Smartphone application-based measurements of stem-base width and plant height in rice seedling.

• A portable smartphone application for stem-base width and plant height measurement. • An easy-to-attain reference object for scale factor determination. • Combing an application and a reference object to measure actual morphological traits. The measurements of stem-base width and plant height in rice seedlings are significantly associated with the assessment of in-field seedling growth quality, the determination of optimal seedling transplanting time, and the decision-making of later fertilization use. At present, the measurements of such two morphological traits heavily rely on the inefficient manual measurement method. Herein, we develop a smartphone application to accomplish these tedious and repetitive measurement tasks. Using the developed smartphone application, the reference object and rice seedling were taken into an entire image and then were performed image pre-processing, thereby segmenting the foreground from the background. The reference object image and rice seedling image in the foreground were respectively separated via a comparison in heights of the foreground contours. The reference object image was used for obtaining a scale factor, while the rice seedling image was used for measuring actual stem-base width and plant height by employing the obtained scale factor. The results of automatic measurements resulted from different conditions were compared with those of manual measurements, accomplishing the assessment of measurement accuracy. Collectively, the smartphone application achieved R2 values of 0.9832 and 0.9796 in measuring stem-base width and plant height respectively, and corresponding RMSE values were 0.0307 mm and 1.2311 mm respectively, when tests were performed in various conditions including three camera resolutions, three photographing heights, and three sunlight conditions. The mean runtime for processing an individual image was about 1796.164 ms when a total of 350 rice seedlings were measured under various conditions. This work not only demonstrates that the smartphone application allows low-cost, fast, and accurate measurements of stem-base width and plant height in rice seedling, which can be used as a reasonable alternative to the traditional manual measurement method; but also offers a valuable reference for farmers or researchers in terms of evaluating in-field seedling growth quality, determining optimal seedling transplanting time, as well as guiding follow-up in-field fertilizer expenditures; and further provides methodological support for relevant researches in other crops or domains. [ABSTRACT FROM AUTHOR]