[Protein and RNA contents of the hypocotyl during steady state growth lengthening in the dark and under the influence of phytochrome (seedlings of sinapis alba L.)].

Inhibition of hypocotyl lengthening by phytochrome can be regarded as a prototype of a "negative" photoresponse. The hypothesis has been advanced (SCHOPFER, 1967) that negative photoresponses are the consequence of a differential gene repression which is exerted by P730, the active phytochrome. This hypothesis is mainly based on experiments with specific inhibitors of RNA- and protein synthesis. -The present paper is part of an experimental program which has been designed to check this hypothesis.-Continuous irradiation with standard far-red has been used to establish a virtually stationary concentration of P730 over the whole period of experimentation (36-60 hours after sowing). To correlate more strictly the growth response of the hypocotyl with "molecular" changes in this organ the axis system without cotyledons has been used (Fig. 1). Even under these conditions the growth rate of the hypocotyl is nearly constant in light (continuous far-red) and dark during the whole period of experimentation (36-60 hours after sowing) (Fig. 2, 3). It is known from earlier experiments that cell division in the hypocotyl are very rare during this period and that there is virtually no increase in the DNA contents of the organ during the period of our experimentation (WEIDNER, 1967). Obviously the number of cells per hypocotyl is virtually constant between 36 and 60 hours after sowing. Organ (i.e. hypocotyl) lengthening is nearly exclusively due to cellular lengthening.-If we follow the protein contents of the hypocotyl we find (Fig. 4) that the total protein of the organ decreases steadily in spite of the fact that the organ grows at a constant rate. There is no significant difference in protein contents between dark-grown and far-red grown systems although the growth rates differ by a factor of 4 (Fig. 2, 3).-The situation is some-what different with respect to total RNA (Fig. 5). The RNA contents eventually decrease in far-red as well as in dark-grown systems but the decrease is significantly faster in the far-red treated systems than in the dark controls.-It is concluded that only a very small part of the total RNA and total protein of a cell can be related to the control of cellular growth. Changes in bulk RNA and bulk protein obviously do not necessarily reflect changes in the growth rate or growth capacity of an organ or a cell.