Airborne ion charge kinetics reveal circadian-range periodicity in detached plant fruits: a continuous, noncontact, and noninvasive measurement system

Abstract

Abstract Background Understanding temporal regulation in plant systems requires measurement approaches that capture physiological kinetics with minimal tissue perturbation. Here, we present a magnetically levitated electrode ionization chamber (MALIC) system for monitoring airborne ion charge kinetics in a continuous, noncontact, and noninvasive manner, enabling the continuous physicochemical observation of plant-derived signals without genetic modification or optical readouts. Results Using detached fruits of two apple cultivars (Yoko and Akibae), Lomb-Scargle periodogram analysis identified dominant periodic components within the circadian range (~ 24–25 h) in airborne ion charge. Phase drift analysis revealed cultivar-dependent differences in temporal stability, with Yoko exhibiting relatively consistent peak timing across successive cycles, whereas Akibae showed greater variability. These differences were further supported by phase coherence analysis, which demonstrated the tighter clustering of phase values in Yoko compared with Akibae. Quantitative analysis showed that Akibae exhibited larger amplitude and a higher coefficient of variation, indicating greater relative variability. Because the MALIC system detects net ion-related signals in the surrounding air rather than intracellular processes directly, the observed oscillations should be interpreted as a proxy for integrated physiological activity. These rhythms are consistent with temporally organized physiological processes but do not establish a direct link to endogenous circadian clock mechanisms. Conclusions The MALIC system enables the continuous, noncontact, and noninvasive measurement of airborne ion charge kinetics exhibiting reproducible circadian-range periodicity in detached plant tissues. This work establishes airborne ion charge as a previously unrecognized temporal signal at the plant–environment interface. Rather than replacing established circadian assays, the MALIC system should be considered a complementary approach that captures signals distinct from transcriptional and photosynthetic readouts. Further validation across species, cultivars, and environmental conditions, together with integrated environmental, molecular, and physiological measurements, will be essential to determine whether airborne ion charge kinetics can serve as reliable indicators of endogenous biological rhythms in plants.

Plant phenotyping relevance

MALICシステムによる植物由来の空中イオン荷電動態を、非接触・連続的に測定する手法の提示と、リンゴ果実での技術的適用・解析が中心である。

Code and data availability

The paper's airborne ion charge time-series datasets (Yoko and Akibae cultivars) and the Python analysis code are not publicly deposited; the authors state data are available only upon reasonable request to the corresponding author. No public URL for data or code is provided.

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