Walnut trees have been shown to be very sensitive to abiotic stresses, especially to soil hypoxia consequence of soil flooding or waterlogging. <...> The aim of study was to search for correlation between polyphenols and tolerance to flooding stress. <...> Two-three years old walnut trees, Juglans regia, J. nigra and interspecific hybrids, grown in pots were subjected to soil flooding in summer at full leaves expansion. <...> In high sensitive J. regia trees, when flooded, the net CO 2 assimilation rapidly decreased and 3-4 days of flooding were enough to block the recovery when soils was drained. <...> In walnut hybrid ( J. nigra x J. regia ) and less sensitive J. regia genotypes, a longer resistance was observed; after 13 days of treatment leaves culd still be green and trees can recovered till to the normal photosynthesis when drained. <...> J. nigra when flooded reduced net photosynthetic rate but maintained leaves without damages and with the capacity to recover CO 2 assimilation. <...> The data showed resistance of J. nigra and hybrid trees and of some J. regia genotypes to the stress. <...> The HPLC analyses of polyphenols showed a modification of the patterns during the stress. <...> The plants less tolerant to hypoxia have higher content of polyphenols distributed in a lot of compounds. <...> Trees more tolerant, J. nigra, showed a very simple HPLC pattern. <...> All samples contained juglone, more in less tolerant genotypes. <...> Hydroxy juglone glucoside was detectable in all genotypes, but only in low quantity, it increased in trees with high resistance to hypoxia and decreased in J. regia. <...> Less flooded tolerant J. regia genotypes have higher polyphenols and juglone content. <...> The metabolism of hydroxyl juglone glucoside could be involved in a mechanism of juglone detoxification during the hypoxia to give stress tolerance.! <...>