Micromorphology and ammonium transmembrane transport characteristics in roots of rice seedlings

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, NH ₄ ⁺ 1.2 mg g ⁻¹ ) and low ammonium concentration (LN, NH ₄ ⁺ 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 NH ₄ ⁺ 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 μm on 6 days after sowing (DAS) to under 500 μm on DAS12. The meristematic zone was the primary site for NH ₄ ⁺ absorption in new root and proved to be particularly sensitive to environmental NH ₄ ⁺ concentration. At DAS 6, the net NH ₄ ⁺ flux rate was highest in the meristematic zone under both HN and LN treatments. Additionally, the external NH ₄ ⁺ concentration influenced the direction of NH ₄ ⁺ flux in the meristematic zone at DAS9, with HN being a net NH ₄ ⁺ influx and LN being an NH ₄ ⁺ efflux, and the net NH ₄ ⁺ 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 NH ₄ ⁺ uptake, exhibiting an infNH ₄ ⁺ 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 NH ₄ ⁺ efflux was observed in the entire seminal root, signaling the loss of NH ₄ ⁺ absorption function 15 days post-emergence. A similar trend was noted in adventitious roots, where NH ₄ ⁺ uptake remained functional for 12-15 days after root emergence. As rice seedlings continued to grow, new adventitious roots replaced old ones, facilitating ongoing NH ₄ ⁺ uptake.