The term walnut covers about 20 species of trees (Juglans spp.) belonging to the Juglandaceae family. In the world, Iran is one of the biggest producers of Persian walnuts. In the north of Iran, there are remnants of wild populations of Persian walnuts in the Hyrcanian Forest. Mass production of plant materials and fast breeding can be possible today through tissue culture techniques. A bottleneck in this field is to produce several biologically active metabolites, such as phenolic compounds, as a limiting factor in walnut tissue culture. In walnut tissue cultivation, explant browning reduces growth at a lower rate and causes death. Most research on walnut tissue culture and the production of phenolic compounds include medium culture, plant growth regulators, and explant sources. In walnut explant, tissue browning can be reduced by methods such as pretreatment with ascorbic acid or citric acid, augmentation of ascorbic acid, melatonin, or citric acid, and prevention of phenolic compound oxidation by adding adsorbents such as activated charcoal or PVP to nutrient media, which connect to phenolic compounds and make them less toxic. Understanding the biochemical pathways of plant phenolic compounds and the key enzymes involved in these pathways can develop effective strategies to counteract their negative effects. After extensive research on the tissue culture of walnuts several decades ago, the production of this plant under in vitro culture was patented, and the growers of this plant complained about the production of phenolic substances in the culture environment and the loss of plants. These issues can be addressed, at least partly, if we analyze most references and compare the results. The present review article deals with the approaches used in in vitro walnut cultivation, the mechanism of action of phenolic compounds, and strategies effective in reducing tissue browning.
