Micronutrient deficiencies and nickel toxicity differentially alter growth, physiology, and leaf ultrastructure in Passiflora edulis seedlings

Micronutrient homeostasis is essential for maintaining plant metabolism and cellular integrity, whereas micronutrient deficiency and excess nickel (Ni) disrupt fundamental physiological processes. However, the cellular responses associated with individual micronutrient deficiencies and Ni exposure remain poorly understood in passion fruit ( Passiflora edulis Sims). This study characterized the morphophysiological and ultrastructural responses of passion fruit seedlings subjected to individual omissions of boron (B), manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), and molybdenum (Mo) or to Ni addition under hydroponic conditions. Seedlings were grown in a complete nutrient solution, Ni-supplemented solution, or solutions lacking individual micronutrients. Visual symptoms, nutrient concentrations, chlorophyll content, gas exchange, root morphology, biomass accumulation, and leaf mesophyll ultrastructure were evaluated. The order of symptom appearance was Fe > Mn > Zn > Cu > Mo > B, indicating a greater sensitivity of seedlings to Fe and Mn deprivation. Each treatment induced a distinct phenotype. Zn and Cu deficiencies caused the greatest reductions in shoot and root biomass, whereas Fe and Mn deficiencies produced the strongest declines in chlorophyll content and photosynthetic performance. Ni-toxicity induced chlorosis, growth inhibition, and pronounced alterations in chloroplast organization. Transmission electron microscopy revealed nutrient-specific ultrastructural changes, including modifications in chloroplast morphology, thylakoid organization, starch granules, plastoglobules, and middle lamella integrity. Overall, micronutrient imbalance and Ni addition differentially affected plant growth, physiology, and mesophyll ultrastructure, providing new insights into the cellular consequences of disrupted micronutrient homeostasis in P. edulis and contributing to the diagnosis of nutritional disorders associated with micronutrient deficiency and Ni toxicity.