This thesis is an investigation of the physiological and genetic bases of photoperiodic injury (PI) in tomato. Based on previous work suggesting the involvement of nitrite accumulation and circadian rhythms in PI in tomato, the involvement of diel activities and transcript levels of nitrate reductase (NR) and nitrite reductase (NiR) in PI were tested in PI-susceptible and PI-resistant tomato lines. The NR transcript level in PI-susceptible line did not exhibit a circadian rhythm under a 24-h photoperiod, whereas the 12-h control maintained the rhythm. In contrast, the NR transcript level in the PI-resistant line exhibited circadian fluctuations under both 12-h and 24-h photoperiods. Continuous light caused tyrosine nitration of protein only for the PI-susceptible cultivar. These results suggest that PI in tomato occurs in part due to nitrite toxicity, which is mediated by proteins nitrotyrosine, and is linked to a loss of circadian coordination between NR and NiR.