CONTROL OF LEAF AND STEM GROWTH IN LIGHT-GROWN PEA-SEEDLINGS BY 2 HIGH IRRADIANCE RESPONSES

The control exerted by light on leaf and stem growth in light-grown ''Alaska'' pea [Pisum sativum] seedlings was studied during the main photoperiod. Two high irradiance responses were observed. The action spectrum for 1 had a single sharp peak at 600 nm. The action spectrum for the other showed a broad peak between 440 and 470 nm. These 2 light responses must be activated simultaneously for any inhibition of stem growth or promotion of leaf growth. Both action spectra may be explained in terms of the high irradiance response of phytochrome.