The key gateway gene of the phenylpropanoid pathway, PAL, which is involved in the biosynthesis of a vast number of phytochemicals and antiox- idants, has a very low expression level in tender plants. Similarly, the expression of other antiox- idant genes studied which are known to have a key role in plant stress and adaptation, namely g-TMT and L-GalDH, was also minimal in these plants. These genes were activated in lettuce when they were exposed to normal growing conditions in a growth chamber. These results suggest that activa- tion of these genes occurs in response to benign normal growing conditions and may have a role in plants’ ability to cope with normal growing condi- tions. Found that daptation of in vitro plants results in increasing the activity of enzymatic antioxidants such as catalase and superoxide dismutase. There is over- whelming evidence that the activation of second- ary metabolism, particularly the pathways that lead to accumulation of antioxidants including phenylpropanoids have been associated with plant stresses such as drought, UV, high light and low temperatures Our results suggest that antioxidants play a role in plant function even under normal growing conditions. There are a number of reports indicating that suppressing genes involved in the synthesis of these phytochemicals leads to many dysfunctions in plants and makes them sensitive to stresses Inhibition of PAL is known to affect the accumulation of a number of downstream phenylpropanoid com- pounds including many phenolic compounds and others with antioxidant properties In our study, plant growth as indicated by biomass accumulation was much slower as a result of PAL inhibition. The plants became more sensitive especially to chilling than did the control plants. In response to chilling,