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Recycling of CO2 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 CO<inf>2</inf>-exchange were studied in watered and droughted greenhouse-grown plants. Watered plants had a typical C3 pattern of CO<inf>2</inf>-exchange. When plants were subjected to drought, nocturnal acid accumulation increased significantly from 0.9 to 13.4 μmol H<sup>+</sup> cm<sup>-2</sup> after 21 days. Water deficit provoked a rapid reduction of daytime CO<inf>2</inf> 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 CO<inf>2</inf> compensation and a small accumulation of acid (idling) was detected during the night. Relative recycling of respiratory CO<inf>2</inf> was approximately 100% for most of the water deficit treatment, the amount of CO<inf>2</inf> recycled showing a high positive correlation with nocturnal acid accumulation. A low rate of nocturnal loss of CO<inf>2</inf> 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 CO<inf>2</inf> 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.