Effects of Nitrogen Fertilizer Application on Photosynthesis, Embryo and Endosperm Development of a Giant Embryo Rice Genotype

Rice bran oil, a valuable edible oil extracted from rice bran with a content of 15–22%, is in high demand around the world because of its various health benefits (LermaGarcia et al., 2009). Rice bran consists mainly of aleurone layer and embryo fractions. Besides a high lipid content, the embryo consists of high amount of protein and vitamins leading to breeding programs trying to increase the size of the embryo. Satoh and Omura (1981), using the method of mutant egg fertilization with N-methyl-N-nitrosourea (MNU), created rice giant embryo mutants with two to three times bigger embryo size. Several genes/quantitative trait loci (QTL) controlling for the giant embryo trait have been detected on chromosome 7 (Koh et al., 1996) and chromosome 3 (Taramino et al., 2003). Recently, some giant embryo varieties have released for cultivation to produce oil and functional food in Japan (Maeda et al., 2001; Matsushita et al., 2008; Ishii et al., 2013) and South Korea (Kim et al., 1991). In a previous report, the promising mutant line “MGE13” with the giant embryo gene Os07g0603700 originating from the high-yielding rice cultivar Mizuhochikara (Miz) was developed (Sakata et al., 2016). The mutant giant embryo had still increased size at 10 days after pollination while the original cultivar had almost developed to its maximum size in the same time period (Itoh et al., 2005). Furthermore, the larger embryo size of mutant type compared to that of the original cultivar rice was found mainly by enhanced cell expansion, but was not significantly related to a larger number of cells in the scutellum (Nagasawa et al., 2013). Also, Yang et al. (2013) discovered the relationship between giant rice embryo development and shoot apical meristem (SAM) activity which is controlled by gene ge for both embryonic and post-embryonic (10 days after pollination) growth promoting plant growth parameters such as the growth rate during the vegetative stage, longer leaves, more tillers, and an increased 1,000-grain weight. Furthermore, embryo development was observed in balance with endosperm development (An et al., 2020). The regulation of endosperm development was related to the auxin and abscisic acid signaling pathways from the embryo (Yi et al., 2016; Zheng et al., 2019), in contrast, the embryo development regulated by the apoplastic nutrient pathway including sugar flow from the endosperm which is mainly a photosynthetic product (Du et al., 2018). Higher nitrogen fertilizer applications have been shown to increase photosynthesis, dry matter accumulation, and grain yield in rice plants (Pham et al., 2003; Tang et al., 2008; Nguyen et al., 2019). Additional nitrogen fertilizer at the time of heading caused the increase of physiological parameters including photosynthesis and amino acid synthesis but reduced the cellulose content in the endosperms (Midorikawa et al., 2014) as well as was involved in chalky tissue formation, C and N metabolism, and the regulation of ribosomal proteins (Lin et al., 2017).