Nitric oxide (NO) is a key signaling molecule in different physiological processes of plants. To study
the metabolism of endogenous NO in growth and development of apple fruit under normal physiological
conditions, the changes of several relative components of NO metabolism including the contents of
NO, l-arginine, S-nitrosothiols (RSNOs) and nitrite plus nitrate, the activities of l-arginine-dependent
nitric oxide synthase (NOS) and nitrate reductase (NR) were analyzed using vegetal ‘Gold Delicious’ and
‘Fuji’ apples as tested materials. The endogenous NO and l-arginine contents, as well as the l-argininedependent
NOS activity in young fruit reached the highest value. The NOS activity was dependent on
l-arginine and -NADPH, and inhibited by a well known NOS inhibitor. The NR activity in apple fruit had
not been detected. Endogenous NO production depended on l-arginine-dependent NOS but not NR. The
content of RSNOs tended to peak in the fruit enlarging period and then declined. Similarly, the nitrate content
increased in early stage of fruit growth, but in late stage, it undulated greatly. Compared to RSNOs
and nitrate, the change of nitrite content was in accordance with that of endogenous NO. In general,
RSNOs and nitrite could reflect the removal mechanism of NO relatively well.