Photosynthetic electron transport and plastid redox. The photosynthetic chain redrawn with each node's quantum chemistry and the evidence behind it. Photosystem I is the strongest structural case in the atlas: charge separation at P700 makes a genuine spin-correlated radical pair, the species the radical-pair mechanism was written for. It is nevertheless only T2, because the one direct field measurement on photosynthesis in the review's bibliography was made in lima bean rather than Arabidopsis, and because that pair is extremely short-lived and its known field effects are seen well above geomagnetic strength. Cyclic electron flow is drawn for the same reason alternative oxidase appears on QBM-01 — it lets a chloroplast rebalance ATP against NADPH without changing linear flux, so measuring linear electron transport alone cannot separate "no effect" from "compensated effect". Four nodes are plastid-encoded (ATCG loci) and will receive no value from poly-A-selected RNA-seq, which is most of OSDR's plant studies. Derived from: Figures 1B and 5 of Porterfield & Barker, "Plant Responses to Near-Null Magnetic Fields" — expanded to gene level and bound to Arabidopsis loci structural_context CHEBI:15377 C00001 spin_relaxation_modulated 1H AT1G76100 AT1G20340 GO:0009543 A mononuclear Cu(II)/Cu(I) centre performing one-electron transfer, so formally paramagnetic in the oxidised state. Deliberately classed as spin_relaxation_modulated rather than as a radical-pair partner. A single copper centre relaxes fast and has no correlated partner, so coherent singlet-triplet mixing is not available here; only a relaxation-based effect could be argued, and none has been shown. semiquinone_radical ros_leak_site 1H ATCG00020 AT1G06680 GO:0009523 1.10.3.9 The QA/QB sites hold a kinetically stabilised plastosemiquinone, and PSII is a major singlet-oxygen source under excess light. Both are one-electron chemistry in a defined pocket, which is the setting a radical-pair argument needs. PSII's dominant ROS species is singlet oxygen from the P680 triplet, not superoxide. Triplet chlorophyll formation is itself spin chemistry, so a field effect here would be more plausible than at most nodes — but it is also a route the review does not develop, and no measurement exists. structural_context CHEBI:28377 C02061 semiquinone_radical radical_pair_donor ros_leak_site 1H CHEBI:28377 The plastid counterpart of ubisemiquinone, and the same argument applies: a protein-bound organic radical with a measurable lifetime is the most credible site for field-dependent branching in the chain. rieske_2Fe2S semiquinone_radical 57Fe 33S 1H AT4G03280 GO:0009512 Structurally and mechanistically the plastid twin of the mitochondrial Rieske centre (QBO:RIESKE_FES), with the same bifurcated Q-cycle chemistry. The review treats both as the same class of candidate target. heme_electron_carrier 57Fe 1H ATCG00540 ATCG00720 GO:0009512 One-electron heme carriers, paramagnetic in the ferric state, adjacent to the semiquinone-generating Q cycle. structural_context AT2G05620 AT4G22890 GO:0009767 iron_sulfur_4Fe4S radical_pair_acceptor triplet_yield_sensitive 57Fe 33S 1H ATCG01060 GO:0009522 1.97.1.12 The geomagnetic field is required for normal lima bean photosynthesis, PSI/LHCI content and ROS production; near-null field conditions altered photosynthetic electron transport. PSI is the strongest structural case in the whole atlas. Charge separation at P700 produces a genuine spin-correlated radical pair (P700·+ A1·−), which is the textbook example of the species the radical-pair mechanism describes — it is studied by exactly that formalism in photosynthesis spectroscopy. Three [4Fe-4S] clusters then carry the electron onward. The P700·+ A1·− pair is extremely short-lived, and the well-characterised magnetic-field effects on it are seen in isolated preparations at fields far above geomagnetic. Structural plausibility is high; relevance at 50 uT is not established. structural_context AT4G04640 GO:0009535 7.1.2.2 iron_sulfur_2Fe2S 57Fe 33S AT1G10960 AT1G60950 GO:0009570 Geomagnetic field conditions modulate iron-sulfur interplay in Arabidopsis, particularly under combined iron and sulfur deficiency. The distribution hub of plastid reducing power. A perturbation here reaches carbon fixation, nitrogen and sulfur assimilation and redox regulation at once, which is why the review nominates ferredoxin biogenesis specifically. flavin_semiquinone ros_leak_site 1H 14N CHEBI:57692 AT5G66190 AT1G20020 1.18.1.2 FNR necessarily transits a flavin semiquinone: it is the step that converts one-electron chemistry into two-electron chemistry, so a half-reduced flavin is obligatory rather than incidental. That makes it the plastid's clearest analogue of the cryptochrome flavin. structural_context CHEBI:57783 CHEBI:58349 C00005 C00006 ros_leak_site CHEBI:18421 CHEBI:16240 AT4G25100 AT2G28190 Reactive oxygen species production differed between geomagnetic and near-null field conditions in lima bean. Near-null magnetic field alters ROS-associated transcriptomic and metabolomic programmes in Arabidopsis. As in the mitochondrion, the amplifier rather than the mechanism: threshold behaviour converts a small change in electron branching into a signal. electron_transfer electron_transfer electron_transfer electron_transfer electron_transfer electron_transfer electron_transfer electron_transfer electron_transfer electron_transfer electron_transfer electron_transfer energy_feedback energy_feedback direct_influence direct_influence redox_feedback redox_feedback