Carbon balance during CAM: an assessment of respiratory CO2 recycling in the epiphytic bromeliads Aechmea nudicaulis and Aechmea fendleri
Abstract The regulation of crassulacean acid metabolism (CAM) under controlled environmental conditions has been investigated for two tropical epiphytes, relating plant water and carbon balance to growth form and habitat preference under natural conditions. Aechmea fendleri is restricted to wet, upper montane regions of Trinidad, while A. nudicaulis has a wider distribution extending into more arid regions of the island. Morphological characteristics of these plants are related to habitat preference in terms of leaf succulence (0.44 and 0.94 kg m−2 for the two species respectively) and a distinct layer of water storage parenchyma in A. nudicaulis In contrast, the thinner leaves of A. fendleri contain little water-storage parenchyma and less chlorenchyma per unit area, but the plants have a more open leaf rosette. The two species differ in expression of CAM, since the proportion of respiratory CO2 recycled as part of CAM had been found to be much lower in A. fendleri This study compared the efficiency of water use and role of respiratory CO2 recycling under two PAR regimes (300 and 120 μnol m−2 s−1) and three night temperatures (12, 18 and 25 °C). Dark CO2 uptake rates for both species were comparable to plants in the field (maximum of 2.3 ± 0.2 μmol m−2s−1± SD, n= 3). Total net CO2 uptake at night increased on leaf area basis with temperature for both species under higher PAR, although under the low PAR regime CO2 uptake was maximal at 18 °C. Water-use efficiency (WUE) increased at 18 °C and 25 °C during dark CO2 uptake (Phase I) and also during late afternoon photosynthesis (Phase IV) in both species. For A. fendleri, dawn to dusk changes in titrable acidity (ΔH +) were similar under high and low PAR, although ΔH+ was correlated to night temperature and PAR in A. nudicaulis. The proportion of ΔH+ derived from respiratory CO2 also varied with experimental conditions. Thus percentage recycling was lower in A. fendleri under high PAR (0–10%), but was only reduced at 18 °C under low PAR. Recycling by A. nudicaulis ranged from 32–42% under high PAR, but was also reduced to 6% under low PAR at 18 °C; at 12 °C and 25 °C, recycling was 37% and 52% respectively. Previous studies have suggested a relationship between the proportion of recycling and degree of water stress. This study indicated that CAM as a CO2 concentrating mechanism regulates both water-use efficiency and plant carbon balance in these epiphytes, in response to PAR and night temperature. However, the precise relationship between respiratory processes and the balance between external and internal sources of CO2 is as yet unresolved.