The production and diffusion of nitric oxide (NO) (white) in the cytoplasm (green) of clusters of conifer cells one hour after mechanical agitation. NO diffuses into the culture medium around cell clusters. The large dark green circle inside each cell is the nucleus. Some nuclei may contain a nucleolus (light green). The small and irregular dark areas outside the nucleus and in the cytoplasm represent the vacuoles (laser confocal photomicrograph ×400). NO was visualized with a fluorescent probe DAF-2 DA (diaminofluorescein diacetate). NO is derived from L-arginine and oxygen by nitric oxide synthase (NOS) activity in conifers and humans. NOS activity and the production of NO in the conifer cells is blocked by Nω-monomethyl-L-arginine.