RECYCLING OF CO2 DURING INDUCTION OF CAM BY DROUGHT IN TALINUM PANICULATUM (PORTULACACEAE)

To investigate the possible induction of Crassulacean acid metabolism (CAM) by drought in Talinum paniculatum ([Jacq.] Gaertn.), a deciduous herb with succulent leaves and lignified stems, nocturnal acid accumu lation and CO2-exchange were studied in watered and droughted greenhou se-grown plants. Watered plants had a typical C3 pattern of CO2-exchan ge. When plants were subjected to drought, nocturnal acid accumulation increased significantly from 0.9 to 13.4 mu mol H(+)cm(-2) after 21 d ays. Water deficit provoked a rapid reduction of daytime CO2 assimilat ion of as much as 92% and a slower increase in night-time fixation. A maximum of 24% of the diel carbon gain was contributed by dark fixatio n in droughted plants. After 34 days of drought, only CO2 compensation and a small accumulation of acid (idling) was detected during the nig ht. Relative recycling of respiratory CO2 was approximately 100% for m ost of the water deficit treatment, the amount of CO2 recycled showing a high positive correlation with nocturnal acid accumulation. A low r ate of nocturnal loss of CO2 in watered plants did not explain the amo unt recycled nightly in droughted plants, implying that respiration in creased with drought. Leaf lamina area was reduced by 49% during droug ht due to rolling. Leaf biomass remained unchanged during the water-de ficit treatment. Neither apparent quantum yield nor light-saturated ph otosynthetic rate differed significantly between control and 14-day wa ter-stressed plants rewatered for 20 h. Chlorophyll content did not ch ange with drought. These results confirm that CAM is induced by drough t in T. paniculatum; the carbon acquired through this pathway only con tributes to maintain, but not to increase, leaf biomass; also, CAM is responsible for a high recycling of respiratory CO2 during the night. Recycling through CAM, plus the reduction of exposed leaf area during drought, may help explain the maintenance of chlorophyll, quantum yiel d and saturated photosynthetic rates in water-stressed plants of T. pa niculatum.