Synergistic Effects of Titanium Dioxide Nanoparticles and Ascorbic Acid on Drought Tolerance in Coffee Plants through Osmoprotection and Antioxidant Defense

Titanium dioxide nanoparticles (TiO₂NPs) and ascorbic acid (AsA) have been shown to enhance plant productivity. However, the effects of applying a combined dose of these substances to plants under drought stress are not well understood. Therefore, this study aimed to assess the effectiveness of applying TiO₂NPs (10 ppm) and AsA (50 ppm) to the leaves of coffee plants (Coffea arabica cv. Khawlani) subjected to drought conditions at 50% and 20% of their field capacity (FC). The study’s findings demonstrated that coffee plants subjected to drought stress exhibited reductions in root and shoot length, as well as decreases in fresh and dry weights, relative water content (RWC), while also showing increases in proline levels, total soluble carbohydrates (TSC), malonaldehyde (MDA) content, and antioxidant activities (superoxide dismutase SOD and catalase CAT activity). However, in drought stress conditions at 50% and 20% FC, TiO₂NPs enhanced shoot length by 23%, 12%, and 33%, while root length improved by 26%, 12%, and 38%. AsA treatments further increased shoot length by 44%, 39%, and 51%, along with root growth of 45%, 16%, and 54%, respectively. Following the application of TiO₂NPs and AsA, these drought-stressed (50% and 20% FC) coffee plants displayed enhanced fresh and dry weights, increased RWC, and higher activities of SOD and CAT, along with decreased levels of MDA, and so on. Taken together, TiO₂NPs and AsA, whether applied individually or in combination, could serve as highly effective components for promoting the development of coffee plants by enhancing their physiological functions.