Assessing the Frequency-Dependent Responses of Nitrate, Pigment Distribution, and Biomass in Hydroponic Lettuce Exposed to Passive Resonant Fields

Nitrate accumulation is a critical quality and safety parameter in leafy vegetables produced under intensive hydroponic conditions. Passive resonant-circuit devices that generate calibrated radio-frequency fields without external power input represent a potentially residue-free biostimulation approach, yet their frequency-specific effects on lettuce remain incompletely characterised. This study evaluated eight single-frequency (5–30 MHz) and eight dual-frequency (paired with a 5 MHz carrier) treatments against an untreated control on Lollo Bionda lettuce grown under uniform NFT conditions across two crop cycles. Morphological traits (height, rosette diameter, leaf number, shoot and root biomass, root length and volume) and biochemical parameters (nitrate concentration, soluble solids, chlorophyll distribution) were quantified. Single-frequency exposure significantly reduced leaf nitrate, with the strongest reduction at 10 MHz (786.7 mg kg−1 FW; −61.0% relative to the control). Dual-frequency treatments produced smaller, non-significant reductions (block mean 1731.7 mg kg−1 FW). Single-frequency regimes increased plant height and rosette diameter, whereas dual-frequency regimes elevated leaf number (up to 34.3) and root length (up to 36.7 cm) while yielding comparable total shoot biomass. Fresh biomass peaked at 160.3 g under 25 MHz versus 99.9 g in the control. Nitrate showed a strong negative correlation with apical chlorophyll (r = −0.747), plant height (r = −0.825) and rosette diameter (r = −0.843). The physiological optima did not coincide: 10 MHz was most effective for nitrate reduction and 25 MHz for biomass production. These frequency-specific responses identify selected single-frequency bands as a promising zero-input tool for hydroponic lettuce. However, the absence of a sham-device control, lack of in situ field-strength measurement, use of an unvalidated portable nitrate meter, and pooling of crop cycles mean that the observed effects cannot yet be attributed exclusively to electromagnetic resonance. Future studies incorporating sham controls, direct field quantification, enzymatic assays of nitrate assimilation, and multi-cycle validation are required to confirm mechanisms and refine commercial application.