Early-stage nitrogen fertilizer raised rice yield and quality

Rice (Oryza sativa L.) is one of the most important cereal crops, and more than half of the global’s population is dependent on rice and its related products to obtain the majority of nutritional requirements [1,2]. Starch, accounting for up to 80% of the total weight of rice grains, constitutes the main caloric source in rice and plays a critical role in cooking quality through its internal structure and other physical properties [3,4]. 

There are two forms of starch granules in rice, single and complex granules. The diameter of starch in complex form is about 10-20 µm [5], which was developed from the singles as the grouting continues [6]. Starch consists of two kinds of homopolymers of α-D-glucosyl units, namely, amylose and amylopectin, which correspond to 0%–30% and 70%–95% of starch, respectively, and their proportion varies in different cultivars [7-9]. Apparent amylose content (AAC), swelling power and solubility are some of the parameters that define the physicochemical properties of starch [7,10,11]. Other factors, such as granule size distribution and crystallinity, are also important indicators for evaluating the quality of cooked rice [12,13].

Image source: Genetic Literacy Project

Genetic, environmental, and agronomical factors are mainly related to rice quality and starch composition [14,15]. Nitrogen is a non-negligible environmental factor that can result in a decreased rice quality in cooking and eating when excessively used. However, proper application of nitrogen in early cereal growth stages can also significantly raise yield and protein content [16,17]. With the increase of nitrogen application rate during rice tillering, the amylose and protein contents of rice gradually increased [18]. Nitrogen application at the heading stage can significantly increase the yield and the protein content, and result in low amylose content [19]. Late stage nitrogen fertilization reduced the cooking quality of rice by increasing amylose content and reducing amylopectin branching degree, which decreased starch crystallinity and aggravated pasting properties [9].

Moreover, Peng et al. [21] have speculated that rice yield and quality could be improved by increasing the amount of fertilizers used at the panicle initiation stage.  According to a recently published paper, nitrogen application at panicle initiation stage can promote starch development and accumulation among different endosperm parts in the endosperm and eventually affected rice agronomic traits [22]. The number of amyloplasts was increased by budding-type division and constricting. Dividing amyloplasts were elongated and generated a beaded structure. The particle size of the SGs from the NTG was smaller than that of the SGs from the CG. Nitrogen significantly decreased AAC and led to an increased order degree at short-rang scale of rice starch. These findings showed that the synthesis and properties of starch were significantly affected by nitrogen fertilizer.

Reference

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