Silicon Sources Differentially Affect Physiological Responses, Nutrient Uptake, and Phenolic Compounds in Sour Passion Fruit

This study evaluated the effects of two silicon sources (silicic acid and Agrisil) and increasing Si concentrations on physiological responses, total polyphenol content, photochemical performance, nutrient uptake, and phenolic metabolism in sour passion fruit (Passiflora edulis Sims) grown under soilless culture conditions. The experiment was conducted in a greenhouse using increasing concentrations of Si applied through the nutrient solution. Gas exchange parameters, chlorophyll index (SPAD), chlorophyll fluorescence variables, leaf temperature, and the contents of Si, nitrogen, and total polyphenols in leaves and roots were evaluated. Moderate Si concentrations enhanced stomatal conductance and transpiration, improving intrinsic water use efficiency, and maintaining higher chlorophyll levels and photochemical performance. In contrast, higher Si concentrations increased Si deposition in leaf tissues, reduced stomatal regulation and transpiration, and increased leaf temperature. These changes were associated with reductions in chlorophyll index and photochemical performance index (PI), as well as increased F0/Fm. Net CO2 assimilation remained relatively stable. Silicon uptake differed between sources, with silicic acid showing faster absorption and Agrisil a more gradual release. Silicon fertilization also increased nitrogen uptake and stimulated the accumulation of phenolic compounds in roots. Overall, moderate silicon supplies enhanced physiological stability, whereas excessive accumulation imposed photochemical constraints.