Corona discharge is a partial electrical breakdown of air that occurs when the local electric field exceeds the dielectric strength of air near electrode surfaces — typically above ~30–35 kV/m in typical ambient conditions, though the exact threshold depends on electrode geometry, humidity, and atmospheric pressure.
What corona generates:
| Species | Formation pathway | Biological relevance |
|---|---|---|
| Ozone (O₃) | O• + O₂ → O₃ | Phytotoxic at > 40 ppb; damages stomata and leaf tissue |
| Nitric oxide (NO) | N₂ + O₂ → 2NO (plasma) | Plant signaling molecule (germination, stomata); toxic at high concentrations |
| Nitrogen dioxide (NO₂) | NO + O₃ → NO₂ | Acidic aerosol precursor; phytotoxic |
| Air ions (+/−) | Direct ionization | Alter atmospheric electricity; independently affect plant physiology |
| Reactive oxygen species | Multiple pathways | Can stimulate or damage depending on concentration |
Why this matters for interpreting electroculture results:
All studies showing adverse plant effects at field strengths above ~35 kV/m face the corona confound. The ozone, NO, and air ions generated by corona are independently capable of:
The Schmiedchen et al. 2018 PRISMA systematic review of 19 plant studies found that zero studies included corona control arms — meaning no study in the reviewed literature successfully isolated a pure electric field effect at high field strengths.
The threshold implication: Studies finding stimulatory effects at < 1.5 kV/m (Mahmood 2014 pea germination) are below the corona threshold and can be more confidently attributed to the EF itself. Studies finding inhibitory effects at > 35 kV/m may be measuring corona chemistry rather than EF biology.
For the post-harvest tomato studies (Wang 2007, Zhao 2011 — -200 kV/m): the antioxidant and shelf-life benefits are large (Cohen’s d = 2–7), but at 200 kV/m, corona effects in a sealed chamber would be extreme. Ozone is itself an antimicrobial and can extend shelf life independently. Until a parallel ozone-matched control is included, the mechanism for the post-harvest effect cannot be confirmed as EF rather than corona.
Design recommendation for future studies: Any electroculture experiment above ~10 kV/m should include: