The production of potato, a major essential crop in Egypt, is declining for biotic and abiotic reasons, even though the area increases each year. Drought stress is primarily brought on by water scarcity and climatic change. Both have negative effects on the yield and quality of potatoes. In the present study, two water regime treatments (irrigation every 10 days and irrigation every 20 days) were done after complete germination (30 days from planting).
Three melatonin foliar treatments (Mel=0.0, Mel=75 ppm,Mel=150ppm( were used two times after 45 and 60 days from planting (stage of potato tuberization) on plant cv. Lady Balfour .The experiment was conducted at the experimental farm of the Horticulture Department at Minia University's Faculty of Agriculture over two consecutive fall seasons (2018/2019 and 2019/2020). the experiment was set up in split plots design using a complete randomized block with three replications.
The obtained results showed that: -
 Irrigation regime had a significant effect on some growth characters such as plant height and plant fresh weight.
The first irrigation regime was the better in comparison to the second one.
 With the first regime (irrigation every 10 days) showing the highest values of plant growth parameters compared to the second regime.
Melatonin treatments also had a significant effect on plant growth parameters. The highest concentration showing the best effect in improving plant growth parameters.
Also, yield and its component addition to tuber quality were affected significantly with irrigation regime and the first one (irrigation every 10 days) was the better for obtained the highest values of these characters. Spraying melatonin particularly with highest concentration had significantly increased total yield and decreased adverse effect of water deficiency. After spraying potato plant with melatonin, some of the water use efficiency i.e., water applied, water saving, and water use coefficient were improved.
In conclusion, the study found that irrigation regime and melatonin treatment had significant effects on various growth parameters of potato plants. The first irrigation regime (irrigation every 10 days) and melatonin treatment at its highest concentration showed the best results in improving plant growth, water use efficiency, productivity, and tuber quality.