shown in fig. 3a, a sharp increase in the aco activity was observed in the control treatments, reached its highest levels on day 6, and then decreased in the following days. however, the aco activity in snp-treated flower petals increased gently only during the first 4 days, and then remained constant, showing that exogenous no inhibited the increase of aco activity in cut rose flowers. furthermore, if cptio was added simultaneously to the snp solution, the effect of no was partly reversed (fig. 3a). similar to the aco activity, the increase of ethylene production rate in control flower petals was much steeper