It is interesting to observe the correlation between the
phenolic content and antioxidant activity between the plant
extracts, since phenolic compounds contribute directly to
antioxidant activity (Duh, 1999). In this study, there was
a distinct correlation between studied parameters (total
phenolic content, total flavonoid content and antiradical
activity) in selected Thai indigenous plant parts. The antiradical
activity (1/EC50), as a function of total phenolic
content and total flavonoid content is shown in Figs. 4(a)
and (b), respectively, and correlation coefficients (r) of
those plots, calculated from linear regression analysis, were
about 0.8 for all ethanolic extracts of plant parts. However,
the literature includes studies reporting a weak correlation
between antioxidant activity and total phenolics (Ka¨hkonen,
Hopia, & Heinonen, 2001; Velioglu et al., 1998).
When the relationship between total phenolic content and
total flavonoid content of all ethanolic extracts was plotted
as shown in Fig. 4(c), the correlation coefficient (r) between
these two parameters was higher than 0.9 indicating that
there is a significant positive relationship between the total
phenolic and flavonoid contents of all plant extracts
selected in this study.