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Recycling of CO<inf>2</inf> during induction of CAM by drought in Talinum paniculatum (Portulacaceae)

Abstract:

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 accumulation and CO2-exchange were studied in watered and droughted greenhouse-grown plants. Watered plants had a typical C3 pattern of CO2-exchange. When plants were subjected to drought, nocturnal acid accumulation increased significantly from 0.9 to 13.4 μmol H+ cm-2 after 21 days. Water deficit provoked a rapid reduction of daytime CO2 assimilation 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 fixation in droughted plants. After 34 days of drought, only CO2 compensation and a small accumulation of acid (idling) was detected during the night. Relative recycling of respiratory CO2 was approximately 100% for most of the water deficit treatment, the amount of CO2 recycled showing a high positive correlation with nocturnal acid accumulation. A low rate of nocturnal loss of CO2 in watered plants did not explain the amount recycled nightly in droughted plants, implying that respiration increased with drought. Leaf lamina area was reduced by 49% during drought due to rolling. Leaf biomass remained unchanged during the water-deficit treatment. Neither apparent quantum yield nor light-saturated photosynthetic rate differed significantly between control and 14-day water-stressed plants rewatered for 20 h. Chlorophyll content did not change with drought. These results confirm that CAM is induced by drought in T. paniculatum; the carbon acquired through this pathway only contributes 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 yield and saturated photosynthetic rates in water- stressed plants of T. paniculatum.