In Ardabil region potato crop needs several irrigation, however ground water supplies are being exhausted due to reduced precipitation and intensive irrigation. In this research drought tolerance of 10 commercial potato cultivars was studied at three irrigation treatments (100%, 80% and 60% of required irrigation water) in Ardabil in a two years field study, 2013-2014. At harvest, marketable tuber yield, tuber weight per plant, mean tuber weight, tuber number per plant, percent tuber dry matter, plant height and the main stem diameter were measured. Also the stress indices such as water stress susceptibility (SSI), tolerance (TOL), mean productivity (MP), geometric mean productivity (GMP) and stress tolerance (STI) were determined and used to select the tolerant cultivars to water deficit. There were significant differences among irrigation treatments, cultivars and cultivars × irrigation treatments for all studied traits; between years and interaction of years × cultivars for marketable tuber yield, tuber weight per plant, mean tuber weight and tuber numbers per plant. The cultivar Satina produced the highest marketable tuber yield, tuber weight per plant, mean tuber weight and plant height and also had higher MP, GMP and STI indices in all three irrigation treatments, therefore, it can be recommended for cultivation under water deficit condition. Cultivar Caesar showed the lowest SSI and TOL indices under both mild and severe water deficit conditions, therefore it can be used in breeding programs for developing drought–tolerant potato cultivars. Under both mild and severe stress conditions a high correlation was found between marketable tuber yield and plant height, but a negative correlation existed between mean tuber weight and tuber number per plant. Cultivar Savalan showed the highest tuber dry matter content.

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