Excessive biuret in fertilizer causes leaf albinism in direct-seeded rice fields. This study aimed to provide a comprehensive understanding of the underlying physiology and molecular mechanisms of leaf chlorosis via biuret using morphophysiological and transcriptome analyses. The induction of biuret in albino rice leaves was examined in a net-growing cultivation bed. Some key morphophysiological indices were measured including biuret content, blade ultrastructure, chlorophyll content, and chlorophyll fluorescence parameters. Candidate genes in the chlorotic leaves under biuret stress were also excavated using transcriptome analysis. Furthermore, physiological and biochemical analyses of the changes in enzyme activities and intermediate metabolite contents in relation to the phenotypic changes in the leaves were carried out. The chlorotic leaves of rice seedlings showed higher biuret accumulation, and the leaves suffered severe damage with higher malondialdehyde contents and low chlorophyll contents. Abnormal chloroplast ultrastructures and thylakoid membrane structure loss were observed in chlorotic leaves under biuret exposure. The related genes involved in the chloroplast development, photosynthesis (including antenna proteins), and carbon fixation pathways were significantly downregulated, which suggests that photosynthesis was destroyed in the chlorotic leaves of rice seedlings. Biuret disturbed the photosynthetic system in chloroplast thylakoid membranes by inhibiting chloroplast development, thereby promoting the formation of the chlorotic leaf phenotype in rice seedlings. Our results promote the understanding of the molecular mechanism of rice in response to biuret toxicity.

Weak seedlings and poor growth uniformity affect the mechanical transplanting of densely sown rice seedlings. To address these issues, seedlings of the conventional japonica rice “Zhehexiang 2” were grown in a traditional flat tray (control), pot-mat tray (26 × 52 bowls; BT(26)), and pot-mat tray (30 × 58 bowls; BT(30)) to compare the effects of different specifications of pot-mat trays (BTs) on the growth and quality of mechanical transplanting of densely sown rice seedlings with 250 g/tray. The BT-raised seedlings showed improved seedling quality, with increases in the shoot and root dry weights by 7.44% and 20.11%, respectively, compared to the flat tray. Under the dense sowing rate, the plant height uniformity of the BT(26) and BT(30) treatments was significantly increased by 6.95% and 3.43%, and the root entwining force of the seedlings was 14.28% and 10.21% higher, respectively, compared with those of the control. The missing hill rate for BT-raised seedlings after mechanical transplanting was significantly reduced by 53.15%. The loss of roots during mechanical transplanting was reduced. Compared with the control, the root length, root surface area, and root number were increased, and a greater number of large roots were retained, which promoted the early development of seedlings after mechanical transplanting. The proportion of holes with two to five seedlings was higher after mechanical transplanting. The pot-mat tray divided the root growth area of seedlings, promoted the growth of the seedlings, and reduced the root loss and missing hill rate under the high sowing rate. Thus, the quality of mechanical transplanting of densely sown seedlings was improved.

The inhibition of low-position tillering in machine-transplanted seedlings affects rice yields. Paclobutrazol (PBZ) is a plant growth regulator that can improve seedling quality and promote low-position tillering in machine-transplanted seedlings. However, the physiological mechanisms underlying the promotion of tiller bud formation induced by exogenous PBZ via sucrose transport remain unclear. Thus, rice cultivar ‘Yongyou 12’ was used to analyze the effects of different seeding rates and the application of exogenous PBZ, gibberellin (GA₃), and water (control) on sucrose transport and metabolism as well as tiller bud development. Exogenous PBZ application combined with a low seeding rate significantly increased the number of tillers as well as seedling fullness (by 42.35%). Increases were also detected for the seedling cytokinin content, chlorophyll content (by 10.55%), and sucrose transport from leaves to the stem base. These changes were associated with the upregulated expression of sucrose transporter genes in leaves and the stem base, as well as increased activities of key sucrose-metabolizing enzymes in the stem base. Notably, the opposite trend was observed after exogenous GA₃ was applied or a high seeding rate was used. Hence, a low seeding rate combined with exogenous PBZ application is useful for controlling seedling height, promoting the formation of low-position tillering, facilitate sucrose translocation from leaves to the stem base, and increasing sucrose metabolism in the basal part of rice plants. These findings provide a theoretical basis for optimizing low-position tillering in machine-transplanted seedlings.