Zinc oxide nanoparticles (ZnO NPs) have garnered increasing attention in agriculture due to their potential to enhance plant growth and nutrient use. This research investigates the concentration-dependent effects of ZnO NPs on young crabapple (Malus robusta) plants, addressing gaps in understanding how different concentrations influence plant development. A hydroponic experiment was conducted, applying foliar treatments of 200 mg L⁻¹ ZnSO₄ (S200) and 200, 500, and 1000 mg L⁻¹ ZnO NPs (N200, N500, N100). The control group (CK) was treated with deionized water (dH₂O). Growth parameters, antioxidant enzyme activity, and nutrient contents were measured to evaluate the impact of ZnO NPs on plant development and nutrient uptake. The results showed that N200 enhanced growth, increasing plant height by 22.64%, total dry weight by 49.36%, and root length by 116.07%. In contrast, N500 and N1000 induced oxidative stress, elevating H₂O₂ and MDA by 32.02~54.43% and inhibiting growth. N200 also improved nutrient uptake, increasing K, Ca, Fe, and Zn uptake fluxes by 84.92%, 112.12%, 185.15%, and 149.92%, respectively, whereas N1000 suppressed overall nutrient uptake but increased root Ca accumulation by 64.59%. These findings suggest that ZnO NPs can enhance plant growth and nutrient utilization at low concentrations, with potential implications for agricultural practices involving nanoparticle (NP)-based fertilizers.

Banana plants require substantial nutrients, and their growth is significantly hindered by nutrient deficiency. This study investigated the influences of Piriformospora indica colonization on the growth of clean river sand-cultivated banana plants under varying phosphorus (P) and potassium (K) levels. Banana plants without (CK) and with P. indica colonization (PI) were watered using Hoagland solutions with four gradients of P or K (100%, 50%, 25%, and 0%). Results showed that P. indica colonization promoted the growth of banana plants under different concentrations of P and K treatments. Further analysis revealed that the pseudo-stem P and K contents were very significantly (p < 0.01) positively and positively correlated with biomass-related parameters (aboveground part fresh weight, root fresh weight, and total plant biomass), respectively. Root starch, sucrose, proline (PRO), and anthocyanins contents, as well as leaf malondialdehyde (MDA) and PRO contents, were positively correlated with most growth-related parameters. However, root and leaf flavonoid contents, total antioxidant capacity (T-AOC), and leaf anthocyanin content showed negative correlations with growth-related parameters. Moreover, a very significant negative correlation (p < 0.01) was identified between root T-AOC and root starch content. Additionally, P. indica altered the P and K reduction-caused starch content change patterns in both leaves and roots. Our study demonstrated that P. indica colonization promoted the growth of banana under different concentrations of P and K treatments by mediating the accumulation of carbohydrates, secondary metabolites, osmoprotectants, and so on.