Diffusion-based dynamic stomatal opening measurement at milli-scale: Toward in-field distributive deployment

Stomata, the micro-size pores on plant leaves, play a central role in the plant-environmental relation that regulates the atmospheric exchange. Through the change of aperture of the stomata poles, the diffusion of CO2 and water is adjusted during the photosynthesis and transpiration processes of plants. Therefore, the stomatal dynamics is important to plant productivity and water use efficiency. Although instruments have been developed to quantify the stomatal state, most of these instruments monitor the averaged steady-state behavior or are not designed for in-field measurement. Moreover, many of these existing in-field devices are expensive and not suitable for large-scale in-field distributive deployment. In this paper, we report the development of a methodology to measure the stomatal dynamics at milli-scale (i.e., measuring stomatal behavior in a leaf area smaller than 1 mm size). The proposed device and methodology are built upon a gas diffusion model that utilizes two miniaturized humidity sensors to accurately quantify the water vapor transpiration rate during the stomatal opening process upon light changes. By housing the two sensors in a transparent tube chamber, a milli-scale closed environment is created to measure the transpiration rate during the stomatal opening process. The developed low-cost, lightweight device is demonstrated through experiment measurement on tobacco and lettuce samples upon light condition changes. The experimental results show that the dynamic characteristics under different light conditions can be accurately measured, and a distinct difference in the stomatal dynamics between the tobacco and the lettuce samples has been observed.