Effect of Biostimulants on Plant Growth and Leaf Functional Compounds of Passiflora Plants

The influence of biostimulant treatments on Passiflora species was assessed by observing changes in the growth and leaf functional compounds of plants exposed to humic acids (HAs) and seaweed extracts (SEs) to determine their optimal application doses. Four different concentrations of HA irrigation (0, 144, 192, and 285 mg·L −1 per pot assigned as HI-1, 2, 3, and 4, respectively), HA spraying (0, 96, 128, and 192 mg·L −1 as HS-1, 2, 3, and 4, respectively), SE irrigation (SI-1∼4 at 0, 1, 1.33, and 2 g·L −1 per pot, respectively), and SE spraying (SS-1∼4 at 0, 0.5, 0.67, and 1 g·L −1 , respectively) treatments were applied to ‘Tainung No. 1’ and Passiflora suberosa cultivars every week to study their responses in plant growth traits and metabolites. Results show that leaf dry weight (DW) and root DW of ‘Tainung No. 1’ plants under SS-3 and HI-4 treatments, respectively, were significantly increased compared with controls. Both HI-3 and HS-3 treatments remarkably increased vine length, fresh weight (FW), and DW of leaves and shoots in P. suberosa plants compared with controls. Furthermore, ‘Tainung No. 1’ and P. suberosa plants respectively subjected to HI-2 and SS-4 treatments contributed to developing biostimulant applications for obtaining higher total phenol and flavonoid content per plant. Both Passiflora species are recommended for treatment with SS-4 and SE to maximize orientin and isovitexin content, and for all their beneficial applications and sustainable use in agriculture.