Exogenous ethephon application promotes nitrogen accumulation by modifying root characters in rice seedlings

Rice (Oryza sativa L.) root characteristics are closely associated with nitrogen (N) uptake, root growth and development are greatly influenced by ethylene. In this study, a hydroponic experiment was conducted using four rice genotypes [Shanyou 63 (SY63) and Zhonghua 11 (ZH11) with well–developed aerenchyma; Yangdao 6 (YD6) and mutant rcn1 from ZH11 with less–developed aerenchyma] to investigate the effects of exogenous ethephon (Eth) on root characteristics, N uptake, dry matter distribution, and clarify the underlying relationship. Compared with YD6 and rcn1, SY63 and ZH11 had higher N accumulation, higher root aerenchyma area to cortex area ratio (ACR), higher NH₄⁺ uptake via the apoplasmic pathway and root-to-shoot NH₄⁺ translocation under no ethephon application (NEth) and Eth treatment, and elevated expression of the three genes (OsAMT1;2, OsAMT2;2, and OsAMT4;1) for ammonium transporters under Eth treatment. Eth treatment increased shoot N and dry matter accumulation, decreased the total root length and root diameter, and increased ACR and the expression of OsAMT genes in four genotypes. In summary, Eth could increase N accumulation via modifying root characteristics in rice, particularly by enlarging root aerenchyma and thinning the roots. The findings provide implications for development of elite rice varieties and green rice production with higher N efficiency.