Presymptomatic visualization of plant-virus interactions by thermography

Salicylic acid (SA), produced by plants as a signal in defense against path ogens, induces metabolic heating mediated by alternative respiration in flo wers of thermogenic plants, and, when exogenously applied, increases leaf t emperature in nonthermogenic plants. We have postulated that the latter phe nomenon would be detectable when SA is synthesized locally in plant leaves. Here, resistance to tobacco mosaic virus (TMV) was monitored thermographic ally before any disease symptoms became visible on tobacco leaves. Spots of elevated temperature that were confined to the place of infection increase d in intensity from 8 h before the onset of visible cell death, and remaine d detectable as a halo around the ongoing necrosis. Salicylic acid accumula tes during the prenecrotic phase in TMV-infected tobacco and is known to in duce stomatal closure in certain species. We show that the time course of S A accumulation correlates with the evolution of both localized thermal effe ct and stomatal closure. Since the contribution of leaf respiration is marg inal, we concluded that the thermal effect results predominantly from local ized, SA-induced stomatal closure. The presymptomatic temperature increase could be of general significance in incompatible plant-pathogen interaction s.