (RQ) after 24–48 h of prolonged darkness, in support of existing models. Further, the apparent fractionation between respiratory substrates and respired CO2 within 1–8 h after the start of the dark period was similar to previous observations. In subsequent experiments, R. communis plants were grown under variable water availability to provide a range in δ13C of recently fixed carbohydrate. In leaves exposed to high light levels prior to the start of the dark period, CO2 respired by leaves was up to 11‰ more enriched than phloem sap sugars within the first 10–15 min after plants had been moved from the light into the dark. The 13C enrichment in respired CO2 then decreased rapidly to within 3–7‰ of phloem sap after 30–60 min in the dark. This strong enrichment was not observed if light levels were low prior to the start of the dark period. Measurements of RQ confirmed that carbohydrates were the likely respiratory substrate for plants (RQ > 0.8) within the first 60 min after illumination. The strong 13C enrichment that followed a high light‐to‐dark transition coincided with high respiration rates, suggesting that so‐called light‐enhanced dark respiration (LEDR) is fed by 13C‐enriched metabolites.
The oxygen isotope composition (δ18O) of leaf water (δ18Oleaf) is an important determinant of environmental and physiological information found in biological archives, but the system-scale ...understanding of the propagation of the δ18O of rain through soil and xylem water to δ18Oleaf has not been verified for grassland. Here we report a unique and comprehensive dataset of fortnightly δ18O observations in soil, stem and leaf waters made over seven growing seasons in a temperate, drought-prone, mixed-species grassland. Using the ecohydrology part of a physically based, 18O-enabled soil–plant–atmosphere transfer model (MuSICA), we evaluated our ability to predict the dynamics of δ18O in soil water, the depth of water uptake, and the effects of soil and atmospheric moisture on 18O enrichment of leaf water (Δ18Oleaf) in this ecosystem. The model accurately predicted the δ18O dynamics of the different ecosystem water pools, suggesting that the model generated realistic predictions of the vertical distribution of soil water and root water uptake dynamics. Observations and model predictions indicated that water uptake occurred predominantly from shallow (< 20 cm) soil depths throughout dry and wet periods in all years, presumably due (at least in part) to the effects of high grazing pressure on root system turnover and placement. Δ18Oleaf responded to both soil and atmospheric moisture contents and was best described in terms of constant proportions of unenriched and evaporatively enriched water (two-pool model). The good agreement between model predictions and observations is remarkable as model parameters describing the relevant physical features or functional relationships of soil and vegetation were held constant with one single value for the entire mixed-species ecosystem.
The combined use of a gas‐exchange system and laser‐based isotope measurement is a tool of growing interest in plant ecophysiological studies, owing to its relevance for assessing isotopic ...variability in leaf water and/or transpiration under non‐steady‐state (NSS) conditions. However, the current Farquhar & Cernusak (F&C) NSS leaf water model, originally developed for open‐field scenarios, is unsuited for use in a gas‐exchange cuvette environment where isotope composition of water vapour (δᵥ) is intrinsically linked to that of transpiration (δE). Here, we modified the F&C model to make it directly compatible with the δᵥ–δE dynamic characteristic of a typical cuvette setting. The resultant new model suggests a role of ‘net‐flux’ (rather than ‘gross‐flux’ as suggested by the original F&C model)‐based leaf water turnover rate in controlling the time constant (τ) for the approach to steady sate. The validity of the new model was subsequently confirmed in a cuvette experiment involving cotton leaves, for which we demonstrated close agreement between τ values predicted from the model and those measured from NSS variations in isotope enrichment of transpiration. Hence, we recommend that our new model be incorporated into future isotope studies involving a cuvette condition where the transpiration flux directly influences δᵥ. There is an increasing popularity among plant ecophysiologists to use a gas‐exchange system coupled to laser‐based isotope measurement for investigating non‐steady state (NSS) isotopic variability in leaf water (and/or transpiration); however, the current Farquhar & Cernusak (F&C) NSS leaf water model is unsuited for use in a gas‐exchange cuvette environment due to its implicit assumption of isotope composition of water vapor (δᵥ) being constant and independent of that of transpiration (δE). In the present study, we modified the F&C model to make it compatible with the dynamic relationship between δᵥ and δE as is typically associated with a cuvette setting. Using an experiment conducted on cotton leaves, we show that the modified NSS model performed well in predicting the time constant for the exponential approach of leaf water toward steady state under cuvette conditions. Such a result demonstrates the applicability of this new model to gas‐exchange cuvette conditions where the transpiration flux directly influences δᵥ, and therefore suggests the need to incorporate this model into future isotope studies that employ a laser‐cuvette coupled system.
H
O enrichment develops when leaves transpire, but an accurate generalized mechanistic model has proven elusive. We hypothesized that leaf hydraulic architecture may affect the degree to which ...gradients in H
O develop within leaves, influencing bulk leaf stable oxygen isotope enrichment (Δ
) and the degree to which the Péclet effect is relevant in leaves. Leaf hydraulic design predicted the relevance of a Péclet effect to Δ
in 19 of the 21 species tested. Leaves with well-developed hydraulic connections between the vascular tissue and the epidermal cells through bundle sheath extensions and clear distinctions between palisade and spongy mesophyll layers (while the mesophyll is hydraulically disconnected) may have velocities of the transpiration stream such that gradients in H
O develop and are expressed in the mesophyll. In contrast, in leaves where the vascular tissue is hydraulically disconnected from the epidermal layers, or where all mesophyll cells are well connected to the transpiration stream, velocities within the liquid transport pathways may be low enough that gradients in H
O are very small. Prior knowledge of leaf hydraulic design allows informed selection of the appropriate Δ
modelling framework.
Thermoregulation of leaf temperature (T
leaf) may foster metabolic homeostasis in plants, but the degree to which T
leaf is moderated, and under what environmental contexts, is a topic of debate. ...Isotopic studies inferred the temperature of photosynthetic carbon assimilation to be a constant value of c. 20°C; by contrast, leaf biophysical theory suggests a strong dependence of T
leaf on environmental drivers. Can this apparent disparity be reconciled?
We continuously measured T
leaf and whole-crown net CO₂ uptake for Eucalyptus parramattensis trees growing in field conditions in whole-tree chambers under ambient and +3°C warming conditions, and calculated assimilation-weighted leaf temperature (T
L-AW) across 265 d, varying in air temperature (T
air) from −1 to 45°C. We compared these data to T
L-AW derived from wood cellulose δ18O.
T
leaf exhibited substantial variation driven by T
air, light intensity, and vapor pressure deficit, and T
leaf was strongly linearly correlated with T
air with a slope of c. 1.0. T
L-AW values calculated from cellulose δ18O vs crown fluxes were remarkably consistent; both varied seasonally and in response to the warming treatment, tracking variation in T
air.
The leaves studied here were nearly poikilothermic, with no evidence of thermoregulation of T
leaf towards a homeostatic value. Importantly, this work supports the use of cellulose δ18O to infer T
L-AW, but does not support the concept of strong homeothermic regulation of T
leaf
Abstract
Enhancing the photosynthetic induction response to fluctuating light has been suggested as a key target for improvement in crop breeding programmes, with the potential to substantially ...increase whole-canopy carbon assimilation and contribute to crop yield potential. Rubisco activation may be the main physiological process that will allow us to achieve such a goal. In this study, we assessed the phenotype of Rubisco activation rate in a doubled haploid (DH) barley mapping population 131 lines from a Yerong/Franklin (Y/F) cross after a switch from moderate to saturating light. Rates of Rubisco activation were found to be highly variable across the mapping population, with a median activation rate of 0.1 min−1 in the slowest genotype and 0.74 min−1 in the fastest genotype. A unique quantitative trait locus (QTL) for Rubisco activation rate was identified on chromosome 7H. This is the first report on the identification of a QTL for Rubisco activation rate in planta and the discovery opens the door to marker-assisted breeding to improve whole-canopy photosynthesis of barley. This also suggests that genetic factors other than the previously characterized Rubisco activase (RCA) isoforms on chromosome 4H control Rubisco activity. Further strength is given to this finding as this QTL co-localized with QTLs identified for steady-state photosynthesis and stomatal conductance. Several other distinct QTLs were identified for these steady-state traits, with a common overlapping QTL on chromosome 2H, and distinct QTLs for photosynthesis and stomatal conductance identified on chromosomes 4H and 5H, respectively. Future work should aim to validate these QTLs under field conditions so that they can be used to aid plant breeding efforts.
Speeding up the photosynthetic induction response to fluctuating light has been highlighted as a key target for crop improvement programmes. We identified significant variation in the photosynthetic induction response after a switch from moderate to saturating light across a barley doubled haploid population. A quantitative trait locus (QTL) for Rubisco activation rate was identified on chromosome 7H, as well as overlapping QTLs for steady-state photosynthesis and stomatal conductance. This is the first report on the identification of a QTL for Rubisco activation rate in planta and the discovery opens the door to marker-assisted breeding to improve whole-canopy photosynthesis of barley.
BACKGROUND AND AIMS: Positive relationships between temperature and soil respiration rate are widely observed, but it remains unclear if the relationships are due to increases in soil organic matter ...mineralisation (R ₒₘ ), or in root and rhizosphere respiration (R ᵣₒₒₜₛ ), or increases in both. This study aims to determine the relative sensitivity of R ₒₘ and R ᵣₒₒₜₛ to temperature in soils with differing properties. METHODS: Taking advantage of the difference in stable carbon isotopic composition provided by C₃ and C₄ plants, we partitioned soil respiration into R ₒₘ and R ᵣₒₒₜₛ for two soils with contrasting clay mineralogy, pH and carbon content over a 24 °C temperature range (from 12 to 36 °C). RESULTS: The Chromosol (dominated by illite, with near neutral pH and low organic carbon content) showed an increase in the proportion of R ₒₘ with temperature, indicating an increase in the decomposition of soil organic carbon. In contrast, the Ferrosol (dominated by hematite and goethite, with acidic pH and high organic carbon) showed no change in the proportion of R ₒₘ with warming, and a negative priming effect at the highest temperature. CONCLUSIONS: The observed positive priming effect for the Chromosol and a negative priming effect for the Ferrosol are consistent with contrasting mineralogy, reflecting the relatively weaker bond strength between soil carbon and illites in the Chromosol compared to the Ferrosol.
H218O enrichment develops when leaves transpire, but an accurate generalized mechanistic model has proven elusive. We hypothesized that leaf hydraulic architecture may affect the degree to which ...gradients in H218O develop within leaves, influencing bulk leaf stable oxygen isotope enrichment (ΔL) and the degree to which the Péclet effect is relevant in leaves. Leaf hydraulic design predicted the relevance of a Péclet effect to ΔL in 19 of the 21 species tested. Leaves with well‐developed hydraulic connections between the vascular tissue and the epidermal cells through bundle sheath extensions and clear distinctions between palisade and spongy mesophyll layers (while the mesophyll is hydraulically disconnected) may have velocities of the transpiration stream such that gradients in H218O develop and are expressed in the mesophyll. In contrast, in leaves where the vascular tissue is hydraulically disconnected from the epidermal layers, or where all mesophyll cells are well connected to the transpiration stream, velocities within the liquid transport pathways may be low enough that gradients in H218O are very small. Prior knowledge of leaf hydraulic design allows informed selection of the appropriate ΔL modelling framework.
We show that leaf hydraulic design contributes to the development of gradients in oxygen isotope composition within leaf water, and that a priori knowledge of leaf hydraulic design can guide selection of appropriate leaf water isotope models.
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