Gibberellic acid and the light inhibition of stem elongation

Abstract

The ability of gibberellic acid (GA₃) to prevent the light inhibition of stem elongation in peas was examined for several varieties under a wide range of irradiation conditions. A saturating dose of GA₃ largely prevented the inhibitory effect of red light on total stem height in “Duke of Albany” (tall), “Alaska” (medium) and “Meteor” (dwarf) although a small, but statistically significant, effect persisted in all varieties after 3 days of light. The growth of the second internode was, however, markedly inhibited by red light even with a saturating dose of GA₃. With gibberellin there was no difference between the effects of continuous red light and 15 minutes per day on height but the second internode was much shorter in the former treatment. The number of internodes present was the same in both cases and, therefore, the upper internodes in continuous light were as long or longer than in the 15-minute treatment. The number of internodes was only slightly fewer in darkness than in light so that, with GA₃, the effect of red light was transient and only the growth of the lower internodes was inhibited. Without GA₃ overall height was less in both red light treatments than in darkness for all three varieties. In blue light, on the other hand, there was no difference depending on whether height or internode length is considered, and even with a saturating dose of GA₃ the growth rate remained depressed in continuous blue light. There was, however, some interaction between blue light and GA₃. Red/far-red reversal experiments showed that in the varieties “Alska” and “Duke of Albany” the far-red stimulation of elongation persisted in the presence of a saturating dose of GA₃ while for the dwarf variety “Meteor” there was a significant interaction between far-red and GA₃. At least a quantitative difference was found between tall and dwarf peas in their response to light. Tall varieties showed a much greater effect of a prolonged exposure to blue and a smaller effect of a short exposure to red than dwarf varieties. Increasing the duration of exposure to red increasingly inhibited the growth of tall varieties. The medium variety “Alaska” grew to approximately the same height in continuous red and blue light.