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To investigate the differential responses of super rice grain filling to low filling stage temperature (LT) and the regulative effect of nitrogen panicle fertilizer (NPF), physiological and molecular experiments were conducted with two super rice varieties (Nanjing 7th: N7 and Nanjing 9108th: N9108) on two different filling stage temperature treatments implemented by applying two sowing dates [Normal filling stage temperature (CK): Sowed on May 30, Tmean = 24.7°C and low filling stage temperature (LT): Sowed on July 1, Tmean = 20.3°C], and two NPF levels (0 and 150 kg N ha−1). In this study, LT, NPF, and simultaneous LT and NPF treatments suppressed the grain filling in all varieties with different levels. Under LT or NPF treatments, the reduction of grain weight, seed setting rate, and filling rate were closely associated with suppressed starch biosynthesis rate in inferior seeds, suggesting that reduced starch biosynthesis rate, expression, and activities of enzymes encoded by related genes, Floury endosperm-4 (FLO4), Starch branching enzyme-I (SBE1), and Starch phosphorylase-L (PHO-l), were responsible for the grain filling reduction. Under LT or NPF treatments, significantly higher grain filling rates and lower variance were found in N9108 compared to that in N7, which were closely related to their higher starch biosynthesis ability, related gene expression, and enzymes activities. One of the probable explanations of the grain filling difference was the variation in the relative amount of key regulative hormones, Abscisic acid (ABA) and 1-aminocyclopropane-1-carboxylic acid (ACC). These results raise a possibility that super rice with higher sink activities has superior adaptability to LT and NPF due to their higher sink activities. [ABSTRACT FROM AUTHOR]

Lodging is a major limiting factor for grain yield and quality, and low light enhances lodging risk by increasing stem elongation and decreasing culm thickness. This study evaluated the effect of exogenous uniconazole (UCZ) on the basal stem physiological mechanism, cell structure, carbohydrates, and lignin-related enzymes in culm tissue under shading conditions. The results showed that shading stress significantly decreased grain yield and stem mechanical strength in 2020 and 2021. Compared with normal light (CK), shading rapidly decreased the breaking strength (M) and section modulus (SM). Exogenous UCZ increased culm wall thickness and improved lodging resistance. Uniconazole application also reduced plant height, gravity height, and second basal internode length by inducing a reduction in the cell length, and caused the cells to be densely arranged under both normal light and shading conditions. Scanning electron microscopy (SEM) revealed that the thickness of the sclerenchyma tissue cell wall increased under both conditions under the UCZ treatment, which induced an increase in the cell wall content. Uniconazole application significantly increased non-structural carbohydrate, lignin, cellulose, and hemicellulose contents in culm tissue. Moreover, UCZ application regulated lignin-related enzymes, such as phenylalanine ammonia-lyase (PAL), 4-coumarate: CoA ligase (4CL), cinnamate 4-hydroxylase (C4H), laccase (LAC) and peroxidase (POD). Thus, exogenous UCZ could enlarge sclerenchyma cell wall thickness, alter cell morphology in the stem, and increase stem strength by increasing carbohydrate content, including non-structural carbohydrates, cellulose, lignin, and hemicellulose, to enhance rice (Oryza sativa L.) lodging resistance under shading conditions. • Shading stress altered slender and loose cell arrangement in the parenchyma tissue of the second basal internode. • Uniconazole enhances lodging resistance of Japonica Rice by increasing sclerenchyma cell wall thickness. • Lignin accumulation and activities of lignin-related enzymes were increased by uniconazole. [ABSTRACT FROM AUTHOR]