Rooting out ultraweak photon emission a-mung bean sprouts

It is well known that life has evolved to use and generate light, for instance, photosynthesis, vision and bioluminescence. What is less well known is that during normal metabolism, it can generate 1–100 photons s⁻¹ cm^–2 known as ultra-weak photon emission (UPE), biophoton emission or biological autoluminescence. The highest generation of these metabolic photons seem to occur during oxidative stress due to the generation and decay of reactive oxygen species (ROS), and their interaction with other components of the cell. To study this further, we have configured a sensitive detection system to study photon emission in germinating mung beans.

Here we investigated growing mung beans over 7 days at a constant temperature of 21 ± 1 °C in a light tight box, using dual top and bottom opposing photomultiplier tubes. Over this time period we showed that in total, mung beans grown from seeds generated an average of 5 ± 1 counts s⁻¹ above background. As the new bean stems grew, they showed a gradual linear increase in emission of up to 30 ± 1 counts s⁻¹, in agreement with previous literature. In addition to this “steady-state” emission we also observe delayed luminescence and drought-stress response emission previously observed in other species. Finally, we also observe episodic increased emission events of between 2 and 15 counts s⁻¹ for durations of around 3 h detected underneath the sample, and assign these to the growing of secondary roots.

We then induce secondary root formation using aqueous solutions of growth hormones hydrogen peroxide (H₂O₂, 167 µM) or 3-indole acetic acid (IAA, 0.5 µM) for watering. Both hormones show prolonged increase in emission above steady-state, over days 3–5 with at least 3 times the number of secondary roots formed compared with water alone. We also observed a significant peak increase in photon emission (474 and 1738 cps vs. 28 and 55 cps for water alone) for the H₂O₂ which we attribute to direct ROS reaction emission as confirmed by measurement on dead plants.

Altogether we have expanded upon and demonstrated an instrument and biological system for reliably producing and measuring intrinsic metabolic photons, first observed 100 years ago by Alexander Gurwitsch.