Anthocyanin Reflectance Index
VC-MET-012 — a spectral estimate of foliar anthocyanin accumulation, a sensitive marker of light, cold, nutrient, and toxin stress that often rises before chlorophyll falls.
Definition
The Anthocyanin Reflectance Index (ARI) is a normalized spectral index, scaled 0–1 for the portal, estimating the concentration of anthocyanin pigments in canopy foliage from the differential absorption in the green and red-edge bands. Anthocyanins accumulate as a protective stress response, so ARI is an increasing-is-worse channel: a rising index flags stress that has not yet damaged chlorophyll. It is derived per cell by the Spectral Canopy Array.
Why it matters
ARI gives CHLORA a stress-anticipation signal independent of greenness: plants synthesize anthocyanins under excess light, chilling, phosphorus deficiency, and certain toxin loads before Fv/Fm or NDVI degrade, so a climbing ARI is an early actionable warning. Because several facility cultivars are bred for elevated alkaloid output, ARI is also cross-read against toxin-load state to separate ornamental pigmentation from genuine stress pigmentation.
Formula
ARI_raw = (1 / ρ550) − (1 / ρ700) normalized to portal index (clamped 0–1): ARI = clamp( ARI_raw / ARI_ref ) ARI_ref = cultivar full-stress reference (per profile) clamp(x) = max(0, min(1, x)) bands (Spectral Canopy Array): ρ550 = green reflectance, 550 nm ρ700 = red-edge reflectance, 700 nm masking: computed only on live-canopy pixels (NDVI > 0.40); 3-pass (3-h) moving mean. Reported to 0.01.
Inputs
| Symbol | Quantity | Reference Band | Source |
|---|---|---|---|
| ρ550 | Green reflectance, 550 nm | 0.08 → 0.20 | Spectral Canopy Array |
| ρ700 | Red-edge reflectance, 700 nm | 0.10 → 0.30 | Spectral Canopy Array |
| ARI_ref | Cultivar full-stress reference | per profile | Cultivar profile |
| NDVI | Live-canopy mask gate | > 0.40 | Spectral Canopy Array |
Units & Scale
Dimensionless index 0–1 (cultivar-normalized), reported to 0.01. Aggregated per cell as the masked median, then leaf-area-weighted to a facility value. Because baseline pigmentation is cultivar-specific, the index is normalized per cultivar so thresholds are comparable across the collection. Confidence degrades when masked-pixel fraction drops below 60% or the cultivar reference is unset.
Sampling & Source
- Derived every 1 hour from Spectral Canopy Array hyperspectral passes.
- Cultivar-normalized so thresholds compare across the collection.
- Smoothed over a 3-h moving mean; cross-read against toxin-load state.
- Stale handling: flagged STALE after 2 missed passes; unset-reference cells low-confidence.
Thresholds
OK
Baseline pigmentation. No action.
WARN
Stress pigmentation rising; CHLORA advisory, find stressor.
CRIT
Heavy stress accumulation; auto-dispatch, intervene.
Recent Trend
Facility leaf-area-weighted ARI, last 14 hours:
Interpretation Guidance
| ARI Band | Reading | Likely Driver | Action |
|---|---|---|---|
| 0.05–0.20 | Baseline | Normal pigmentation | None; reference state. |
| 0.21–0.30 | Healthy | Mild diurnal light response | None; normal range. |
| 0.31–0.45 | Soft rise | Excess light, cool nights, or P deficit | Check PAR, temp Δ (VC-MET-013), nutrient EC. |
| 0.46–0.60 | WARN stress | Sustained abiotic stress or toxin load | Identify & correct stressor. |
| > 0.60 | CRIT heavy | Severe stress accumulation | Auto-dispatch; cultivation intervention. |
Related Metrics
Fv/Fm Fluorescence
Often nominal while ARI rises.
VC-MET-002NDVI
Greenness; ARI leads its decline.
VC-MET-011Senescence Fraction
Downstream tissue-loss confirmation.
VC-MET-013Canopy Temp Δ
Chilling/heat driver of pigmentation.
VC-MET-001Canopy Vitality Index
Early-warning context for CVI.
VC-MET-210Toxin Yield Rate
Cross-read for stress vs. ornamental colour.
Related SOPs
- SOP Library — ARI cultivar-reference calibration procedure.
- Early-stress (anthocyanin) investigation & WARN/CRIT triage — Operations SOP set in the SOP Library.
- Toxin cross-referral & alarm escalation — Monitoring Systems.