Hibiki Igarashi ¹ Takeshi Baba ¹ Kairi Takemura ¹ Takahiro Kono ² Hirofumi Kadono ³ Jun Yamada ¹ Uma Maheswari Rajagopalan ^1*

• ¹ Shibaura Institute of Technology, Minato, Japan
• ² Tokyo Metropolitan University, Hachioji, Tōkyō, Japan
• ³ Saitama University, Saitama, Saitama, Japan

With rapid climate change, it has been increasingly difficult to grow different crops, and as an alternative method, artificial cultivation in controlled environments has evolved into a new sustainable agriculture practice. However, the cost of having a controlled environment has become a major issue, and investigations have been conducted to develop cost-saving and efficient cultivation techniques. One research focus is on the utilization of stimulating appropriate photoreceptors for a certain time by far-red (FR) light to influence plant development.Here, we propose a novel laser biospeckle method, a nondestructive and real-time measurement method for speedy evaluation of FR effects with argula (Eruca sativa Mill) plants. Laser biospeckles are formed from the interference of scattered light from the organelles within the biological tissue, and the intensity of such speckles varies due to displacements within the tissue.In the experiment, while illuminating with FR and red (R) LED light of, respectively, 735 nm and 630 nm for a duration of 120 s to 300 s, leaves of an arugula plant were irradiated simultaneously with laser light of 852 nm to obtain biospeckles. Movies were recorded by a CMOS camera at 15fps for 20 s. Correlations between the first and the subsequent frames were calculated to investigate the differences in the internal activity with exposure to FR and were characterized with a parameter called biospeckle activity (BA). Experiments were done with the healthiness and the age of the plant as parameters.It was found that depending on the healthiness of the plants, under short durations of 120s FR, BA and thus, the internal activity within the leaves increased compared to the long duration of 300s FR. Further, a one month old plant was found to have a faster decay of correlation and thus a steep increase in BA compared to that for a three month old plant. Our results suggest that BA could be used as a measure to investigate the effects of FR or FR plus R in plant development within a timeframe of a few minutes, thus, can be employed as a complementary measurement technique for speedier investigation of FR effects of plants.