PARENT SESSION
Posters P8C C4 and CAM. Abstracts (685-698)

Effects of elevated CO₂ on photosynthesis, respiration and mitochondrial number in the CAM plant Opuntia ficus-indica. Núria Gómez-Casanovas¹, Xavier Huete², Julia Baeder¹, Joaquim Azcón-Bieto^*,1, ¹ Unitat de Fisiologia Vegetal, Departament de Biologia Vegetal, Barcelona, Spain² Departament I+D, Barcelona, Spain

ABSTRACT- Prickly pear cactus (Opuntia ficus indica (L.) Mill.) is a highly productive CAM plant which shows a large growth increase when it is exposed to elevated atmospheric CO₂ concentration (see review by Drennan and Nobel, 2000. Plant Cell Environ. 23: 767-781). The responses of photosynthesis and other processes (dark CO₂ fixation, etc.) to elevated CO₂ have been quite well studied in this species, but there is practically no information in the literature about possible effects of elevated CO₂ on dark respiration in cactus pear, and in CAM plants in general (Drennan and Nobel, 2000). Therefore, we have measured the rate of dark O₂ uptake of cladode slices from plants grown during four months at 800 ppm CO₂ in a Conviron chamber. The rate of net O₂ production in the light was also measured, as an estimate of net photosynthesis in the absence of an external source of carbon dioxide. Shoot (basal and new cladodes) and root dry biomass production significantly increased in 'elevated CO₂' plants when compared to 'ambient' control plants, consistently with earlier work (see above). The rate of net photosynthetic O₂ production in the light presented a small increase in 'elevated CO₂' cladodes. In the case of respiration, the rate of cladode dark oxygen uptake presented an important reduction (20-24%) when plants were grown under elevated CO₂. Organic nitrogen content was slightly reduced in elevated CO₂ plants, but mainly in cladode photosynthetic tissue. Finally, we have measured the number of mitochondria of these tissues by developing a digital three-dimensional view of the cells with advanced microscopy techniques, and preliminar results indicate that an important reduction of the number of mitochondria has taken place in photosynthetic cells from 'elevated CO₂' cladodes. To our knowledge, this is the first study showing a substantial reduction of mitochondrial respiration in a CAM plant grown under elevated carbon dioxide. We believe that this effect could have an impact in improving the carbon balance of cactus pear under elevated atmospheric CO₂. This work has been supported by research grants from the Spanish Government (ref. BFI 2003-09680) and the Catalan Government (ref. 2001-SGR-00094).

KEY WORDS: dark respiratory oxygen uptake rate, prickly pear cactus, elevated carbon dioxide, number of mitochondria