Abstract: Taking two-year old Malus hupehensis as test material, this paper studied its root nitric oxide (NO) production under waterlogging, and the effects of  exogenous NaNO₃ on this production. Waterlogging for 3-9 days promoted the NOproduction significantly. Within the 12 days of waterlogging, the NO production, nitrite reductase (NR) and nitric oxide synthase (NOS) activities, and malondialchehyche (MDA) content in roots increased first, and decreased subsequently. Under waterlogging, the application of 10 mmol NaNO₃·L^-1 inhibited the increase of MDA content and NOS activity while improved the NR activity significantly. After applying NaNO_3, the root NO production increased in the first three days, but decreased significantly after the 6th day.

Key words: Malus hupehensis, waterlogging, NaNO₃, nitric oxide, root, tasseling stage, root lodging, stem lodging, yield loss, summer maize.