Net Photosynthetic Rate (A)
VC-MET-008 — the net rate of CO₂ assimilation per unit leaf area, the direct measure of carbon-fixation productivity that ultimately funds growth and toxin yield.
Definition
Net photosynthetic rate (A) is the net CO₂ assimilation flux per unit leaf area, in µmol CO₂·m⁻²·s⁻¹ — gross carbon fixation less photorespiration and mitochondrial respiration. Positive values mean the leaf is a net carbon sink. At GreenWorld Sector-7 it is measured by PhytoSense Mesh closed-cuvette infrared gas analysers (IRGA) on tagged reference leaves, reported per cultivar cohort at the prevailing light and CO₂ conditions.
Why it matters
Net assimilation is the productivity bottom line: it caps the carbon available for biomass accrual and for the alkaloid synthesis driven in the Phyto-Synthesis Bay. It is co-limited by photochemistry (Fv/Fm) and stomatal supply (gsw), so reading A alongside both isolates whether a productivity drop is photochemical, diffusional, or biochemical. CHLORA uses A to forecast growth and toxin throughput and to tune CO₂ enrichment set-points.
Formula
A = (u · ΔC / A_leaf) − E · C_s where: u = cuvette molar flow rate (µmol·s⁻¹) ΔC = CO2 mole-fraction drawdown (inlet − outlet) A_leaf = enclosed leaf area (m²) E = transpiration flux (dilution correction) C_s = CO2 mole fraction at leaf surface conditions logged with each sample: Q = incident PAR (µmol·m⁻²·s⁻¹) Ca = cuvette CO2 (µmol·mol⁻¹, enrichment target 800) T_l = leaf temperature calibration: IRGA zero/span on CO2-free N2 and span gas each shift. Reported to 0.1 µmol·m⁻²·s⁻¹, light-normalized for trending.
Inputs
| Symbol | Quantity | Reference Band | Source |
|---|---|---|---|
| ΔC | CO₂ drawdown (inlet−outlet) | 5 → 60 µmol·mol⁻¹ | PhytoSense Mesh IRGA |
| Q | Incident PAR | 400 → 900 µmol·m⁻²·s⁻¹ | PhytoSense Mesh |
| Ca | Cuvette CO₂ (enrichment) | 700 → 900 µmol·mol⁻¹ | CO₂ Enrichment Control |
| T_l | Leaf temperature | 22 → 28 °C | PhytoSense Mesh |
Units & Scale
µmol CO₂·m⁻²·s⁻¹, reported to 0.1. Strongly light- and CO₂-dependent, so trending uses a light-normalized A at the reference PAR/CO₂ to separate plant state from environment. Aggregated per cohort as the median; facility value is leaf-area-weighted. Confidence degrades when cuvette PAR falls below 400 µmol·m⁻²·s⁻¹ (light-limited, uninformative) or when IRGA span drift exceeds 2%.
Sampling & Source
- Sampled every 15 minutes by PhytoSense Mesh IRGA cuvettes.
- Thresholds evaluated only above the 400 µmol·m⁻²·s⁻¹ PAR light-compensation floor.
- Light-normalized A computed for cross-period trend comparison.
- Stale handling: flagged STALE after 30 min; span-drift fault routes to Metrology.
Thresholds
OK
Productive carbon fixation. No action.
WARN
Depressed assimilation; CHLORA advisory, diagnose limitation.
CRIT
Near-zero net fixation; auto-dispatch, productivity at risk.
Recent Trend
Facility leaf-area-weighted, light-normalized A, last 14 samples:
Interpretation Guidance
| A Band | Reading | Likely Driver | Action |
|---|---|---|---|
| 22–28 | Peak productivity | Optimal light, CO₂, water | Maintain enrichment set-points. |
| 12–21 | Healthy | Normal operating range | None; normal range. |
| 8–11 | Soft decline | Light, CO₂, or mild stress | Check PAR & CO₂; review Fv/Fm. |
| 4–7 | WARN depressed | Stomatal or photochemical limit | Run limitation analysis. |
| < 4 | CRIT stalled | Severe stress or biochemical block | Auto-dispatch; cultivation triage. |
Related Metrics
Fv/Fm Fluorescence
Photochemical co-limit of A.
VC-MET-004Stomatal Conductance
Diffusional co-limit of A.
VC-MET-009Relative Growth Rate
Downstream of carbon supply.
VC-MET-001Canopy Vitality Index
Productivity context for CVI.
VC-MET-210Toxin Yield Rate
Carbon funds alkaloid synthesis.
VC-MET-105CO₂ Concentration
Enrichment driver of A.
Related SOPs
- SOP Library — IRGA zero/span calibration & cuvette light protocol.
- Photosynthesis limitation analysis & WARN/CRIT triage — Operations SOP set in the SOP Library.
- CO₂ enrichment tuning & alarm escalation — Monitoring Systems.