Abstract
Agaves exhibit the water-conserving crassulacean acid metabolism (CAM) photosynthetic pathway. Some species are potential biofuel feedstocks because they are highly productive in seasonally dry landscapes. In plants with CAM, high growth rates are often believed to be associated with a significant contribution of C3 photosynthesis to total carbon gain when conditions are favourable. There has even been a report of a shift from CAM to C3 in response to overwatering a species of Agave. We investigated whether C3 photosynthesis can contribute substantially to carbon uptake and growth in young and mature Agave angustifolia collected from its natural habitat in Panama. In well-watered plants, CO2 uptake in the dark contributed about 75% of daily carbon gain. This day/night pattern of CO2 exchange was highly conserved under a range of environmental conditions and was insensitive to intensive watering. Elevated CO2 (800 ppm) stimulated CO2 fixation predominantly in the light. Exposure to CO2-free air at night markedly enhanced CO2 uptake during the following light period, but CO2 exchange rapidly reverted to its standard pattern when CO2 was supplied during the subsequent 24h. Although A. angustifolia consistently engages in CAM as its principal photosynthetic pathway, its relatively limited photosynthetic plasticity does not preclude it from occupying a range of habitats, from relatively mesic tropical environments in Panama to drier habitats in Mexico.
Highlights
Most, if not all, of the approximately 200 species in the New World genus Agave exhibit the nocturnal uptake of CO2 and accumulation of malic acid characteristic of crassulacean acid metabolism (CAM) (Nobel, 2003, 1996a)
We investigated whether C3 photosynthesis can contribute substantially to carbon uptake and growth in young and mature Agave angustifolia collected from its natural habitat in Panama
Despite an ability to occupy contrasting habitats, photosynthetic flexibility in A. angustifolia does not appear to be exceptional in terms of the proportional contributions to carbon gain of CO2 uptake in the dark and light
Summary
If not all, of the approximately 200 species in the New World genus Agave (family Asparagaceae, subfamily Agavoideae; Chase et al, 2009; Govaerts et al, 2013) exhibit the nocturnal uptake of CO2 and accumulation of malic acid characteristic of crassulacean acid metabolism (CAM) (Nobel, 2003, 1996a). As biomass becomes an increasingly valuable commodity for energy generation, there is realization that the seasonally dry or semi-arid landscapes inhabited by Agave, whilst not optimal for growing traditional water-demanding food crops, may be suitable for biomass generation (Borland et al, 2009; Chambers and Holtum, 2010; Davis et al, 2011; Holtum et al, 2011; Yan et al, 2011) In such landscapes, water-use-efficient Agave can accumulate biomass at annual rates that approach those produced by C4 plants like sugar cane and Miscanthus in higher rainfall regions (Nobel, 1991, 1996a; Somerville et al, 2010). A. tequilana is being trialed in the seasonally dry Australian tropics as a biofuel feedstock (Chambers and Holtum, 2010; Holtum et al, 2011)
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