Anthocyanin Expression and Phenyalanine Ammonia-lyase Activity in Sorghum Bicolor as Influenced by Temperature and Plant Age

Tanabe, Michael J.
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The effects of temperature and plant age on anthocyanin production and Phenylalanine ammonia-lyase (PAL) activity in etiolated Sorghum bicolor seedlings were investigated in this study. The etiolated seedlings were exposed to various temperature combinations and sucrose in the presence and absence of light. After these treatments, they were analyzed for anthocyanin content or PAL activity. Anthocyanin formation was not affected by temperature or sucrose in the absence of light. This indicated that light was required for the activation and/or synthesis of anthocyanin synthesizing enzymes. This was not the case for PAL which was stimulated in darkness when sucrose was added to the medium. It was speculated that the exogenous source of carbohydrates served as a substitute stimulus for light. Anthocyanin formation was stimulated by temperature in the presence of and following exposure to light. Limited quantities of anthocyanin were produced at 30C 4 hours after seedling exposure to light. The effect was of a much greater magnitude 24 hours after exposure to light. The additional time period required for maximal anthocyanin production was attributed to the presence of a lag phase. It was further assumed that temperature affected the production and/or activity of anthocyanin synthesizing enzymes during this lag phase; a higher temperature being more effective than a lower temperature. Endogenous carbohydrate supply was also considered to be important. Anthocyanin production was high only when the seedlings had been exposed to low (IOC) temperatures prior to light exposure. The low temperature was thought to decrease respiration rates of the seedlings and therefore make more substrates (e.g. endogenous carbohydrates) available for anthocyanin formation during and following exposure to light. PAL activity was greatly stimulated by a temperature of 30C 4 hours after the initiation of the light treatment but its activity decreased markedly 24 hours following exposure to light. It was assumed that light and high temperatures functioned as stimuli for the synthesis and/or activity of a PAL deactivating enzyme system. The effectiveness of this system to deactivate PAL was thought to be greatly increased during a 24 hour incubation period after exposure to light. This was the case only when the temperature was high (30C), therefore it was assumed that the activity and/or synthesis of the PAL deactivating enzyme system was hampered by low (IOC) temperatures. Sucrose had a greater stimulatory effect on PAL activity at the lower temperatures. This might have been a result of the absence or low activity of a PAL deactivating enzyme system at low temperatures and/or the greater availability of endogenous substrates due to lower respiration rates. As plant age increased, anthocyanin production and PAL activity decreased. It was postulated that this was a result of lower endogenous substrate levels due to longer respiration times. The older plants also may have produced greater quantities of anthocyanin and PAL inhibitors. Maximal PAL activity was found to precede maximal anthocyanin production by a few hours. The trend was similar for minimal PAL activity and anthocyanin production. A lag phase was speculated as being responsible for this relationship. If this assumption is correct, our studies indicate that PAL activity might have some regulatory control over anthocyanin production.
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