Comparative physiological and anatomical responses of Tagetes erecta L. and Zinnia elegans Jacq. seedlings to AgNO3 and ZnSO4 metals and their nanoparticles

Abstract

Heavy metals (HMs) pose a significant ecological risk due to their enduring presence in the ecosystem. They become more prevalent as a result of urbanization and industrialization, disrupting critical functions of plant life such as photosynthesis, seedling growth, and germination. Understanding the impact of HMs and nanoparticle exposure on plants is crucial for developing effective strategies to mitigate their potential hazards. Therefore, an effort was made to elucidate the impact of biosynthesized Ag NPs (nanoparticles) and ZnO NPs versus their precursor metal salts, AgNO3 and ZnSO4, respectively on two Asteraceous plants, Mexican marigold (Tagetes erecta L.) and Zinnia (Zinnia elegans Jacq.) in hydroponic solution by monitoring the changes in photosynthetic pigments and their associated photochemistry of photosystem II along with the anatomical attributes. In the present study, two concentrations (50 μM and 100 μM) for each treatments viz., AgNO3, Ag NPs, ZnSO4 and ZnO NPs were given to the seedlings. Metal salts were found to be more toxic than their nanoparticles, and this was manifested in terms of reduction in photosynthetic pigments, especially chlorophylla content (by 32% in marigold and 27% in Zinnia upon exposure to 100 µM of AgNO3, respectively) in both test seedlings. Reduction in chlorophyll fluorescence parameters was also found under metal as well as nanoparticle toxicity. In contrast, energy flux parameters such as absorption of photon per active reaction centre (ABS/RC), trapped energy flux per active reaction centre (TR0/RC), electron transport flux per active reaction centre (ET0/RC) and energy dissipation flux per active reaction centre (DI0/RC) increased significantly to cope with stress toxicity. Similarly, metals and nanoparticles also distorted the anatomy of the root and leaf, and the effect was more intense under AgNO3 treatment than with ZnSO4 in Zinnia. In conclusion, Zinnia seedlings were more resilient towards all the tested conditions, which might be attributed to their adaptability to various environmental factors, indicating their potential for thriving in diverse climates.