Effects of spent mushroom substrate cultivation under greenhouse conditions on growth and fruiting of passion fruit

Introduction This study aimed to develop a sustainable spent mushroom substrate (SMS)-based compound substrate for greenhouse soilless cultivation of passion fruit ( Passiflora edulis Sims.), to reduce SMS accumulation pollution and overcome continuous cropping obstacles in traditional soil-based systems. Methods A greenhouse pot experiment was conducted using grafted ‘Qinmi No. 9’ passion fruit seedlings. Substrate physicochemical properties, plant growth, fruit yield and quality parameters were assessed, and rhizosphere bacterial and fungal communities of eight representative treatments were profiled by 16S rRNA and ITS amplicon sequencing. Results Among all formulations, T4 ( Pleurotus geesteranus SMS: seaweed mud: river sand = 3:1:1) exhibited the best overall performance. Compared with the control, T4 significantly increased single fruit weight by 8.2%, fruit number per plant by 24.8% (from 35.0 to 43.6 fruits/plant) and yield per plant by 43.1% (from 2.69 to 3.85 kg/plant). Fruit quality was also improved, with soluble solids, soluble sugar and vitamin C contents increased by 11.3%, 27.6% and 30.6%, respectively, while acidity was reduced by 19.3%. Mechanistically, T4 maintained more stable substrate bulk density, pH and electrical conductivity throughout the cultivation period, and significantly enriched beneficial rhizosphere genera ( Niastella and Rhodanobacter ) that were positively correlated with plant growth and fruit quality traits. In contrast, pure SMS formulations (T5, T15) exhibited higher nutrient levels but poorer plant performance, likely due to excessive electrical conductivity and unstable substrate structure. Conclusions The T4 formulation is an optimal sustainable substrate for passion fruit, balancing physical structure, salt buffering and microbial benefits, offering a practical solution for SMS reuse.