Foliar application of nano carbon dots increases grain yield in rice via enhancing leaf photosynthesis and stem assimilates translocation under nitrogen deficiency

Rice is one of the world's most important food crops, and the excessive use of fertilizers to achieve high rice yields has led to global environmental concerns. Therefore, it is critical to reduce nitrogen fertilizer input, a problem for which nano carbon dots (NCD) provide a promising solution. However, few studies have examined the effect of NCD on rice yield formation under reduced nitrogen input conditions. The aim of this study is to evaluate the effects of NCD on the photosynthetic characteristics, accumulation and translocation of stem assimilates, and yield of rice under reduced nitrogen conditions. A two-year field experiment was conducted on rice using two varieties, Yongyou2640 and Y liangyou2, with five treatments: N1 (270 kg N ha −1 ), N2 (216 kg N ha −1 ), N3 (162 kg N ha −1 ), CN2 (216 kg N ha −1 +foliar application of NCD), and CN3 (162 kg N ha −1 + foliar application of NCD). Leaf photosynthetic enzyme activity, stem and grain starch synthesis-related enzyme activity, sucrose translocation-related enzyme and gene expression, and yield and yield components were measured at five time points from the jointing stage to maturity. Under the condition of reducing nitrogen fertilizer by 40 %, the foliar application of NCD could significantly increase rice yield, with average increases of 13.2 % and 12.3 % for both varieties in 2022 and 2023, respectively, reaching the yield achieved with normal nitrogen levels. The foliar application of NCD significantly increased the leaf photosynthetic rate by increasing rubisco enzyme activity, resulting in a 8.5–61.5 % increase in the photosynthetic rate for both varieties at different growth stages across the two years. In addition, the foliar application of NCD prior to heading increased the stem activity of ADP-glucose pyrophosphorylase (AGP) and starch synthase (StS), thereby increasing the accumulation of NSC in the stems before flowering. The average increases for both varieties were 21.2 % and 17.8 % in 2022 and 2023, respectively. After heading, the activities of α-amylase, β-amylase, and sucrose phosphate synthase (SPS) in the stems, as well as the activities of AGP, StS, and cell wall invertase in the grains, were significantly increased. The expression of key genes involved in sucrose phloem loading in the stem, OsSUTs and OsSWEETs, and genes related to sucrose phloem unloading in the grains, OsSUTs , OsSWEETs , and OsCINs, were upregulated, thereby promoting the translocation of assimilates from the stems to the grains, which improved the grain filling percentage and consequently increased the grain yield under conditions of nitrogen deficiency. In summary, under conditions of nitrogen deficiency, the foliar application of NCD to rice can increase the activities of photosynthetic enzymes, starch metabolism enzymes in stems and grains, and sucrose translocation-related enzymes; increase sucrose translocation-related gene expression; regulate the accumulation and translocation of assimilates; and improve grain filling and grain yield, and also increase economic profit. • Nano carbon dots improved rice yield by increasing photosynthetic capacity, carbon metabolism and translocation. • Nano carbon dots enhanced the leaf photosynthetic rate by increasing rubisco activity. • Nano carbon dots increased the activities of enzymes related to carbon metabolism in stems. • Nano carbon dots upregulated the expression of the genes related to sucrose phloem loading and unloading.