Optimal concentration of melatonin enhances drought stress tolerance in fenugreek

Fenugreek (Trigonella foenum-graecum L.) is a valuable medicinal plant in farm and pharmaceutical industry, and on the other hand, drought stress is one of the major problems of crop production in most countries. To explore the effect of drought stress on fenugreek, we first examined three levels of water deficit (− 3.0, − 5.0, − 7.0 Bar water potentials using polyethylene glycol) and assessed plant responses to polyethylene glycol-induced stress. In the second experiment, the effect of different concentrations of melatonin (50, 100, 300 and 500 µM) on the alleviation of detrimental effects of water stress was investigated in the highest level of drought stress (− 7 Bar PEG). The finding in the first experiment (drought stress applying) demonstrated that water deficit decreased length, fresh and dry weight of shoot and root, chlorophyll content and antioxidant enzyme activities, such as ascorbate peroxidase, catalase, superoxide dismutase and peroxidase, and also led to an increase in proline content. Also, melatonin treatment (especially 100 mM) improved photosynthetic performance by increase in net photosynthesis (PN) and transpiration rate (E), reducing chlorophyll degradation but increasing the antioxidant enzyme activities. These results revealed that fenugreek tolerated water deficit by increasing the endogenous melatonin and trigonelline, as well as the general physiological responses.