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Long-term and short-term effects of phytochrome on the levels ("tissue contents") of NAD, NADH 2  , NADP and NADPH 2  were measured in the cotyledons of the mustard (Sinapis alba L.) seedling. It was found that long-term far-red light (which is considered to operate exclusively via active phytochrome, P fr ) strongly increases the levels of NADP and NADPH 2  , whereas this light treatment suppresses the levels of NAD and NADH 2  below the levels present in the cotyledons of the dark grown seedling. The high levels of NADP and NADPH 2  as well as the low levels of NADP 2  (and probably also NAD) are actively being maintaned by the far-red light. In experiments with light pulses it was found that a red light pulse causes a rapid but transient rise in the level of NADPH 2  under all circumstances (dark-grown seedlings and seedlings pretreated with long-term far-red lig h t). The operational criteria for the involvement of phytochrome (P fr ) are fulfilled. In the case of NADP a significant rise of the level following a red light pulse could only be observed if the seedlings were pretreated with long-term far-red light. Dark-grown seedlings do not significantly respond. In the case of NAD and NADP 2  no significant changes could be induced by light pulses either in dark-grown seedlings or in seedlings pretreated with long-term far-red light. It is concluded that NADPH 2  does neither originate from NADP 2  nor from NADP. It is further concluded that it is unlikely that nicotinamide nucleotides are links in any causal chain originating from P fr  and leading to phenomena of photomorphogenesis. We favour the con­cept that the phytochrome-mediated changes caused by light pulses occur in the plastids.

Control by Phytochrome of the Level of Nicotinamide Nucleotides in the Cotyledons of the Mustard Seedling