Effects of prolonged darkness on the sensitivity of leaf respiration to carbon dioxide concentration in C-3 and C-4 species

Predicting responses of plant and global carbon balance to the increasing c oncentration of carbon dioxide in the atmosphere requires an understanding of the response of plant respiration to carbon dioxide concentration ([CO2] ). Direct effects of the carbon dioxide concentration at which rates of res piration of plant tissue are measured are quite variable and their effects remain controversial. One possible source of variation in responsiveness is the energy status of the tissue, which could influence the control coeffic ients of enzymes, such as cytochrome-c oxidase, whose activity is sensitive to [CO2]. In this study we compared responses of respiration rate to [CO2] over the range of 60 to 1000 mu mol mol(-1) in fully expanded leaves of fo ur C-3 and four C-4 herbaceous species. Responses were measured near the mi ddle of the normal 10 h dark period, and also after another 24 h of darknes s. On average, rates of respiration were reduced about 70% by the prolonged dark period, and leaf dry mass pet unit area decreased about 30%. In all s pecies studied, the relative decrease in respiration rate with increasing [ CO2] was larger after prolonged darkness, In the C-3 species, rates measure d at 1000 mu mol mol(-1) CO2 averaged 0.89 of those measured at 60 mu mol m ol(-1) in the middle of the normal dark period, and 0.70-times when measure d after prolonged darkness. In the C-4 species. rates measured at 1000 mu m ol mol(-1) CO2 averaged 0.79 of those at 60 mu mol mol(-1) CO2 in the middl e of the normal dark period, and 0.51-times when measured after prolonged d arkness. In three of the C-3 species and one of the C-4 species. the decrea se in the absolute respiration rate between 60 and 1000 mu mol mol(-1) CO2 was essentially the same in the middle of the normal night period and after prolonged darkness. In the other species, the decrease in the absolute rat e of respiration with increase in [CO2] was substantially less after prolon ged darkness than in the middle of the normal night period. These results i ndicated that increasing the [CO2] at the time of measurement decreased res piration in all species examined, and that this effect was relatively large r in tissues in which the respiration rate was substrate-limited, The large r relative effect of [CO2] on respiration in tissues after prolonged darkne ss is evidence against a controlling role of cytochrome-c oxidase in the di rect effects of [CO2] on respiration. (C) 2001 US Government.