Module 3: Photosynthesis & C3 Physiology

In C₃ photosynthesis CO&sub2; diffuses from the atmosphere through stomata and the mesophyll intercellular airspace to the chloroplast stroma, where Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) fixes it onto ribulose-1,5-bisphosphate (RuBP) to give two molecules of 3-phosphoglycerate (3-PGA), the three-carbon entry compound that gives the pathway its name. The Calvin–Benson cycle then regenerates RuBP at the expense of ATP and NADPH produced by the light reactions.

Rubisco is a notoriously promiscuous enzyme: it also catalyses the oxygenation of RuBP, producing one 3-PGA and one 2-phosphoglycolate (2-PG). 2-PG must be salvaged via the photorespiratory C&sub2; cycle (peroxisome + mitochondrion), costing ATP, reducing power, and releasing one CO&sub2; per 2-PG. Under ambient temperature and CO&sub2; concentration, photorespiration destroys ~25% of the carbon that Rubisco fixes — a major yield penalty that C₄ plants (maize, sorghum, sugarcane) avoid by concentrating CO&sub2; around Rubisco.

Rubisco Stoichiometry

The partitioning between carboxylation and oxygenation is set by the specificity factor Sₛ/ₒ (= kₛᵧKₒ / kₒᵧKₛ). Higher plants have S-values of 80–100; wheat is ~92, typical for a temperate C3 monocot. Cyanobacteria have lower S (~40) but compensate with CO&sub2;-concentrating mechanisms; some red algae reach S ~ 250.

The cornerstone of modern quantitative plant physiology, the FvCB model (Farquhar, von Caemmerer & Berry 1980), expresses net CO&sub2; assimilation A as the minimum of three biochemical limitations:

• Rubisco-limited, Aₛ: active site saturation at low internal CO&sub2; (Cᵢ).
• RuBP-limited, Aⱼ: electron transport rate J limits RuBP regeneration.
• Triose-phosphate-utilisation-limited, Aₚ: triose-phosphate export from chloroplast cannot keep up with fixation at very high Ci.
• Rₔ: mitochondrial (“dark”) respiration, subtracted from gross A to give net A.

Rubisco-Limited Regime

Here Vₛ,ₘₐₓ is the maximum carboxylation rate, Kₛ and Kₒ are the Michaelis constants for CO&sub2; and O&sub2;, Γ* is the CO&sub2; photocompensation point (the Ci at which carboxylation and oxygenation balance), and O is the chloroplastic O&sub2; partial pressure (~210 mbar under atmospheric conditions).

RuBP-Limited Regime

where J, the electron transport rate, is an increasing saturating function of absorbed PAR. The factor 4 accounts for the 4 electrons per CO&sub2; reduction; the factor 8 in the denominator captures the extra electron cost of the oxygenation side-reaction.

Typical Wheat Parameters at 25 °C

• Vₛ,ₘₐₓ = 70–100 µmol m⁻² s⁻¹
• Jₘₐₓ = 140–180 µmol m⁻² s⁻¹
• Kₛ = 270 µbar, Kₒ = 165 mbar
• Γ* = 37 µbar, Rₔ = 1.0 µmol m⁻² s⁻¹
• S/O specificity factor = 92 (Bracher 2017)

Under ambient Ci ≈ 280 µbar and saturating light, wheat operates mostly in the Rubisco-limited regime. Lifting ambient CO&sub2; (free-air CO&sub2; enrichment, FACE) progressively pushes the leaf toward the RuBP-limited regime, reducing the marginal benefit of extra CO&sub2; — the well-documented diminishing returns at high CO&sub2; (Long 2006).

At ambient conditions, Rubisco oxygenation wastes ~25% of the carbon it has just fixed. The photorespiratory C&sub2; cycle operates across three organelles: chloroplast, peroxisome, and mitochondrion. 2-PG is oxidised via glyoxylate, serine, and glycerate back to 3-PGA, with one CO&sub2; released for every two 2-PGs processed. Net consumption: 2 ATP, 1 NADPH, and 1 ½O&sub2; per 2-PG salvaged, plus the forgone carbon.

Why does plant evolution tolerate this? Two reasons. First, Rubisco’s active site presumably evolved when atmospheric O&sub2; was much lower than today (~2.4 billion years ago, before the Great Oxidation Event); it is now stuck with inherited chemistry. Second, photorespiration may help dissipate excess reducing power under high-light, low-CO&sub2; stress, preventing photooxidative damage. In any case, several plant lineages have independently evolved CO&sub2;-concentrating mechanisms (C₄ Hatch–Slack, CAM, cyanobacterial carboxysomes) to bypass the problem.

Rubisco Bioengineering

Parry et al. (2013) and Long et al. (2015) propose several routes to improve wheat photosynthesis:

• Improve Rubisco kinetics: replace wheat Rubisco with a higher-S variant (e.g., from red algae, Galdieria, or engineered mutants).
• Photorespiratory bypasses: introduce bacterial glycolate catabolism pathways (E. coli glycolate dehydrogenase + Kebeish bypass) directly in the chloroplast (South 2019: 40% field yield increase in tobacco).
• Transfer a CO&sub2;-concentrating mechanism: reconstitute cyanobacterial carboxysomes in higher-plant chloroplasts (Price 2013, Hanson 2016).
• Install a C₄ scaffold: the multi-lab C4 Rice Project aims to convert rice, a model for wheat, to C4 photosynthesis.
• Enhanced Rubisco activase: Rubisco activase (Rca) restores catalytically-competent Rubisco; thermotolerant Rca variants protect photosynthesis under heat stress (Carmo-Silva 2015).

The leaf cannot simply open its stomata to maximum and let CO&sub2; in: every open stomate also lets water vapour out. In wheat, water-use efficiency (WUE, mol CO&sub2; gained per mol H&sub2;O lost) ranges from 2–6 under optimal conditions; plants must close stomata under drought to avoid cavitation. Stomatal aperture is regulated by a host of signals (light, CO&sub2;, humidity, ABA) that can be summarised phenomenologically by the Ball–Berry or Medlyn models:

Here gₛ is stomatal conductance to water vapour, A is net photosynthesis, h is leaf-surface relative humidity, Cₐ is ambient CO&sub2;, and D is the leaf-to-air vapour-pressure deficit. Wheat typical values: g₀ = 0.02 mol m⁻² s⁻¹, g₁ = 9 (Ball–Berry), with higher g₁ in well-watered conditions.

Supply-Demand Intersection

The operating point of the leaf is the intersection of two curves in the (A, Ci) plane: the biochemical demand curve (FvCB A vs Ci, saturating) and the physical supply curve (A = gₛ(Cₐ − Cᵢ)/1.6, linear in Ci). Under drought, gₛ falls, the supply curve pivots down, and the operating Ci falls into progressively more Rubisco-limited (and more photorespiratory) territory. Diagnosis of “stomatal limitation” vs “biochemical limitation” of A is a central exercise in A–Ci curve interpretation (Long & Bernacchi 2003).

A single leaf reaches its light-saturation plateau at PAR ≈ 1000 µmol m⁻² s⁻¹; further light is wasted. A crop canopy uses its vertical structure to distribute light among many leaves: layer by layer the photon flux falls, and the lower leaves, shaded but still above the light-compensation point, continue to contribute to carbon gain.

The extinction coefficient k depends on leaf-angle distribution. Wheat is erectophile: flag leaves stand near-vertical, so k ≈ 0.40–0.55 at solar noon (lower than the planophile k ≈ 0.7 typical of legumes). Erectophile canopies allow deeper penetration of light and tolerate higher LAI without saturation at the top. Modern wheat cultivars are bred for upright flag leaf to maximise LAI-integrated canopy photosynthesis.

Flag Leaf Dominance

The flag leaf (the uppermost leaf on the tiller) is the dominant source of assimilate for grain filling. During the post-anthesis phase it contributes ~50% of grain N and ~30% of grain C; a healthy flag leaf sustained for 45–50 days post-anthesis is a breeding target. Flag-leaf senescence is accelerated by heat, drought, and nitrogen deficiency, and its premature decline is the main cause of terminal-heat yield loss in south Asian wheat systems.

Chlorophyll Ratio and Pigment Composition

Wheat leaves carry chlorophyll a, chlorophyll b, and carotenoids (β-carotene, lutein, violaxanthin, zeaxanthin). The chl a : chl b ratio is ~3:1 in normal leaves, rising to ~4:1 in high-light-acclimated canopies (more core antenna relative to LHCII) and falling to ~2:1 in shade (more LHCII peripheral antenna). Xanthophyll-cycle turnover (V→Z via violaxanthin de-epoxidase) dissipates excess excitation energy as heat under high light — non-photochemical quenching (NPQ) — and its rate of relaxation in the field has been proposed as a breeding target for improving photosynthesis (Kromdijk 2016).

Monteith (1977) decomposed yield into three multiplicatively acting terms:

PAR is the seasonal incident PAR (MJ m⁻²); fₑᵢₙₜ the season-integrated fractional interception; RUE the biomass produced per unit intercepted PAR (g DM MJ⁻¹); HI the harvest index (grain / total biomass).

Typical wheat figures: seasonal PAR 1200–1600 MJ m⁻² in temperate cereal-growing regions; fₑᵢₙₜ ~ 0.85 at peak LAI; RUE = 1.35 g MJ⁻¹ (Fischer 2007, Calderini 1997); HI = 0.45–0.50 in modern semi-dwarfs. Multiplying gives potential yields of 8–12 t/ha, consistent with record farm yields in the UK, Germany, and New Zealand.

Boyer’s Yield Gap

Boyer (1982) famously documented the gap between “record” farm yields and average commercial yields, attributing it overwhelmingly to abiotic stresses, especially water and nitrogen limitation. The Monteith decomposition localises the loss: water limitation hits fₑᵢₙₜ (canopy closure) and RUE (stomatal closure); N limitation hits fₑᵢₙₜ and leaf area duration; heat stress hits RUE (Rubisco activase inactivation) and HI (shortened grain-filling window).

Leaf-Temperature Optimum

Wheat photosynthesis shows a broad thermal optimum at 20–22 °C leaf temperature. Above 28 °C Rubisco activase begins to thermo-inactivate (Salvucci 2004), Jₘₐₓ declines, and photorespiration accelerates because Γ* rises with T. Above 35 °C sustained leaf temperature, assimilation collapses. Terminal heat stress during grain filling — short bursts of >30 °C days after anthesis — is the principal yield hazard in the Indo-Gangetic plains and is projected to intensify with climate change (Asseng 2015).

Enclosure chambers (growth rooms, open-top chambers) consistently showed ~30% yield gains when wheat is grown at 550 µbar CO&sub2; relative to 370 µbar. But chamber artefacts (reduced wind, altered micrometeorology) biased the results upward. Free-air CO&sub2; enrichment (FACE) experiments, which fumigate open field plots with a ring of perforated pipes, settle the question under realistic field conditions. Long et al. (2006) compiled a meta-analysis showing FACE yield enhancements of 13% for wheat — about half the chamber estimate.

Why the Reduced FACE Response?

• Photosynthetic down-regulation: sustained high CO&sub2; reduces Rubisco content (sugar-triggered signalling), capping the gain.
• Nitrogen limitation: biomass gain requires additional N to maintain canopy protein; field-realistic N supply rarely supports the chamber enhancements.
• Stomatal closure: elevated CO&sub2; closes stomata by ~25%, improving WUE but also limiting transpirational cooling, which can tip canopies into heat stress.
• Grain quality decline: FACE wheat has 5–15% lower grain protein and reduced micronutrient density (Fe, Zn, Mg) — hidden nutritional loss.

Solar Radiation Use Efficiency

RUE values for wheat cluster tightly: 1.30–1.45 g DM MJ⁻¹ PAR, reaching ~1.6 in the best experiments. The ceiling is set by the quantum requirement for carbon fixation (8–12 photons per CO&sub2;, plus photorespiratory overhead) and the reflectance/absorbance losses at the canopy surface. Photosynthetic engineering that reduces photorespiration (e.g., glycolate bypass) would lift RUE; better Rubisco kinetics or C₄ grafting would push the upper bound of potential yield.

Rubisco is catalytically inactive until a specific lysine (K201 in wheat) is carbamylated by CO&sub2; and the carbamate is stabilised by Mg²⁺. Sugar phosphates (RuBP itself, but also 2-carboxyarabinitol-1-phosphate, CA1P, and xylulose-1,5-bisphosphate) bind the uncarbamylated active site and lock it inactive. The enzyme Rubisco activase (Rca) is an AAA+ ATPase that removes these inhibitory sugar phosphates, allowing spontaneous carbamylation and activation.

Rca is exquisitely heat-sensitive: its half-life collapses above 32 °C, and the resulting loss of Rubisco activation state is the principal cause of the short-term heat sensitivity of wheat photosynthesis (Salvucci 2004). Wheat varieties differ in Rca thermotolerance, with North African and Indian genotypes carrying Rca-α alleles (one of the two wheat Rca isoforms) that retain activity to higher temperatures. Crossing heat-tolerant Rca alleles into temperate germplasm is an active breeding target (Scafaro 2018).

Diel Cycle of Activation State

Leaves track diel light intensity by modulating Rubisco activation state: at low light only a fraction of enzyme is activated, rising to near-fully-activated at saturating light. This avoids futile ATP consumption by keeping activation state matched to electron-transport capacity. Sudden sunfleck events (cloud clearance) require rapid Rca action to lift activation state in <1 min; slow Rca limits photosynthesis in fluctuating-light canopies, a loss estimated at 10–30% of daily carbon gain in real field canopies (Taylor & Long 2017).

Canopy temperature is the net result of radiative heat balance: absorbed shortwave radiation + absorbed longwave + metabolic heat = transpiration cooling + convective loss + emitted longwave. In a well-watered wheat canopy at saturating irradiance, transpiration can depress canopy temperature 3–5 °C below air temperature (“transpirational cooling”). Under drought, stomatal closure eliminates this cooling and canopy temperature rises sharply.

Infrared thermometry (thermal imaging, canopy-temperature depression ΔT measurements) has become a standard phenotyping tool for drought and heat tolerance (Reynolds 2015). Cooler canopies indicate active transpiration and more open stomata, usually associated with higher yield under well-watered conditions. Under drought the ranking can reverse: cultivars that maintain low canopy temperature via profligate water use may run out of soil moisture before grain filling completes. The optimal breeding target depends on the target environment’s water-supply regime.

Terminal Heat Stress

A terminal heat stress event (3–5 days of Tₘₐₓ > 33 °C during grain filling) shortens grain filling by 5–10 days, reduces grain weight by 10–30%, and impairs grain quality (reduced dough strength, altered gliadin:glutenin ratio). Acclimation responses include synthesis of HSP101 and small HSPs, accumulation of glycine-betaine and proline as osmoprotectants, and thermotolerant Rca variants. Marker-assisted selection for terminal heat tolerance targets the TaHsfA6e, Rca-α, and grain-filling-rate QTLs.

Asseng Projection

The multi-model ensemble of Asseng et al. (2015) projects a 6% reduction in global wheat yield per 1 °C of local mean temperature rise, in the absence of adaptive management. Adaptation strategies (earlier sowing, higher-RUE cultivars, terminal-heat-tolerant germplasm) can offset part but not all of this loss. Under RCP8.5 warming to 4 °C by 2100, roughly a quarter of current major wheat-growing regions would become marginally productive; conversely, northward expansion into Canada, Russia, and Scandinavia could compensate by ~50%.

Photosynthesis is the biophysical engine that drives the entire wheat yield pipeline: Rubisco’s kₛᵧ and S/O set the biochemical ceiling; Ball–Berry couples that biochemistry to the water economy; canopy architecture integrates leaf-level A into a community-level carbon gain; Monteith’s RUE and HI translate intercepted PAR into grain. Every step offers levers for improvement, and the modern pipeline from photosynthetic engineering to field testing (RIPE, C4 Rice, HB4 drought, see Module 7) illustrates the payoff.

But photosynthesis is inseparable from water. Every mole of CO&sub2; fixed costs ~200 moles of H&sub2;O lost through transpiration. Module 4 turns to the hydraulics: xylem transport, root-to-shoot signalling, cavitation physics, and the osmotic/ABA machinery that keeps wheat alive through the increasingly frequent drought episodes of a warming world.

The biochemical FvCB model presumes an adequate supply of ATP and NADPH from the light reactions of the thylakoid membrane. Photosystem II (PSII) oxidises water, releasing O&sub2; and delivering electrons via plastoquinone to the cytochrome b₆f complex; plastocyanin shuttles electrons to Photosystem I (PSI), whose reduced ferredoxin finally passes electrons to ferredoxin–NADP⁺ reductase to generate NADPH. ATP is produced by chemiosmotic coupling at the ATP synthase, powered by the thylakoid proton gradient.

ATP:NADPH Stoichiometry and Cyclic Electron Flow

Linear electron transport yields an ATP:NADPH ratio of ~1.28. Calvin cycle and photorespiration together demand an ATP:NADPH ratio of ~1.5–1.6; the shortfall is met by PSI-driven cyclic electron flow (CEF), which produces extra ATP without reducing NADP⁺. In wheat, CEF is catalysed by the NDH complex (plastid analogue of mitochondrial complex I) and the antimycin-sensitive PGR5/PGRL1 pathway. CEF capacity correlates with photosynthetic robustness under fluctuating light and is an emerging breeding target.

Quantum Yield & Chlorophyll Fluorescence

The quantum yield Φₚₛₙₙ of photosystem II can be measured non-invasively by chlorophyll fluorescence. Under dark-adapted conditions, the maximum quantum yield Fᵛ/Fₘ ~ 0.83 in healthy leaves; values below ~0.75 indicate photoinhibition or senescence. The PAM fluorometer (pulse-amplitude modulation) allows real-time tracking of PSII operating efficiency Φₚₛₙₙ′, non-photochemical quenching NPQ, and electron transport rate ETR under any light/stress regime.

CO&sub2; entering the leaf through stomata must still diffuse through the intercellular airspace, across the mesophyll cell wall, through the plasma membrane, across the cytoplasm and chloroplast envelope, and finally into the stroma before reaching Rubisco. This liquid-phase path resistance is summarised as the mesophyll conductance gₘ. In wheat gₘ ~ 0.2–0.4 mol m⁻² s⁻¹, of the same order as gₛ. The chloroplast CO&sub2; concentration C₳ is then lower than Ci:

Typical Cₐ − Cᵢ drop is 70 µbar (stomatal), and Cᵢ − C₳ drop is another 60 µbar (mesophyll), so C₳ ~ 200 µbar at ambient. This effectively lowers the supply of CO&sub2; to Rubisco and pushes the leaf toward photorespiration. Engineering higher gₘ — e.g., by overexpressing plasma-membrane aquaporins (PIP1;2) that also conduct CO&sub2; — is another active target (Flexas 2016). Increased aquaporin CO&sub2; permeability has been shown to raise Rubisco-site CO&sub2; concentration and photosynthetic rate in tobacco and rice trials.

Isotopic Discrimination (δ¹³C)

Rubisco preferentially fixes ¹²CO&sub2; over ¹³CO&sub2;, giving C₃ photosynthate a characteristic δ¹³C of −27 to −30‰ vs PDB. C₄ photosynthate is ~−12‰ because PEP carboxylase discriminates less. This isotopic fingerprint — measured by isotope-ratio mass spectrometry on dry grain or leaf tissue — reports cumulative WUE integrated over the lifetime of the tissue. Carbon-isotope discrimination is a widely used selection tool for improved WUE in wheat breeding (Condon 2004).