Changes in the concentration of indole‐3‐acetic acid during the growth of etiolated lupin hypocotyls

The longitudinal distribution of unaltered radioactive indole‐3‐acetic acid (IAA), after application of [5‐3H]‐IAA to decapitated etiolated lupin hypocotyls. exhibited a wave‐like pattern similar to that obtained with endogenous IAA. Waves of radioactive IAA were localizated both in the elongation zone and in the non‐growing basal region of the hypocotyl. These IAA waves were transient because of basipetal polar transport and metabolism of IAA. The level of endogenous IAA in different zones of the hypocotyl varied with age, following a wave‐like pattern. During the elongation period of each zone, IAA was parallel to the bell‐shaped curve of the growth rate. In addition, a role in secondary cell wall deposition is suggested for the other IAA wave that appeared after the cell elongation period, since an electron microscopic morphometric analysis of the cell wall showed that the cell wall thickness increased once the cell elongation ceased. As the oscillation of endogenous IAA level occured in both space (distribution along the hypocotyl) and time (variation with age), it is suggested that the level of IAA really depended on the growth status of the cells. The response of the cells to the positional information submitted by the auxin waves as regards the growth status of the cell is discussed.