Irrigation methods and nitrogen-form interactions regulate starch-metabolising enzyme activity to improve rice yield and quality

Nitrogen management and irrigation methods play crucial roles in determining rice's grain yield and quality (Oryza sativa L.). However, limited knowledge exists on how interactions between nitrogen forms and irrigation regimes regulate starch-metabolising enzyme activity to influence rice yield and quality. A soil-growth experiment was conducted using a high-lodging-resistance rice cultivar under three irrigation methods, namely, submerged irrigation (0 kPa), alternate wetting and moderate drying (-20 kPa), and alternate wetting and severe drying (-40 kPa), as well as three nitrogen forms, namely, ammonium nitrogen (NH4+-N), mixed ammonium + nitrate (50:50), hereafter denoted as 50:50, and nitrate nitrogen (NO3--N). Results indicated that compared with the other treatments, alternate wetting and moderate drying interacted with 50:50 treatment, resulting in the following: improved grain yield by 11.7-21.0%, milling, appearance, eating and cooking, and nutritional qualities including milled-rice and gel consistency; and decreased chalky rice, chalky size, chalky degree, amylose content, and protein content by 20.0-23.1, 29.6-33.3, 44.1-48.5, 6.2-9.6 and 10.1-13.9%, respectively. The activities of adenosine phosphate glucose pyrophosphorylase (AGPase), starch synthase (SS), starch-branching enzyme (SBE), and adenosine triphosphate (ATP) enzyme in the grains also improved, with an increase of 20.0-35.0, 11.8-20.0, 13.6-26.3 and 21.2-39.6%, respectively. Conversely, severe drying and NO3--N treatment negatively impacted grain yield and quality due primarily to decreased SS activity in grains under each irrigation method. Correlation analysis showed that starch-metabolising enzyme (AGPase, SS and SBE) activity at 14 days after anthesis (DAA) and 28 DAA exhibited a positive correlation with grain yield, milling quality and gel consistency, whereas negatively correlated with appearance and nutritional qualities. In summary, the adoption of alternate wetting and moderate drying and 50:50 interaction treatment can synergistically boost grain yield by increasing the filled-grain rate and 1 000-grain weight and enhance grain quality of rice by upregulating the activities of starch-metabolising enzyme activity.