Evaluation of sub-nanometric plant growth activities under far-red light illumination using Statistical Interferometry Technique (SIT)

Abstract

Statistical Interferometry Technique (SIT) developed in our laboratory is a unique optical interferometry method using the statistical property of a complete randomness of fully developed speckle field. SIT enables non-invasive and realtime measurement of short-time sub-nanometric growth behaviors of plants. Applying SIT to the measurement of plant leaf elongation, we found that plants grow with very small fluctuations at nanometric scale, named as Nanometric Intrinsic Fluctuation (NIF) that formed the basis of our research. Our previous studies have demonstrated that NIF seems to reflect the healthiness of plants sensitively. In other words, SIT could assess an environmental condition through a plant NIF. However, the origin of NIF is still unknown. In this study, we focus on the effect of far-red light (FR) of wavelength 700-800 nm and evaluate how it affects NIF to infer the origin of NIF from the mechanism of FR-induced growth. LED (wavelength of 730 nm) was employed as a FR source, and fresh rice seedlings (2-4 weeks age) were used in the experiment. FR illumination at 50 µmol/m 2 /s increased the standard deviation of NIF of rice seedlings approximately 50% within 30 minutes. According to the literatures, FR increase endogenous auxin level via phytochrome signaling, and auxin-induced growth may occur within a few tens of minutes. In addition, it was found in our previous study that low concentration of exogenous auxin increase the magnitude of NIF. Hence, this results implies that not only FR has a positive effect on plant growth but also the increase may be caused by auxin-induced growth.