HIGH AMBIENT CARBON-DIOXIDE DOES NOT AFFECT RESPIRATION BY SUPPRESSING THE ALTERNATIVE, CYANIDE-RESISTANT PATHWAY

Total dark respiration (v(t)), cytochrome pathway (v(eyt)), alternativ e pathway (v(alt)) and residual (v(res)) respiration were measured in Lemna gibba plants, by the use of pathway inhibitors. NaCN was used to inhibit v(eyt) and SHAM (salicylhydroxamic acid) to inhibit v(alt). R esidual respiration (v(res)) was about 5% of v(t). The effect of high (100 Pa) and low (0 Pa) carbon dioxide partial pressure ([CO2) on v(t) , v(cyt) and v(alt) was determined from both CO2 efflux and O(2)uptake measurements. The higher [CO2] suppressed v(t) by about 30%. When res piration operated through the cytochrome pathway only. in the absence of v(alt), it was suppressed by about 12% as measured by the O-2 uptak e of submerged Lemna fronds or by about 40% as measured by CO2 efflux from Boating fronds. The higher [CO2] treatment had only a small effec t oil respiration, when v(alt) alone operated. There was no evidence o f a specific suppression of the v(alt) pathway by high [CO2]. Succinic dehydrogenase activity of the mitochondria of roots of Medicago sativ um was reduced by 18%, when the mitochondria were pre-treated with 120 as compared to 34 Pa [CO2]. There was no such effect on cytochrome c oxidase activity of mitochondria under the same conditions. It is conc luded that there is no evidence for the hypothesis that the high [CO2] suppression of respiration is a result of a CO2 effect on the non-pho sphorylating alternative respiration pathway. (C) 1995 Annals of Botan y Company