Phosphorus (P) and potassium (K) stress significantly affect the growth, physiological characteristics, and nutrient uptake of rice plants. This study investigated the photosynthetic nitrogen (N) metabolism, N uptake, and N utilization of plants under varied P and K supplies. Two local conventional high-yield rice varieties (Shennong 265 and Liaojing 294) were used. These varieties were subjected to the following hydroponic experimental treatments: HPHK (normal P and K concentrations), HPLK (normal P and 1/20 normal K concentration), LPHK (normal K and 1/20 normal P concentration), and LPLK (1/20 normal P and K concentrations). The results showed that the mesophyll cells had a relatively complete nuclear and chloroplast structures, and the antioxidant enzymes of the plants were significantly reduced under the HPHK treatment. Compared to the LP treatments (LPHK and LPLK), the HPHK treatment was found to have the following potential effects: effectively optimize plant configuration; promote leaf development (Pn, E, Ci, and Tr, chlorophyll, and leaf area index); significantly increase the N-metabolism-related enzyme activity of leaves and roots and the accumulation of N in the plant in the main growth stages; and significantly increase the rice yield and N-related efficiency. In conclusion, the HPHK treatment was found to be beneficial in improving the plant configuration, promoting photosynthetic N metabolism, and increasing grain yield and N-related utilization efficiency.

Northeast China is an important commercial grain base for China, but also the largest japonica rice production area. However, N, and K fertilizer application and unreasonable application times are prominent contradictions that restrict the development of japonica rice. This study aimed to investigate how to rationally apply N and K fertilizers to affect grain filling and ultimately increase the quality of the rice. In this field study, two N application levels and three K application ratios were set in 2020 and 2021 using Shennong 265 (SN265) and Meifengdao 61 (MF61). We found that the final seed growth and filling rate of SN265 were higher than those of MF61, and their filling characteristics were slow in the early stage and fast in the later stage, with large fluctuations. Appropriate reductions and increases of N and K fertilizer applications, respectively, in the early stage could improve grain filling. Compared with SN265, MF61 had a 3.2% increase in head rice rate, lower amylose and protein content, a decrease of chalkiness degree and chalkiness percentage by 23.96 and 34.00%, respectively, and more reasonable protein components. With the N application increase, the processing quality improved, the amylose and protein content and chalkiness increased, the protein components increased except for the milled rice glutelin, and the rice taste value decreased. At low N levels, increasing the proportion of K application was consistent with the effect of increasing N. The taste value of SN265 decreased linearly with the increase in the ratio of N application to pre-application of K, the highest taste value was obtained when the N fertilizer was applied at a rate of 180 kg ha⁻¹, and the ratio of before and after K fertilizer application was about 1:2. The taste value of MF61 decreased linearly with the N application increase and showed a trend of increasing and then decreasing with the K application increase in the early stage. The taste peak gradually shifted back with the N application increase, and the highest taste value was obtained when N fertilizer was applied at 180 kg ha⁻¹; the ratio of before and after K fertilizer application was about 3:2. By constructing the grain-filling quality evaluation system, the characteristic parameters of superior and inferior grains at the early and late stages of grain filling, respectively, greatly affected the rice taste value. Additionally, the percentage of the rice grain weight at the maximum grain-filling rate to the final rice grain weight (I) of superior grains, glutelin content, and value of the RVA profile characteristics were all critical reference indicators for rice taste quality.

Selenium (Se) is a trace element that is beneficial in enhancing the quality of rice production. However, research on the effects of Se on rice quality under varying nitrogen (N) levels is limited and requires further investigation. This experiment utilized a randomized block design, incorporating an N fertilizer reduction and efficient application mode, with two N levels, CN (225 kg·hm⁻²) and LN (180 kg·hm⁻²), and three Se levels, HSe (0.12 kg·hm⁻²), LSe (0.06 kg·hm⁻²), and 0Se (0.00 kg·hm⁻²). The results indicated that the effects of Se on rice processing quality differ under different N levels. Selenium adversely affected the processing quality under the CN level, whereas it demonstrated some improvement at the LN level. Furthermore, Se application increased the Se content in rice by 46.48–141.82% and enhanced the taste value by 14.88–22.73%. It significantly improved the nutritional and cooking qualities of rice and positively influenced its appearance. Although N levels induced variations, their overall impact remained beneficial. Considering various indicators, applying 0.06 kg·hm⁻² of Na₂SeO₃ under the LN level yielded optimal results. This study provides valuable insights into the effects of Se on rice quality under different N levels. It provides a more scientific basis for the application of selenium fertilizer in rice.