Abstract:This paper reports that Fe2+ has non-competitive inhibition on the PODs of the cytoplasm, SECW (salt Extractable cell wall) and RCW (residual cell wall) from the excised and aerated cotyledons of the cucumber which have been incubated with the 1/15 mol/l phosphate Na+-K+ buffer containing 10 ppm 6BA, pH 6.0, for 36 hours in dark. The exogenous ‘Fe2+ promoted the expansion of the excised and aerated cotyledons of the cucumber, and simultaneously decreased the Hyp/Pro and the Idt/Tyr of RCW, what is more, the higher the Fe2+ concentration, the higher the percentage of promotion, and the less both the Hyp/Pro and the Idt/Tyr. Conversely, the exogenous POD of RCW as tested with Fe2+ enormously inhibited this expansion and increased the Idt/Tyr of RCW, but the PODs of the cytoplasm and SECW haven‘t such obvious effect as the POD of RCW. It suggests that the POD of RCW not only can convert the Pro of extension to Hyp, and cross-link with the polysaccharides of ceil wall via the Hyp-o-Ara, but also catalyze coupling of two tyrosine units to form Idt (isodityrosine) which is an inter-polypeptide cross-link. So the extensibility of cell walls is decreased and the growth of plant cell is limitted.