Volatile Compounds from Waste Shiitake Fungi Beds Enhance Rice Growth, Yield, and Performance Under High-Temperature Field Conditions

Agricultural waste streams represent an underutilized source of bioactive compounds with potential to enhance crop resilience under climate stress. We previously showed that volatile compounds (VCs) emitted from waste shiitake fungi beds (WSFBs) promote early rice seedling growth under controlled conditions. Here, we evaluated whether these early-stage effects persist after transplanting and translate into agronomic benefits under field conditions, including the record high temperatures (HTs) of the 2023 growing season in Niigata, Japan. Seedlings of two japonica cultivars, Nipponbare and Koshihikari, were exposed to WSFBs-derived VCs using a non-contact system and subsequently grown in paddy fields across two seasons (2023–2024). WSFBs-VCs-treated (+VCs) plants exhibited enhanced seedling vigor, increased tiller and panicle numbers, higher grain yield per plant, greater 1000-grain weight, and reduced grain chalkiness. Gas exchange measurements at the reproductive stage during the 2023 record HT showed that +VCs plants maintained higher net photosynthetic rate, stomatal conductance, intercellular CO2 concentration, and transpiration rate, while intrinsic water-use efficiency showed a modest decline consistent with transpirational cooling. Controlled-environment assays revealed enhanced physiological stability supported by upregulation of cytokinin and stress-responsive genes under acute heat stress. Together, these results demonstrate that short-term exposure to WSFBs-derived VCs enhances rice performance under field conditions, including during extreme heat, and highlight their potential as low-cost, waste-derived biostimulants that support sustainable, circular, and climate-resilient rice production.