Physiological responses and nutrition in passion fruit seedlings biostimulated with humic substances

The production of high-quality passion fruit ( Passiflora edulis ) seedlings is essential for the establishment of commercial orchards, yet excessive mineral fertilization can compromise plant development. This study evaluated physiological, nutritional, and growth responses of yellow passion fruit seedlings biostimulated with different sources of humic substances (HS) under mineral fertilization regimes. A completely randomized 4 × 2 factorial design was used, with four HS sources (no HS, leonardite, forest growing medium HS, and organic cultivation growing medium HS) combined with two mineral fertilization levels (with and without), totaling eight seedlings with eight replications. Mineral fertilization markedly increased fresh aerial phytomass (+ 186%), shoot dry mass (+ 181.66%), and total chlorophyll content (+ 30.66%). HS application enhanced growth only in fertilized seedlings, with leonardite and forest growing medium HS increasing fresh aerial phytomass by 57.70 and 36.50%, respectively. Leonardite HS promoted root fresh phytomass (+ 78%), while organic cultivation growing medium HS maximized the Dickson quality index (+ 121.71%). In fertilized seedlings, forest growing medium HS significantly increased net photosynthetic rate (11.22 µmol CO 2 m −2 s −1 ), water use efficiency (+ 117.09%), intrinsic water use efficiency (+ 180.71%), and instantaneous carboxylation efficiency (+ 149.34%). HS from organic cultivation growing medium also improved these efficiencies. Foliar N and K contents were adequate in fertilized seedlings, but P remained below critical levels. These findings confirm the synergistic effect of rational mineral fertilization and HS application, particularly from tropical growing medium sources, to enhance seedling vigor and physiological performance. Future research should investigate to elucidate long-term effects across the full crop cycle and post-harvest fruit quality.