Researchers confirmed potential for new plantations in drylands under constraint environmental conditions.

Two CAM species are examined here as potential bioenergy crops: Opuntia ficus-indica and Euphorbia tirucalli. Both show the high degree of drought tolerance typical of CAM plants and produce promising yields with low rainfall. Even CAM plants in semi-arid areas may have opportunity costs in terms of lost agricultural potential, but an alternative approach to bioenergy may allow the food value of land to be increased whilst using the land for energy. Global power generation from gas is around 5 PW h per year. The data suggests that 5 PW h of electricity per year could be generated from CAM plants cultivated on between 100 and 380 million hectares of semi-arid land, equivalent to between 4% and 15% of the potential resource.

The CAM pathway of photosynthesis enables the temporal separa- tion of CO2 fixation and assimilation during the day–night cycle. This allows plants to absorb CO2 from the atmosphere during the night when air temperatures are lower – so reducing evaporative losses – and to metabolise the stored carbon during the day when solar energy is available. Although this is the defining characteristic of CAM plants, as a group they exhibit a significant degree of plasticity in their photosynthetic behaviour, and many can supplement their nocturnal CO2 fixation with diurnal CO2 uptake when water availability is high.

This flexibility makes them attractive candidates for use as energy crops in semi-arid and variable rainfall areas.

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While a projected 42 million of the blue-tinted, spikey-leaved plants are needed this year to supply 140 registered companies, only 17.7 million planted in 2011 are ready for harvest, according to Tequila Regulatory Council and the National Tequila Industry Chamber. Agave plants take seven to eight years to reach maturity. As a result, the price of agave has risen sixfold in two years, from about 21 cents a kilo in 2016 to $1.18 currently.

Coal is the world’s biggest source of electricity, with approxi- mately 9 PW h generated in 2011. The second biggest source of power was gas, at almost 5 PW h.69 To achieve 5 PW h from CAM plants would require somewhere between 4% and 15% of the 2.5 billion hectares of potentially available semi-arid land, depending on the yield and gas production assumptions used.

Authors in this and previous studies, have been reporting on potential development and new a growing demand for new plantations that may combine environmental benefits derived form positive land use effects (green covers against erosion and ongoing desertification).