Control by phytochrome of the level of nicotinamide nucleotides in the cotyledons of the mustard seedling.

Long-term and short-term effects of phytochrome on the levels (“tissue contents”) of NAD, N A D H ,, NADP and NADPH, 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, Pfr) strongly increases the levels of NADP and NADPH2 , whereas this light treatment suppresses the levels of NAD and NADH2 below the levels present in the cotyledons of the dark grown seedling. The high levels of NADP and NADPH2 as well as the low levels of NADP2 (and probably also NAD) are actively being maintaned by the far-red light. In experi­ ments with light pulses it was found that a red light pulse causes a rapid but transient rise in the level of NADPH2 under all circumstances (dark-grown seedlings and seedlings pretreated with long-term far-red light). The operational criteria for the involvement of phytochrome (Pfr) are ful­ filled. 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 NADP2 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 NADPH2 does neither originate from NADP, nor from NADP. It is further concluded that it is unlikely that nicotinamide nucleotides are links in any causal chain originating from Pfr and leading to phenomena of photomorphogenesis. We favour the con­ cept that the phytochrome-mediated changes caused by light pulses occur in the plastids.