Lunar cycle gates Sphagnum riparium sensitivity to the geomagnetic field.

Despite the increasing body of knowledge concerning the influence of the lunar cycle on various components of the biosphere, how living organisms respond to environmental factors across this cycle remains unclear. Recent evidence that cryptochromes are involved in the perception of both magnetic fields and moonlight suggests a potential mechanistic link, whereby plant sensitivity to the geomagnetic field may vary with lunar phase. Here, we test the hypothesis that geomagnetic sensitivity of plant growth is modulated during the lunar cycle. Growth dynamics of Sphagnum riparium were analysed using a long-term field dataset obtained from Karelian mires (Russia). Monitoring was conducted at 2-day intervals over 10 full growing seasons, yielding 1795 growth rate estimates derived from measurements of 245860 shoots. Although no significant relationship between growth rate and the geomagnetic Kp index was detected across the full dataset, phase-resolved analysis revealed a different pattern. Near both the full moon and the new moon, the growth response to geomagnetic variations became significant and exhibited a characteristic flip-flop pattern, with a positive response preceding the lunar phase and a negative response immediately following it. These responses were confined to narrow time windows. The results support the hypothesis of lunar modulation of geomagnetic sensitivity in plants and demonstrate that this sensitivity is temporally gated within specific phases of the lunar cycle. The emergence of a half-lunar (~14.8-day) periodicity in geomagnetic response, absent in the growth rate itself, suggests that this mechanism may function as a regulatory layer synchronizing plant-environment interactions rather than directly controlling growth.