Melatonin Enhances Drought Tolerance and Recovery Capability in Two Contrasting Fenugreek (Trigonella foenum-graecum L.) Landraces Through Improved Photosynthetic Apparatus Protection and Carboxylation Efficiency

Drought stress is a major challenge for agriculture and horticulture, limiting crop growth and productivity. Melatonin, a naturally occurring hormone in plants, has been shown to have potential as a stress reliever and growth regulator. Nonetheless, limited research has perused the effect of melatonin on different morpho-physiological and biochemical parameters during prolonged drought stress and subsequent recovery in contrasting landraces. The study aimed to determine whether the exogenous application of melatonin to two contrasting landraces of fenugreek (Trigonella foenum-graecum L.) can improve drought adaptations through drought stress tolerance and/or drought recovery capability and to find the primary parameters associated with this likely improvement in superior treatments by examining photosynthetic pigment contents, chlorophyll fluorescence, gas exchange parameters, and plant growth and water relation indices. Our study revealed that the plants present different responses according to the intensity of drought stress (moderate and severe) and the time elapsed since re-watering (initial and full-recovery). Severe drought stress caused more damage to the drought-sensitive landrace (Shushtar), but at the same time, melatonin treatment was most effective in improving the performance of stressed plants in this landrace. Our findings further demonstrated that the melatonin function is landrace-specific. The drought tolerance response in sensitive landraces was associated with an increase in the chlorophyll content, improved chlorophyll fluorescence and carboxylation efficiency, and cellular membrane protection. This is the first study to report the advantage of melatonin in drought recovery capability, which was attained in tolerant landraces (Varamin) by a decrease in light reactions of photosynthesis and an improvement in dark reactions of photosynthesis and mitochondrial respiration.