Abstract: The changes of rootmitochondrialmembrane potential (Δψm) , H₂O₂ concen
tration and the activity of caspase23 /7, the key enzyme of p rogrammed cell death were studied
under 20%-PEG6000 treatment. The results showed that root mitochondrial Δψm declined
slowly in the first six days of PEG treatment, then declined rap idly. The content of mitochon
drial H₂O₂ increased slowly in the first six days, then accumulated speedy inside of mitochon
dria. TUNEL assay was carried out and root paraffin slices disp layed distinct positive reaction
spots, which were more and more with the p rolonging treatment. The activity of caspase23 /7 in
root kep t at low level from the first day to the sixth day, and then rose sharp ly on the ninth and
twelfth day of PEG treatment. These indicate that 20% PEG6000 can induce p rogrammed cell
death inM alus hupehensis Rehd. roots at a certain treating2phase.

Key words: Malus hupehensis Rehd., Root, Water stress, Mitochondria, Programmedcell death, Caspase