Abstract: The seedlings of wheat (‘ML7113') were exposed to He-Ne laser irradiation (L) with 5 mW·mm^-2 power density after enhanced UV-B radiation (10.08 kJ·m^-2·d^-1) to determine the mechanism of the action of nitric oxide on stomatal movement in wheat seedlings.The leaves and epidermal strips of wheat underwent different treatments by experiments for pharmacology and laser scanning confocal microscopy.The results showed that (1) Enhanced UV-B radiation not only induced stomatal closure,but also increased the levels of NO significantly in the guard cells and leaves of wheat.In addition,NO scavenger cPTIO obviously inhibited stomatal closure induced by UV-B radiation and the levels of NO evidently reduced in guard cells and leaves of wheat.(2) There were obvious differences in the effect of nitric oxide synthase (NOS) inhibitor L-NAME on stomatal aperture,NO content in guard cells and leaves of wheat exposed to enhanced UV-B radiation compared with nitrate reductase (NR) inhibitor NaN₃.This suggested that nitric oxide synthase was the main way to produce NO in wheat leaves by UV-B radiation.(3) For stomatal aperture,L>CK>BL>B.For the NO level,B>BL>CK>L.For activity of NR and NOS,NR activity in the UV-B radiation treatment group was the lowest and He-Ne laser irradiation treatment group was the highest;however,NOS activity between B and L showed the opposite pattern compared with NR acti-vity.The stomatal opening changes induced by UV-B radiation and He-Ne laser irradiation were associated with high and low NO content in guard cells and leaves of wheat.Decreases and increases in stomatal aperture were consistent with increases and decreases in NO content.Furthermore,NO generation was the same as nitric oxide synthase in wheat leaves by He-Ne laser individual irradiation.