Red and blue laser light drives photosynthesis through dynamic changes of stomatal aperture

Amid the growing global demand for sustainable crop production, plant factories with artificial lighting (PFALs) have gained attention as the systems provide stable, optimal growth environments for crops, and are largely unaffected by climate change. However, a limitation of widely used light-emitting diodes (LEDs) in PFALs is their decreased energy conversion efficiency at high output levels, prompting the search for more efficient light sources. This study focused on laser diodes (LDs), which has shown superior energy conversion efficiency, as an alternative. We developed an LD lighting system capable of mixing red and blue light at arbitrary ratios and mounted it onto a commercial gas-exchange measurement system. Using this system, photosynthetic parameters in rice were obtained under conditions of red light alone and as well as combined red and blue light illumination. Under red LD illumination alone, steady-state CO₂ assimilation rate, stomatal conductance, and transpiration rate in rice were significantly higher than those under red LED illumination, whereas intrinsic water-use efficiency decreased due to a relatively greater increase in stomatal conductance. Notably, stomatal conductance and transpiration rate exhibited pronounced temporal oscillations with a period of approximately 900 s, which closely corresponded to oscillations in stomatal aperture confirmed by microscopic observations. Under combined red and blue LD illumination, steady-state photosynthetic parameters did not differ significantly from those under LED illumination; however, the dominant oscillatory frequency observed under red LD alone was not detected, and some photosynthetic capacity parameters tended to decline. Furthermore, CO₂ response analyses revealed that, despite lower CO₂ assimilation, stomatal conductance responded more strongly to changes in intercellular CO₂ concentration under combined red and blue LD illumination. Taken together, these results demonstrate that LD lighting, particularly red LD, enhances stomatal dynamics and induces characteristic oscillatory behavior compared with LED lighting. While red LD appears to be a promising cultivation light source for PFALs capable of maintaining high photosynthetic activity, the physiological impacts associated with blue LD, including potential reductions in photosynthetic capacity, require further study to optimize blue-light proportions for rice cultivation.