Effects of phosphorus fertilizer application rate on grain quality and starch synthase activity of different low-phosphorus-tolerant varieties of rice

Phosphorus (P), an essential nutrient for plant growth and development, significantly affects rice yield and quality. A pot experiment was conducted to examine the influence of different phosphorus fertilizer application rates on the quality of different low-phosphorus-tolerant rice varieties and the enzyme activities of superior and inferior grains. Using ‘Lianjing 7’ (low-phosphorus sensitive) and ‘Yongyou 26040’ (low-phosphorus tolerant) as the experimental rice varieties, four phosphate fertilizer application rates were set: 0, 60, 120, and 180 kg(P₂O₅)∙hm⁻², denoted as P0, P60, P120, and P180, respectively. The relationship between quality and enzyme activity related to starch synthesis in superior and inferior grains of different varieties was analyzed. This study aimed to provide a reference for high-quality rice cultivation. The results demonstrated the following: 1) compared with no phosphorus application, phosphorus fertilization improved rice quality. An increase in the phosphorus application rate resulted in initial increases followed by decreases in brown rice rate, milled rice rate, and head rice rate. The chalky grain rate, chalky area, degree of chalkiness, alkali spreading value, and amylose content decreased and then increased. The rice quality of ‘Lianjing 7’ and ‘Yongyou 2640’ were optimized under P120 and P60 treatment, respectively. For example, the brown rice rate, milled rice rate, and head rice increased by 19.6%, 25.3%, and 79.7% (‘Lianjing 7’ under P120 treatment) and 17.8%, 23.5%, and 38.4% (‘Yongyou 2640’ under P60 treatment), respectively, compared with P0 treatment. The quality of ‘Yongyou 2640’ was better than that of ‘Lianjing 7’. 2) Compared with no phosphorus application, phosphorus fertilization enhanced the activities of enzymes related to starch synthesis in rice. With an increase in the phosphorus application rate, the adenosine diphosphate glucose pyrophosphorylase (AGPase), soluble starch synthase (StSase), sucrose synthase (SuSase), starch branching enzyme (Q-enzyme), and adenosine triphosphatase (ATPase) activities of superior and inferior grains increased and then decreased. Generally, the enzyme activities of ‘Lianjing 7’ and ‘Yongyou 2640’ were highest under P120 and P60 treatment, respectively, under which the AGPase and StSase activities of superior grains in the middle stage of grain filling increased by 18.4% and 51.1% (‘Lianjing 7’) and 20.0% and 51.5% (‘Yongyou 2640’), respectively, compared with P0 treatment. The enzyme activities of ‘Yongyou 2640’ were higher than those of ‘Lianjing 7’. 3) Correlations among protein content, amylose content, and starch synthesis-related enzyme activities were found to vary between different low-phosphorus-tolerantt varieties. The activities of enzymes related to starch synthesis were significantly affected by the protein content of ‘Lianjing 7’, but less affected by the amylose content, while they were significantly affected by the amylose content of ‘Yongyou 2640’, and less affected by its protein content. ‘Lianjing 7’ and ‘Yongyou 2640’ showed the highest enzyme activities of superior and inferior grains under the P120 and P60 treatments, respectively, promoting the synthesis and accumulation of grain starch, giving rise to improved rice quality.