and indeed other products such as enzymes and lactic and succinic acid (Perez‐Pimienta et al., 2017). Current quantities grown are small compared with the most common alternatives based around corn, sugar cane and wheat, and most of this is in Mexico. However, the much greater productivity of Agave (Figure 3.18), and the high water use efficiency it derives from its CAM metabolism, highlights its great potential for the future.
Figure 3.18 Agave species (CAM plants) exhibit high productivity due to their high water use efficiency. Comparison of the productivities of various species of Agave (double entries reflect separate studies) and four selected bioenergy feedstocks (raw products for the production of biofuels). Corn stover consists of the remains of plants left in the field after corn (Zea mays) has been harvested. Sugarcane bagasse is the fibrous matter than remains after sugarcane (Saccharum spp.) has been crushed to extract its juice.
Source: After Perez‐Pimienta et al. (2017).
APPLICATION 3.3 Genetic engineering of CCMs into crops
Another way to exploit the potential of CAM would be to engineer its molecular machinery into C3 plants (Borland et al., 2014). Indeed there is great interest in general in engineering a wider range of CCMs, taken from C4 plants and from microbes, into commercial crops. Complex metabolic alternatives, such as CAM, typically require a whole suite of co‐adapted changes, and implementing them will therefore be a massive challenge. However, promising results may be obtained from less ambitious approaches. Figure 3.19, for example, illustrates the results when the photosynthetic rate of a standard variety of soybean, Glycine max, was compared with that of a transgenic variety that had been transformed to express the ictB gene taken from cyanobacteria, where it plays a crucial role in carbon uptake and is essential for their survival at anything other than very high CO2 concentrations. In both varieties, photosynthetic rates increased with the concentrations of CO2 in plant tissues but levelled off at higher concentrations. However, these rates, and especially the maximum rate, were significantly higher in the transgenic variety (Figure 3.19). Crucially, this difference translated into significant improvements in yield in the transgenic variety. As the authors of the study conclude, work such as this indicates that even single genes can contribute to the enhancement of yield in a major commodity crop ‘… and point to the significant role that biotechnological approaches to increasing photosynthetic efficiency can play in helping to meet increased global demands for food’.
Figure 3.19 Bioengineering of a gene from cyanobacteria into soybean increases its rate of photosynthesis. A comparison of soybean (Glycine max) wild type (blue) and a variety genetically engineered to express the cyanobacterial membrane protein ictB (red) in terms of the effect of a plant’s internal CO2 ([CO2i]) concentration on its net rate of photosynthesis (Anet). The fitted lines were non‐rectangular hyperbolas reflecting current understanding of the underlying physiology.
Source: After Hay et al. (2017).
the evolution of C4 and CAM
The evolution of the C4 and CAM pathways, and of CCMs generally (that increase the concentration of CO2 around RuBisCO), has been reviewed by Raven et al. (2008). They describe the very strong evidence that these mechanisms are evolutionarily primitive (the earliest appearing at least 300 million years ago) but also that the C4 and CAM systems must have arisen repeatedly and independently during the evolution of the plant kingdom – the most recent being the appearance of C4 in land plants only 20–30 million years ago. This prolonged coexistence of multiple paths to carbon fixation foreshadows a pattern we will see many more times in later chapters – of coexisting species utilising the same resources but in different ways.
3.4.2 The response of plants to changing atmospheric concentrations of CO2
Of all the various resources required by plants, CO2 is the only one that is increasing on a global scale. This rise is strongly correlated with the increased rate of consumption of fossil fuels and the clearing of forests. As Loladze (2002) points out, while consequential changes to global climate may be controversial in some quarters, marked increases in CO2 concentration itself are not. High mixing rates in the atmosphere mean that these are changes that will affect all plants. Plants now are experiencing around a 30% higher concentration compared with the preindustrial period – effectively instantaneous on a geological timescale. Trees living now may experience a doubling in concentration over their lifetimes – effectively instantaneous on an evolutionary timescale.
changes in geological time
Putting this in a wider context, though, there is also evidence of large‐scale changes in atmospheric CO2 over much longer timescales. A range of models suggest that during the Triassic, Jurassic and Cretaceous periods (around 250–70 million years ago), atmospheric concentrations of CO2 were four to eight times greater than at present (and both lower and very much higher prior to that), falling after the Cretaceous from between 1500 and 3000 μl l−1 to below 1000 μl l−1 in the subsequent Eocene, Miocene and Pliocene, and fluctuating between 180 and 280 μl l−1 during more recent glacial and interglacial periods (the last 400 000 years; Figure 3.20). The steady rise in CO2 since the Industrial Revolution is therefore a partial return to pre‐Pleistocene conditions, more than 2.5 million years ago.
Figure 3.20 Estimates of atmospheric CO2concentrations over the past 600 million years (Ma). The red line (with estimated range of error in pink) is the output of the GEOCARB III model, which uses estimates of rates of geological weathering, emission and burial modified by a range of factors including global temperature and continental size. The blue line is the result of averaging four proxies of CO2 concentrations, including isotopic compositions of minerals and plankton and the distribution of stomata in plant leaves.
Source: After Royer et al. (2004).
what will be the consequences of current rises?
When other resources are present at adequate
