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 prono unced temperature elevation by stimulation of the alternative respirat ory pathway. We have studied the thermal response of tobacco (Nicotian a tabacum L.) leaves, a non-thermogenic tissue, to exogenous SA, and i ts relation to alternative respiration. A reproducible increase in sur face temperature of 0.5-1.0 degrees C was registered with high-resolut ion infrared cameras. The same phenomenon was observed when 2,6-dihydr oxybenzoic acid, an active analogue of SA, was used. Non-active SA ana logues, such as 3- and 4-hydroxybenzoic acid, did not induce thermogen icity. The thermal effect of SA was abolished with inhibitors of the a lternative 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 compar ed with non-treated controls. Therefore, we postulate that, as in ther mogenic 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.