Real-time, continuous young seedling-growth measurement improves plant factory stabilization and productivity. Projected leaf area (PLA) based on seedling top-view images is a useful growth index, and easy to measure continuously for large seedling populations. However, it is difficult to automatically determine PLA with a high degree of accuracy, because RGB image color-balance fluctuates with plant growth, leaf movement, and environment. Therefore, we developed a technique for determining PLA on nursery-grown lettuce seedlings. Using a Raspberry Pi 3 microcomputer with a camera module placed above the seedlings, RGB images of 153 seedlings were obtained every 20 min from day 6 to 15 after sowing. Seedling PLA images were obtained by binarization and separation of the leaves from the background. To assess binarization accuracy, we used an Intersection over Union (IoU) index to compare the standard Excess Green (ExG) method, an optimized ExG method (O-ExG), and the artificial neural network U-Net method. Results showed that O-ExG was optimal under the experimental conditions tested. PLA and circadian rhythm amplitude extracted from PLA-image time-series data were independent, implying they can be used together for growth prediction. These findings improve the accuracy of imagebased growth prediction and have practical application in plant factories.

Acoustic emission (AE) can be used to evaluate drought stress (water stress) in plants. Although bubble motion in plant vessels is suspected of being the source of AE generated by plants (burst-type AE), this has not yet been demonstrated experimentally. This study examined whether burst-type AE is caused by bubble motion by comparing the detected frequency with the theoretical frequency estimated based on a bubble motion equation. Four experimental treatments with different soil dry densities and water supply regimes were established, and the effect of the treatment conditions on AE characteristics was clarified. AE induced by water stress was extracted by focusing on the maximum amplitude and the centroid frequency calculated by wavelet transform analysis. The results showed that the measured frequency and the estimated frequency were almost equal, which demonstrates that the model agreed with the actual measurements and that the burst-type AE can be attributed to bubble motion. The AE induced by water stress had a smaller maximum amplitude and a higher centroid frequency than the AE detected during the water supply regime. Therefore, focusing on the centroid frequency may facilitate the extraction of AE induced by water stress.

The growth and morphogenesis of leaf lettuce in response to gradual changes in light intensity over time were investigated. Seedlings of leaf lettuce (Lactuca sativa L. var. crispa “Greenwave”) were grown in a blue to red light-emitting diode (LED) ratio of 0.33 and daily light integral (DLI) of 8.7 mol m^-2 d^-1. The first experiment was conducted with nine treatments, in which light intensity varied every 15 min with its peak during the day and a consistent dark period of 8 h. In the second experiment, three dark period lengths and three irradiation changes were combined based on the irradiations that produced different plant growth responses in the first experiment. Based on the results of the two experiments, most of the irradiation methods with the same DLI and dark period resulted in similar leaf lettuce growth in terms of plant weight and shape. However, plant weight was reduced when a high level of light intensity was employed for a long duration during the light period. Shorter dark periods can accelerate the growth of leaf lettuce. Consequently, the results from this study propose a new irradiation method with a non-constant light intensity in relation to time.

To investigate whether dry matter production depends on the growth stage of cucumber (Cucumis sativus L.), different growth stage plants of two cultivars (‘Project X’ [PX] and ‘Fresco dash’ [FD]) were cultivated hydroponically, and their growth and yield were evaluated based on dry matter production during the same period. Old and young developmental stage plants (OSP; 200 d after transplanting [DAT], YSP; 43 DAT) were compared for yield components. No significant differences were observed in fruit yield obtained in FD between different developmental stages. However, OSP had a higher yield than YSP in PX. For both cultivars, there was a difference in the LAI between OSP and YSP. In contrast, total dry matter (TDM) showed a decrease in OSP in FD, whereas PX did not show a significant decrease. The fraction of fruit was significantly higher in OSP than in YSP for both cultivars. No significant differences in light use efficiency (LUE) and photosynthetic rate were observed at different developmental stages, while water use efficiency showed improvement in the old developmental stage in both cultivars.

To produce doubled haploid homozygous male-sterile line of eggplant with the cytoplasm of Solanum grandifolium, anther culture of the male-fertile (MF) BC₅ in the S. grandifolium induced CMS system was done. Anthers were cultured in an embryo induction medium and were heated to 35℃ for 3 d in the dark and then transferred to 25℃ under a 16 h photoperiod. After 20 d the anthers were transferred to a germination medium. The presence of pollen in the anther, number of chromosomes in the root tip cells, size of the stomatal guard cells and pollen fertility of the regenerated plants were investigated. The frequency of embryo formation in the MF line was 1.02%. From the embryos, one regenerated plant showed pollen non-formation type male sterility. Root tip chromosome number and stomatal guard cell size confirmed that the regenerated male-sterile plant is a microspore originated doubled haploid through spontaneous chromosome doubling. The present study demonstrates the first successful production of a doubled haploid homozygous male-sterile line in a CMS system of eggplant with the cytoplasm of S. grandifolium. Therefore, this material could be a useful breeding material for eggplant and improve the speed of conventional breeding.