Photobiologie Du Mesocotyle d'AVOINE I‐ Action Des Radiations Rouge Clair Et Rouge Sombre

Red light (around 6,550 Å.) and far‐red light (around 7,300 Å) have been administered to excised first internodes of etiolated oat seedlings, and their effects upon growth by elongation have been studied. It has been found that red light inhibits much more the elongation produced bv gibberellic acid (GA) than that produced by 3‐indolylacetic acid (IAA). Far‐red light acts synergistically with GA in promoting elongation, but not with IAA. The effect of far‐red is localized in the node and in the first 4 mm below the node, at least when the seedlings are 63 hr old. The stimulatory effect of the far‐red radiation is observed when seedlings are younger than 65 hr or older than 69 hr after sowing. Except for a gradual decrease with age, there is no such two‐peaked pattern for the sensitivity to red light. Red light promotes the elongation of the node and of the 2 mm‐zone of the first internodes immediately below it. It inhibits the elongation of the zone comprised between 2 and 10 mm below the node. Various types of experiments could not demonstrate a true reversal of the red effect by far‐red light and vice‐versa. They indicated rather an additive effect of the inhibitory and stimulatory properties characteristic of each type of radiation.