Agronomic biofortification of water spinach ( Ipomoea aquatica Forssk.) with zinc through silicon root treatment and foliar zinc application

Zinc (Zn) deficiency is an emerging global health issue. This study aimed at enhancing Zn accumulation in the leaf tissues of water spinach through foliar application of Zn, supplied either as zinc sulfate (ZnSO4) or zinc oxide nanoparticles (Zn-NPs), in combination with root application of silicon (Si) in the form of monosilicic acid at the germination stage. Two-weeks seedlings with the first pair of true leaves were treated at the roots with 0 and 0.025% (w/v) monosilicic acid, and then water spinach plants (4 wk old) were sprayed with either distilled water (control) or Zn solutions. Plant morphological, physiological, and biochemical traits were evaluated. Application of 0.5% Zn-NPs resulted in a 296% increase in Zn concentration and a 165% increase in iron (Fe) concentration in leaf tissues. In contrast, Si application did not enhance Si content, indicating limited uptake or translocation. The integrated application of 0.025% Si and 0.5% Zn-NPs improved net photosynthetic rate (Pn), while the combination of 0.025% Si and 0.5% ZnSO4 maximized γ-tocopherol, α-tocopherol, and total tocopherol levels. Moreover, the Si + ZnSO4 combination resulted in the highest shoot fresh weight, shoot dry weight, and shoot dry matter. Overall, Zn-NPs significantly increased Zn biofortification in leaves, whereas the Si and ZnSO4 combination enhanced biomass production and total tocopherol content. These findings suggest that the co-application of Si and Zn is a novel strategy for improving biomass and biofortification of water spinach.