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.