Flavonoid Compounds and Photosynthesis in Passiflora Plant Leaves under Varying Light Intensities

Functional constituents in the leaves of Passiflora plants contain antidepressant and antianxiety effects which are beneficial to human health and fitness. The objective of this study was to investigate leaf growth, physiological parameters, and secondary metabolite contents of Tainung No. 1 variety (P. edulis × P. edulis f. flavicarpa.) and P. suberosa in response to three light intensity conditions, including 100% light intensity (LI-100), 50% light intensity (LI-50), and 15% light intensity (LI-15) for 2 months. The leaf number, length, width, area, dry weight (DW), minimal fluorescence (Fo), maximal fluorescence (Fm), maximum photochemical efficiency of photosystem II, and soil-plant analysis development (SPAD) values of all tested plants increased with a decreasing light intensity, except for the leaf number and DW of P. suberosa plants. Low values of the net photosynthetic rate, transpiration rate, and stomatal conductance of Tainung No. 1 leaves in the LI-15 treatment showed the acclimation capacity of these plants. These observations together with high values of leaf growth traits of Fo, Fm, SPAD, and the intercellular-to-atmospheric CO₂ concentration ratio indicate their physiological plasticity, which is of fundamental importance when cultivating plants in environments with different light availabilities. Wide variations occurred in total phenol (TP), total flavonoid (TF), orientin (OR), and isovitexin (IV) contents of the two Passiflora varieties, and P. suberosa contained higher TP and TF contents than did Tainung No. 1 in each light treatment but IV content of P. suberosa was lower than that of Tainung No. 1 in the LI-15 treatment. Moreover, increases in TF, OR, and IV contents of Tainung No. 1 and P. suberosa were clear in the LI-50 and LI-100 treatments, respectively, compared to LI-15 treatment. Leaf growth, physiological parameters, and secondary metabolite accumulations in Passiflora species can be optimized for commercial production via lighting control technologies, and this approach may also be applicable to leafy vegetables to produce a stable industrial supply of high leaf yields and metabolite contents.