Decapitation of Arabidopsis floral stern suppresses elongation and any associated rate oscillation. This growth inhibition also appeared after application on intact plants of IAA transport inhibitors such as NP A. A subsequent IAA paste application on the cut surface restored growth and circadian oscillations in extension rate. These results indicate that the upper zone of the inflorescence has a major influence on floral stem extension rate and give a particular importance to auxin. Furthermore, measurement of endogenous contents of IAA and IAA-aspartate conjugate displayed that they were subjected to a circadian rhythm during Arabidopsis floral stern growth. All together, these data point out lAA as an essential factor in circadian growth rhythm regulation of Arabidopsis floral stem