SALICYLIC-ACID ENHANCES THE ACTIVITY OF THE ALTERNATIVE PATHWAY OF RESPIRATION IN TOBACCO-LEAVES AND INDUCES THERMOGENICITY

A rise in the level of endogenous salicylic acid (SA) during flowering of the thermogenic voodoo lily, Sauromatum guttatum, leads to a pronounced temperature elevation by stimulation of the alternative respiratory pathway. We have studied the thermal response of tobacco (Nicotiana tabacum L.) leaves, a non-thermogenic tissue, to exogenous SA, and its relation to alternative respiration. A reproducible increase in surface temperature of 0.5-1.0 degrees C was registered with high-resolution infrared cameras. The same phenomenon was observed when 2,6-dihydroxybenzoic acid, an active analogue of SA, was used. Non-active SA analogues, such as 3- and 4-hydroxybenzoic acid, did not induce thermogenicity. The thermal effect of SA was abolished with inhibitors of the alternative pathway, such as salicylhydroxamic acid and propyl gallate. Polarographic measurement of the respiratory activity, including that of the alternative pathway in SA-treated plants, showed a significant increase of both total respiration and the alternative pathway compared with non-treated controls. Therefore, we postulate that, as in thermogenic species, SA enhances the activity of total respiration and of the cyanide-resistant pathway in tobacco leaves, subsequently leading to an elevation in surface temperature.