[The inhibition of longitudinal growth by phytochrome -a process requiring substrate uptake?].

The growth of the mustard seedling hypocotyl is a phytochrome controlled steady state process (36-60 hrs after germination, 25°C). This paper describes experiments indicating that with respect to hypocotyl lengthening, phytochrome and substrates (externally applied ions and sucrose, as well as substrates from the cotyledons which are translocated into the organ hypocotyl) act as independent factors in a multiplicative system. This is true for the intact seedling as well as for the isolated hypocotyl. This means: phytochrome depresses the growth rate of the organ not through a control of substrate uptake. In the light of these data the hypothesis of KöHLER et al. (1968) does not seem to be generally applicable. In their view "light ... hampers growth of internodes [of pea seedlings] by inhibiting uptake of essential growth material delivered from the cotyledons".