Abstract The escalating global population necessitates the continuous enhancement of agricultural productivity. However, environmental stressors such as drought pose significant challenges to achieving this goal. Research exploring the application of light-emitting diodes (LEDs) across diverse plant species and stress situations holds considerable potential for optimizing this technology in the context of modern agriculture. This study investigated the effects of varying LED light spectra on the yield, morphological, physiological, and biochemical characteristics of hyssop (Hyssopus officinalis L.) under drought stress conditions. This research aimed to assess the impact of LED light spectra on the morphological, yield, physiological, and biochemical characteristics of hyssop under drought stress conditions. A combine analysis in completely randomized design with three replicates was implemented in a controlled pot culture environment. Five light treatments were employed: 100% red, 100% blue, 75% red-25% blue, and 100% white LED light, alongside natural greenhouse light as a control. Two moisture levels were established: full irrigation and 70% irrigation to simulate drought stress. This design allowed for the eva‎luation of the differential effects of light spectra and water availability on various aspects of hyssop plant performance. Drought stress generally resulted in a decline in various growth parameters, including canopy size, plant height, internode length, leaf width, and fresh and dry weight. Additionally, stress indicators such as ion leakage and physiological and biochemical parameters like chlorophyll a, carotenoid content, greenness index, chlorophyll fluorescence, total proline, phenol and flavonoid content, antioxidant activity, and essential oil percentage all increased. Interestingly, the red-blue LED treatment, which exhibited the most significant influence on antioxidant activity, also displayed the highest levels of these specific compounds (phenols and flavonoids). Hyssop exhibited a pronounced response to the red-blue LED combination. Photosynthetic factors, growth parameters, and yield characteristics all displayed the highest values under this light treatment. Furthermore, the highest essential oil percentage and specific components were observed in the red-blue light combination. Generally, despite the adverse effects of drought stress on growth and yield in natural greenhouse light, the red-blue then red and blue LED treatments promoted these parameters. While some growth and physio-biochemical parameters exhibited marginal improvements under red or blue light alone, a significant majority demonstrated superior trends and statistically significant enhancements when exposed to a combination of these wavelengths. This suggests a synergistic effect of the red and blue light combination, which potentially improved plant resilience to moisture stress. Consequently, the combined red a d blue light treatment may enhance yield and essential oil production under water-limited conditions.

مهدي رحيم ملك , مريم حقيقي

محمدرضا سبزعليان دستجردي

علي نيك بخت , هاجر شكرچي زاده