We demonstrate the use of laser photoacoustic detection (LPAD) as a highly sensitive method to detect in planta nitric oxide ( . NO) production from tobacco (Nicotiana tabacum . NO 1 O 3 / NO 2 1 O 2 ). The utility of the LPAD method was shown by examination of a nonhost hypersensitive response and a disease induced by Pseudomonas syringae (P. s.) pv phaseolicola and P. s. pv tabaci in tobacco.. NO was detected within 40 min of challenge with P. s. pv phaseolicola, some 5 h before the initiation of visible tissue collapse. The wildfire tobacco pathogen P. s. pv tabaci initiated. NO generation at 2 h postinfection. The use of . NO donors, the scavenger CPTIO ([4-carboxyphenyl]-4,5-dihydro-4,4,5,5-tetramethyl-3-oxide), and the mammalian nitric oxide synthase inhibitor L-NMMA (N G -monomethyl-L-arginine) indicated that . NO influenced the kinetics of cell death and resistance to both avirulent and virulent bacteria in tobacco. These observations suggest that .