Arginine decarboxylase (ADC), one of the key enzymes of polyamine metabolism in plants, was investigated inChenopodium rubrumL. seedlings under constant and alternating temperature and lighting conditions. With seedlings grown at constant temperature, ADC activity of the whole seedling increased rapidly between the second and the third day after sowing. This effect was always higher under continuous light than in continuous darkness. From the third to the seventh day after sowing, there was a marked decrease in ADC activity of the whole seedling almost down to the level of the second day. Under“normal”lighting and temperature conditions (32.5°C/10°C, light/dark) there was a marked increase in ADC activity when plants were transferred to 10°C and a rapid decrease when they were transferred to 32.5°C. The same time course was observed when an“inverse”light-temperature program (32.5°C/10°C; dark/light) was applied. This means that the time course of ADC activity in the seedlings is slightly light-dependent, but strongly temperature-dependent. The data are discussed with respect to the chronopathological effects of the“inverse”light-te
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