Size dependent efficacy of zinc oxide nanoparticles in zinc biofortification of basmati rice

Zinc (Zn) deficiency poses a significant global health concern, particularly in regions where rice is a staple food. Biofortification, the process of enhancing the nutrient content of crops, offers a sustainable solution. This study investigates the potential of soil applied zinc oxide nanoparticles (ZnO-NPs) of different sizes (30, 40, and 95 nm) as an alternative to enhance Zn uptake, growth, and grain quality in two popular basmati rice cultivars (Pusa Basmati-1121 and Pusa Basmati-1509). Results showed that ZnO-NPs treatments performed significantly but among the various sizes, the 30 nm ZnO-NPs performed the best and increased the photosynthetic rate by 21.5–23.4%, stomatal conductance by 35.7–38.5%, chlorophyll a, b, and total content by 21.7–47%, and carotenoids by 38.2–46.2% compared to the control. Similarly, the performance of ZnO-NPs was significantly higher for metabolites especially protein, proline, and antioxidant enzymatic activities such as superoxide dismutase (SOD), and catalase (CAT), particularly 30 nm ZnO-NPs increased the most by 18.1–36% compared to the control. Soil amendment with ZnO-NPs significantly (p < 0.05) improved root length, surface area, volume, and average root diameter compared to the control. Additionally, ZnO-NP treatment increased tillers per hill (46%), productive tillers per hill (25% ), panicle length (5–20%), grain weight per panicle (33.3–36.3%), and yield per hill (29.2–32.1%) over the control, with the 30 nm ZnO-NPs performing the best among the three sizes. Other sizes of NPs (40 and 95 nm) also showed a significant improvement in crop yield attributes. ZnO-NP soil amendment significantly increased Zn density in roots and grains, with the 30 nm nanoparticles resulting in the highest increase (~ 57%) in grain Zn content in both cultivars. Furthermore, soil amended with ZnO-NPs significantly (p < 0.05) reduced phytic acid content in the grains of both rice cultivars. These findings demonstrate that ZnO-NPs, especially in smaller sizes (30 nm), can serve as an effective nano-biofortification strategy, addressing Zn deficiency in rice . The long-term effects of ZnO-NPs on soil health, microbial balance, and nutrient cycling, as well as assessing bioaccumulation risks in ecosystems, could be examined in future studies.