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Improving early seedling growth is crucial for direct-seeding rice cultivation. In this study, field and laboratory experiments were conducted to determine the effects of Plant Growth Regulators (PGRs) such as gibberellic acid (GA 3) and ethephon (ET) on the early growth of direct-seeded rice. Seeds were treated with the PGRs and pre-germinated at 30°C in the dark. For the field experiment, two Flooding Depth (FD) regimes were established during the growth period: 0 cm (drained condition) and 2 cm (flooded condition). Treatment of seeds with ET or GA 3 alone promoted the growth of seedling organs compared to that of the control at both the FDs in some cases. However, combined application of ET and GA 3 (ET+GA 3) synergistically improved the growth of different seedling organs at both the FDs in most cases with slight exception: the coleoptile and mesocotyl plus coleoptile (mesocotyl+coleoptile) length were not different across the PGR treatments under drained condition. Further, the growth of the second node tiller was stimulated by the application of ET alone under drained conditions. The laboratory experiment revealed that ET+GA 3 increased the growth rate of mesocotyl+coleoptile and that of the first leaf. These results suggest that the combined application of ET and GA 3 might be useful for improving the growth performance of rice seedlings in the direct seeding method.

This study introduced a data screening method for comparing the reflectances in middle latitude forest regions collected by a Geostationary Earth Observing (GEO) satellite and a Low Earth Orbit (LEO) satellite. This method attempts to reduce the differences between the relative azimuth angles of the GEO and LEO observations. The method, called relative azimuthal-angle matching (RAM), takes advantage of the high temporal resolution of the GEO satellites, which enables collection of a wide range of relative azimuth angles within a day. The performance of the RAM method was evaluated using data in the visible and near-infrared bands collected by the Himawari-8/Advanced Himawari Imager (AHI) and the Terra/Moderate Resolution Imaging Spectroradiometer (MODIS). The consistency of the reflectance pairs of MODIS and AHI selected by the RAM method was compared with the consistency of the reflectance pairs acquired simultaneously by the two sensors. The data were matched pixel-by-pixel after applying atmospheric corrections and cloud screening. The results show that RAM improved the reflectance ratio by approximately 10% for the red and NIR bands on average relative to the simultaneous observations. Significant improvements in the two bands were observed (20%), especially among data collected in the fall and winter. Performance of RAM depends largely on season. Especially in summer, the reflectance pair chosen by RAM showed less consistency than solar zenith-angle matching (SZM). The results also indicated the relatively large influence of the spectral response functions on the green and red bands of the two sensors.

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Early seedling growth, including seedling emergence and vigor, is crucial in direct seeded rice. In this study, we examined the effects of gibberellic acid (GA 3) and ethephon (ET) on the early growth of direct seeded rice, and on α-amylase activity and soluble sugar content in the seeds. Rice seeds were treated with water (control), ET, GA 3 , or ET+ GA 3 and were allowed to grow for 21 days. The results showed that ET and GA 3 synergistically improved the lengths of mesocotyl, coleoptile, mesocotyl+ coleoptile, and first leaf. Moderate synergistic effects of ET and GA 3 were detected in the second and third leaves. ET+ GA 3 treatment induced higher α-amylase activities during early post-germination growth, which is consistent with the longer coleoptile, mesocotyl, mesocotyl+ coleoptile, and first leaf. In addition, ET+ GA 3 induced higher α-amylase activity in the seeds, and consequently, higher sucrose and glucose concentrations than other treatments. These results partially explain the regulatory mechanism underlying the synergistic interaction of ET and GA 3 on rice seedling growth in terms of starch and sugar metabolism in the seeds, and this knowledge is expected to facilitate the practical use of ET and GA 3 in direct seeded rice systems.