Although the growth of intact plants is inhibited by irradiation with blue light, the growth rate of isolated stem segments is largely unaffected by blue light. We hypothesized that this loss of responsiveness was a result of ethylene production as part of the wounding response. However, we found no interaction between ethylene-and blue light-induced growth inhibition in dark-or red light-grown seedlings of pea (Pisum sativum L.). Inhibition of growth begins in dark-grown seedlings exposed to blue light within 3 min of the onset of blue light, as was known for red light-grown seedlings. By contrast, ethylene-induced inhibition of growth occurs only after a lag of 20 to 30 min or more (dark-grown seedlings) or 60 min (red light-grown seedlings). Also, the inhibition response of red light-grown seedlings is the same whether ethylene is present from the onset of continuous blue-light treatment or not. Finally the spatial distribution of inhibition following blue light was different from that following ethylene treatment.Blue light irradiation can inhibit stem elongation in dicotyledonous seedlings that have been grown in the dark or in red light by as much as 80% (1,2,6,8 ethylene induced a growth inhibition of approximately 60%. The inhibition began about 6 min after ethylene application in 4-d-old etiolated pea (Pisum sativum) seedlings. In comparison, we found that the latent time for blue light-induced growth inhibition in 6-d-old red light-grown pea seedlings was approximately 2 to 3 min (6). Both ethylene-and blue light-induced growth inhibitions are relieved after the stimulus is removed (6, 11).In addition, it has been observed that when a blue-light response is seen in cut sections, the degree of inhibition caused by blue light is related to physiological conditions known to affect the rate of ethylene production. Shinkle and Jones (10) found that when cucumbers were grown in red light, blue light inhibited the growth of sections by approximately 50%, a much greater degree of inhibition than was seen in sections excised from dark-grown plants (2). Previous experiments with etiolated pea seedlings have demonstrated that exposure to red light results in the suppression of endogenous ethylene production (5). Such results are consistent with the view that blue light might act by increasing ethylene production. In such a case, the wound ethylene produced as a result of excising sections from the stem might saturate the inhibition. Such a saturation would cause an apparent lack of blue-light sensitivity in cut sections. MATERIALS AND METHODSAlaska pea (Pisum sativum L.) seedlings were grown for 6 d in continuous red light at a fluence rate of 1 gmol m-2 s-' as described by Laskowski and Briggs (6)