Multiscale use of fluorescence for an enhanced understanding of the vegetation carbon uptake during drought – FACET

WP1: Fluorescence yield of photosystem I (F(PSI)) and NPQ in abiotic stress conditions

Under field conditions, and especially abiotic stress conditions, the fluorescence yield of PSII (F(PSII)) is largely quenched due to NPQ. When F(PSII) is low, F(PSI) becomes relatively more important, especially under conditions when the acceptor side of PSI is limiting and PSI is in a “closed state”. Furthermore, changes in the absorption cross section by alterations of the antenna size, the so-called state transitions, will increase F(PSI) in comparison to F(PSII).
Wild-type plants, Arabidopsis as a plant model for C3 species and maize as a plant model for C4 species, will be subjected to drought stress at control and elevated temperatures and transient heat waves up to 40°C. Arabidopsis mutants affected in cyclic electron flow and photosynthetic control and in ferredoxin and ferredoxin-NADP+ oxidoreductase will be included. In the latter mutants, PSI is expected to be in a closed state since acceptor-side limitation of PSI is highly likely. Furthermore, state transition mutants will be studied to demonstrate whether the antenna size of PSI is the key for variability of F(PSI). ChlaF at room temperature will be detected at ? < 710 nm and ? > 710 nm, and at low temperature to see alterations in antenna size. Linear and cyclic flows will be monitored by ChlaF and transient absorption. In maize, changes in F(PSI) will be monitored upon exposure to abiotic stress in mesophyll and bundle sheath cells using the microscopy version of Imaging-PAM.


WP2: Modelling with ORCHIDEE

We will first implement within ORCHIDEE a new leaf-level model (JB) that allows for a variable distribution of excitation energy between PSII and PSI, for C3 and C4 species. Based on results from WP1, we will adapt this framework to possibly integrate a variable quantum yield of ChlaF for PSI, and improve our NPQ sub-model. The radiative transfer model in ORCHIDEE will allow to upscale these leaf-level improvements brought to the variables of interest (GPP, SIF) to the canopy scale. The new estimates will be evaluated at canopy level using in situ GPP and SIF measurements over various biomes with different structures, and at global scale against independent reference datasets, with an emphasis on drought events. We will specifically examine whether the JB model still holds in drought-stress conditions, when plants close their stomata so that CO2 intercellular concentration decreases and the excess energy increases. We will then optimise ORCHIDEE parameters related to various processes using data assimilation techniques. The recent ESA TROPOSIF product, that provides a daily global coverage at a 3.5x5.5 km2 resolution, is a suitable candidate for assimilation. We will also perform projections based on two IPCC socioeconomic scenarios: the sustainable and the “business-as-usual” ones, and quantify the impact of the newly optimised model on the estimation of future carbon and water fluxes.

Predicted climate changes will affect photosynthetic CO2 assimilation and thereby plant growth and crop productivity. Plants will be exposed to more frequent periods of increased temperatures and droughts, with negative consequences on photosynthesis. Numerical land surface models are used to simulate gross primary production (GPP), the quantity of carbon assimilated by the continental vegetation through photosynthesis. This GPP estimate is still largely uncertain, especially for drought events. The chlorophyll a fluorescence (ChlaF) induced by the sun (SIF) in the near infra-red domain is detected by remote sensing and used as a GPP proxy. For a better modelling of multiscale fluorescence and photosynthesis, experimental data are required that need an interdisciplinary research environment. Notably, the distribution of excitation energy between Photosystem (PS) II and PSI, and a putative variable fluorescence yield for PSI have to be quantified. This could help improving the estimations of SIF and GPP, and explaining the observed decoupling of SIF and GPP during stress periods. The aim of this project is thus to improve the modelling of SIF and GPP first at leaf scale, especially in drought conditions, and specifically to understand the distribution of excitation energy between PSII and PSI, and to determine the impact of variations in the yield of PSI fluorescence on the overall fluorescence detected in the near infrared domain. To accomplish these objectives, we will use interdisciplinary approaches including modelling of processes at the leaf level, and experimental methodologies like measurements of ChlaF and transient absorption spectroscopy, at the cellular and leaf levels. The improved variables SIF and GPP will be upscaled at canopy and global levels thanks to the land surface model ORCHIDEE, in situ observations of SIF and GPP for various biomes, and SIF satellite data.