Foliar Application of Nano‐Selenium Enhances Tea Quality and Selenium Biofortification by Promoting Theanine Synthesis and Nitrogen Metabolism

ABSTRACT Selenium (Se) biofortification improves crops' nutritional value, while nano‐selenium (nano‐Se) offers enhanced bioavailability over traditional Se fertilisers. The quality of tea ( Camellia sinensis ) depends critically on nitrogen (N) metabolism and amino acid balance, particularly of theanine. This study assessing growth and quality, elucidating molecular mechanisms and evaluating long‐term persistence explored the effects of nano‐Se on tea through three different experiments. The results showed that nano‐Se significantly increased the chlorophyll content, photosynthetic parameters and non‐structural carbohydrates, indicating an enhanced photosynthetic capacity. Nitrogen uptake and metabolism were promoted, along with an increased leaf nitrogen content, NO 3 − accumulation and upregulation of ammonium transporter genes, thus providing further evidence. Theanine significantly increased in roots and leaves and upregulated theanine transporter genes, while polyphenol and catechin contents decreased, lowering the polyphenol‐to‐amino acid ratio of tea. The benefits of nano‐Se persisted across three harvest times, with a sustained selenium content and an increased root theanine level, although leaf theanine and quality improvements varied. Overall, nano‐Se promoted theanine accumulation via a coordinated regulation of N transporters and biosynthetic genes, while concurrently enhancing photosynthesis and Se biofortification. Its long‐lasting efficacy positions nano‐Se as a sustainable strategy for producing high‐quality, Se‐enriched tea, particularly in Se‐deficient regions.