Zhang, X.; Xu, Y.; Chen, S.; Chu, G.; Xu, C.; Yang, Y.; Wang, D.

During the seedling stage, rice root exhibit rapid growth and absorb significant quantities of nutrients. This study involved cultivating rice seedlings in two different seedling-raising mediums characterized by high ammonium concentration (HN, NH4+ 1.2 mg g-1) and low ammonium concentration (LN, NH4+ 0.006 mg g-1). The morphology of distinct root zones (root cap, meristematic zone, elongation zone, and maturation zone) was observed under a microscope throughout root development, and variations in transmembrane flux rates of NH4+ in different root zones were assessed using Non-invasive micro-test technology (NMT). Results showed that the root caps of both seminal and adventitious roots became separated from the root tip between 9 and 12 days after root emergence. During this time, the root tip transformed from a sharp to a round shape, and the distance from the maturation zone to the root tip progressively decreased from over 900 um on 6 days after sowing (DAS) to under 500 um on DAS12. The meristematic zone was the primary site for NH4+ absorption in new root and proved to be particularly sensitive to environmental NH4+ concentration. At DAS 6, the net NH4+ flux rate was highest in the meristematic zone under both HN and LN treatments. Additionally, the external NH4+ concentration influenced the direction of NH4+ flux in the meristematic zone at DAS9, with HN being a net NH4+ influx and LN being an NH4+ efflux, and the net NH4+ influx in the meristematic zone persisted for 3 days longer in the HN treatment compared to the LN treatment. In mature roots, the root hair zone emerged as the primary site of NH4+ uptake, exhibiting an infNH4+ influx rate of 40-140 pmol cm-1 s-1, and this low rate of uptake could be sustained for up to 12 days after root emergence. By DAS15, a net NH4+ efflux was observed in the entire seminal root, signaling the loss of NH4+ absorption function 15 days post-emergence. A similar trend was noted in adventitious roots, where NH4+ uptake remained functional for 12-15 days after root emergence. As rice seedlings continued to grow, new adventitious roots replaced old ones, facilitating ongoing NH4+ uptake.