Control of Phenylalanine Ammonia-lyase and Ascorbate Oxidase in the Mustard Seedling by Light and Hoagland’s Nutrient Solution

Hoagland’s nutrient solution (HS) * was used as a tool to investigate whether or not the phyto­ chrome-mediated appearance of phenylalanine ammonia-lyase (PAL) * and ascorbate oxidase (AO) * in the mustard seedling cotyledons is simply a modulation of the appearance of these enzymes in the dark (basal level). HS was applied to the mustard (Sinapis alba L.) seedling at sowing instead of distilled water. It was observed that HS causes a stimulation of enzyme dis­ appearance (interpreted as degradation) in light and dark. This effect becomes obvious at ap­ proximately 48 h after sowing. On the other hand, however, HS exerts specific effects on the ap­ pearance of the enzymes in continuous far-red light (which is considered to operate exclusively via phytochrome). In the case of PAL there is a strong stimulation of the rate of enzyme appearance; however, the onset of activity increase remains precisely at 29 h after sowing irrespective of treat­ ment. In the case of AO the onset of activity increase is shifted by approximately 6 h ; however, the rate of enzyme appearance remains the same (up to 54 h after sowing). The data suggest that HS specifically affects phytochrome-mediated enzyme syntheses whereas syntheses of the same enzymes in the dark are not affected. It is concluded that the appearance of PAL and AO in the dark and phytochrome-mediated appearance of these enzymes are independent phenomena. Some consequences of these conclusions for the interpretation of density labelling data obtained with “inducible” enzymes are discussed.