Effects of Prolonged Darkness on the Sensitivity of Leaf Respiration to Carbon Dioxide Concentration in C3and C4Species

Abstract

Predicting responses of plant and global carbon balance to the increasing concentration 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 respiration 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 coefficients 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 1000μmol mol−1in fully expanded leaves of four C3and four C4herbaceous species. Responses were measured near the middle of the normal 10h dark period, and also after another 24h of darkness. On average, rates of respiration were reduced about 70% by the prolonged dark period, and leaf dry mass per unit area decreased about 30%. In all species studied, the relative decrease in respiration rate with increasing [CO2] was larger after prolonged darkness. In the C3species, rates measured at 1000μmol mol−1CO2averaged 0.89 of those measured at 60μmol mol−1in the middle of the normal dark period, and 0.70-times when measured after prolonged darkness. In the C4species, rates measured at 1000μmol mol−1CO2averaged 0.79 of those at 60μmol mol−1CO2in the middle of the normal dark period, and 0.51-times when measured after prolonged darkness. In three of the C3species and one of the C4species, the decrease in the absolute respiration rate between 60 and 1000μmol mol−1CO2was essentially the same in the middle of the normal night period and after prolonged darkness. In the other species, the decrease in the absolute rate of respiration with increase in [CO2] was substantially less after prolonged darkness than in the middle of the normal night period. These results indicated that increasing the [CO2] at the time of measurement decreased respiration in all species examined, and that this effect was relatively larger in tissues in which the respiration rate was substrate-limited. The larger relative effect of [CO2] on respiration in tissues after prolonged darkness is evidence against a controlling role of cytochrome-c oxidase in the direct effects of [CO2] on respiration.