Effects of three allelopathic phenolics on chlorophyll accumulation of rice (Oryza sativa) seedlings: II. Stimulation of consumption-orientation

The effects of three allelopathic phenolics, o-hydroxyphenyl acetic, ferulic and p-coumaric acids, on the chlorophyllase activity of rice leaf (Oryza sativa cv. TN67) were investigated. Ten-day-old green seedlings of rice were cultured in greenhouse for 16 d in Kimura's culture solution, which was changed every 4 days, with or without 50, 100 or 200 ppm of the phenolic compounds. Just before changing the culture solution, leaves were harvested to determine their chlorophyll (Chl) and chlorophyllide (Chlide) contents, and their chlorophyllase a and b activities. While the Chl and Chlide contents decreased and increased, respectively, causing the molar ratio of Chlide/Chl to increase, as the phenolic concentrations increased; the chlorophyllase a and b activities drastically increased. This suggests that the consumption-orientation of Chl was significantly stimulated by the exogenously applied phenolics. The order of inhibition of growth of the rice seedlings is: ferulic acid>p-coumaric acid>o-hydroxyphenylacetic acid. The order of inhibition effect on Chl accumulation is: p-coumaric acid>o-hydroxyphenylacetic>ferulic acid. The order of stimulation effect on chlorophyllase a activity is: o-hydroxyphenylacetic acid>ferulic acid>p-coumaric acid. The order of promotion effect on chlorophyllase b is: ferulic acid>o-hydroxyphenylacetic acid>p-coumaric acid. The different responses of chlorophyllase a and b activities to the same concentrations of allelochemical phenolics suggest that they may be two different enzymes. It is apparent that the three phenolics may enhance the activities of enzymes, such as chlorophyllase and Mg-dechelatase, responsible for the Chl degradative pathway. A combination of the present and the preceding data strongly suggest that the three allelopathic phenolics may comprehensively affect the biosynthetic and degradative pathways of Chl.