Molecular Plant Breeding 2025, Vol.16, No.4, 250-260 http://genbreedpublisher.com/index.php/mpb 251 2 Agronomic Traits of Japonica Rice Varieties 2.1 Morphological and physiological features relevant to transplanting The agronomic traits of japonica rice, such as plant height, tiller number, effective panicles, panicle grains, leaf area size, and root development, will directly affect its adaptability to different transplanting methods. Studies have found that applying more nitrogen fertilizer appropriately and increasing planting density can boost the number of effective panicles and grains per panicle. However, if too much is used, it may instead affect the output (Ren et al., 2024). In addition, whether the root system grows well or not, such as the number of roots, how deep they grow, and how vigorous they are, will also affect the ability to absorb water and nutrients. These will in turn affect the growth and yield after transplanting (Chu et al., 2019; Zhu et al., 2020). In adverse environments such as saline-alkali land or drought, some japonica rice varieties show good resistance, such as delayed heading time, shorter plants, shorter panicles, and reduced effective panicles and seed setting rate (Ahmad et al., 2021). 2.2 Genetic diversity and responsiveness to cultivation environments The varieties of japonica rice vary greatly in terms of genetics and responses to the environment. Japonica rice in the south generally has wider and thicker grains, while that in the north has longer grains and a higher seed setting rate (Yan et al., 2024). Genomic association studies have found that traits such as plant height, panicle length, and grain shape all have corresponding quantitative trait loci (QTLs), and this genetic information can help explain why different varieties behave differently in different environments (Hori et al., 2017; Wang et al., 2025). Some good alleles can also increase yield, quality or disease resistance, and their adaptability varies in different regions (Zhang et al., 2022; Jiang et al., 2023). 2.3 Yield potential versus quality stability The yield of japonica rice is closely related to some of its key traits, such as the number of effective panicles, the number of grains per panicle, plant height, and growth period, etc. (Li et al., 2019; Zou et al., 2024). High-yield varieties often increase the total yield by increasing the number of grains per panicle and the capacity of the yield reservoir. However, if one blindly pursues yield, it may lead to a decline in quality or weakened disease resistance. Nowadays, genomic selection methods have begun to be used in breeding to balance the relationship between high yield, good taste and disease resistance. For example, new japonica rice varieties like XY99 and JXY1 not only have high yield, but also good quality and strong disease resistance (Figure 1) (Xiao et al., 2021; Jiang et al., 2023). There are also some varieties that perform stably under conditions such as saline-alkali, drought, and low nitrogen, with the yield not easily dropping and the quality maintained. They are suitable for promotion in different regions and under different planting methods (Ahmad et al., 2021; Ren et al., 2024). 3 Classification and Characteristics of Transplanting Methods 3.1 Manual transplanting Manual transplanting is a traditional method, which requires people to plant the seedlings one by one into the field. It is flexible to operate and can adjust the row spacing and density according to the size and shape of the field. The seedlings are easy to survive and grow well in the early stage. However, this method is too laborious and inefficient, and is not suitable for large-scale rapid planting (Wu et al., 2022). 3.2 Mechanical transplanting There are two common types of mechanical rice transplanting: basket-shaped seedlings and pot-shaped seedlings. Mechanical transplanting of blanket seedlings (MC) is to lay the whole seedling in the field. It is fast and very suitable for large-scale planting, and also saves labor (Bian et al., 2018; Wu et al., 2022). Mechanical transplanting of pot seedlings (MT/PS) is to raise seedlings in small grids. Each seedling is relatively strong, grows fast after transplanting, turns green early, can distribute more nutrients to the spike and grain in the later stage, has a high yield, and has a stronger ability to absorb fertilizer (Hu et al., 2020; Fu et al., 2021). This type of transplanting method is particularly suitable for field cultivation where there are few people and high yield is desired (Hu et al., 2018; Wu et al., 2022). 3.3 Tray nursery and seedling throwing methods The method of throwing seedlings involves first raising seedlings in seedling trays and then scattering them into the field either manually or by machine. This method is convenient and labor-saving, and is suitable for irregular
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