Enhancing Drought Tolerance in Marigold (Tagetes erecta L.) with Harpin Treatment

Abstract

Marigolds are plants of high aesthetic value that can grow in various climatic conditions. However, increasing drought conditions threaten their use in sustainable landscape designs. Developing strategies to reduce drought stress is essential. Elicitors like harpin proteins have been used to boost plant stress tolerance, yet studies on seasonal flowers such as marigold remain limited. This study investigated the effects of foliar-applied harpin solutions (0.5 ppm, 2.0 ppm, and 10.0 ppm) on morphological, biochemical, and physiological traits of Tagetes erecta L. F1 'Bali Orange' under drought. After three harpin treatments, seedlings were subjected to either 100% or 40% field capacity (FC). The experiment concluded when more than 50% of untreated plants under 40% FC showed visible drought stress. Parameters analyzed included plant height, leaf number, leaf area, flower diameter, chlorophyll content, relative leaf water content (LRWC), proline content, malondialdehyde (MDA) content, and membrane injury index. Marigolds exposed to limited irrigation exhibited significant inhibition of plant growth parameters compared to plants grown under well-irrigated conditions. However, harpin-treated seedlings under drought showed improvements in all traits compared to untreated plants, with statistically significant differences (p≤0.05) depending on the harpin concentration. Harpin alleviated stress by reducing membrane damage and sustaining LRWC and chlorophyll, thereby positively impacting morphology. In particular, the 0.5 ppm harpin treatment resulted in higher plant height (30.33 cm), leaf area (395.00 mm²), and leaf number (83.00 leaves plant⁻¹) compared with the untreated drought-stressed plants. Many traits in harpin-treated plants under drought resembled those in well-irrigated plants. Considering all parameters, the 0.5 ppm harpin concentration proved to be the most effective treatment. These results suggest that applying elicitors like harpin before planting in landscapes can improve stress tolerance and prolong aesthetic function. Further studies on different species are advised to optimize concentration and application timing.