Abstract: To clarify the effects of exogenous indole-3-acetic acid (IAA) and its competitive inhibitor p-chlorophenoxyisobutyric acid (PCIB) on stem elongation of rapeseed (Brassica napus L.) seedlings, and provide a theoretical basis for strong seedling cultivation and plant architecture regulation of rapeseed, 5-day-old seedlings of rapeseed cultivar Dadi 199 were used as plant materials. Isolated stem segments, intact seedling stems and de-rooted seedling stems were treated with different concentrations of IAA and PCIB for 48 h, and stem elongation was measured to compare concentration response differences. The results showed that IAA exhibited a promoting effect at low concentrations and an inhibitory effect at high concentrations on elongation of isolated stem segments. Stem elongation was increased by 70.02% under 0.50 μmol/L IAA treatment, whereas it was decreased by 41.44% under 10.00 μmol/L IAA treatment. Exogenous IAA at all concentrations inhibited elongation of intact seedling stems and de-rooted seedling stems. PCIB showed weak promotive effects on isolated stem segments, but significantly stimulated elongation of intact seedling stems and de-rooted seedling stems at appropriate concentrations, with optimal effects observed at 20 μmol/L and 10 μmol/L, respectively. It was concluded that auxin signaling-dependent stem elongation of rapeseed seedlings varied with material types. Isolated stem segments were likely deficient in endogenous auxin, and appropriate IAA could accelerate their elongation. Intact seedling stems and de-rooted seedling stems were probably under supraoptimal auxin levels or hypersensitive to auxin, and moderate PCIB could attenuate excessive auxin signaling and thereby facilitate stem elongation.

Key words: rapeseed (Brassica napus L.), indole-3-acetic acid (IAA), p-chlorophenoxyisobutyric acid (PCIB), seedling, stem elongation, auxin threshold