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These variations in CO₂ availability, along with associated variations in, for example, the difficulties of capturing CO₂ while avoiding the loss of water, have led to the widespread evolution of carbon concentrating mechanisms (CCMs) that increase the availability of CO₂ at the metabolic sites where it is required. Hence, while one might expect a process as fundamental to life on earth as carbon fixation in photosynthesis to be underpinned by a single unique biochemical pathway, in fact, even in higher plants there are three such pathways (and variants within them): the C₃ pathway (the most common), the C₄ pathway and CAM. The ecological consequences of the different pathways are profound, especially as they affect the reconciliation of photosynthetic activity and controlled water loss (see Section 3.3.1). Even in aquatic plants, where water conservation is not normally an issue, and most plants use the C₃ pathway, there are many CCMs that serve to enhance the effectiveness of CO₂ utilisation (Griffiths et al., 2017). These CCM‐based pathways are of profound importance. The C₄ and CAM pathways account for 18–30% of the 60 Pg (approximately) of carbon assimilated each year on land; while CCMs in cyanobacteria and algae account for more than half the 50 Pg of carbon assimilated each year in the oceans (Raven et al., 2008).

the C₃ pathway

In the C₃ pathway, the Calvin–Benson cycle, CO₂ is fixed, through combination with ribulose 1,5‐biphosphate (RuBP), into a three‐carbon acid (phosphoglyceric acid) by the enzyme RuBisCO (ribulose‐1,5‐biphosphate carboxylase‐oxygenase), which is present in massive amounts in the leaves (25–30% of the total leaf nitrogen). This same enzyme can also act as an oxygenase, as its name indicates, and this activity (photorespiration) can result in a wasteful release of CO₂ – reducing by about one‐third the net amounts of CO₂ that are fixed. Photorespiration increases with temperature with the consequence that the overall efficiency of carbon fixation declines with increasing temperature.

The rate of photosynthesis of C₃ plants increases with the intensity of radiation, but reaches a plateau. In many species, particularly shade species, this plateau occurs at radiation intensities far below that of full solar radiation (see Figure 3.4). Plants with C₃ metabolism have low water‐use efficiency compared with C₄ and CAM plants (see later), mainly because in a C₃ plant, CO₂ diffuses rather slowly into the leaf and so allows time for a lot of water vapour to diffuse out of it through the open stomata.

The rate of photosynthesis of C₃ plants also increases with the concentration of CO₂ within the plant, and because of the slow rate of diffusion, with the concentration of CO₂ in the atmosphere (see later). However, this rate is limited by the ability of C₃ plants to regenerate RuBP with which CO₂ can be combined, and therefore levels off as CO₂ concentrations increase.

the C₄ pathway

In the C₄ pathway, the Hatch–Slack cycle, the C₃ pathway is present but it is confined to cells deep in the body of the leaf. CO₂ that diffuses into the leaves via the stomata meets mesophyll cells containing the enzyme phosphoenolpyruvate (PEP) carboxylase. This enzyme combines atmospheric CO₂ with PEP to produce a four‐carbon acid. This diffuses, and releases CO₂ to the inner cells where it enters the traditional C₃ pathway. PEP carboxylase has a much greater affinity than RuBisCO for CO₂. There are profound consequences.

First, C₄ plants can absorb atmospheric CO₂ much more effectively than C₃ plants and the rate of photosynthesis is therefore much less dependent on CO₂ concentrations (but see later). Also, because of the reduced need to keep stomata open, C₄ plants may lose much less water per unit of carbon fixed. Furthermore, the wasteful release of CO₂ by photorespiration is almost wholly prevented and, as a consequence, the efficiency of the overall process of carbon fixation does not change with temperature. Finally, the concentration of RuBisCO in the leaves is a third to a sixth of that in C₃ plants, and the leaf nitrogen content is correspondingly lower. As a consequence of this, C₄ plants are much less attractive to many herbivores and also achieve more photosynthesis per unit of nitrogen absorbed.

It may seem surprising that C₄ plants, with such high water‐use efficiency, have failed to dominate the vegetation of the world, but there are clear costs to set against the gains. The C₄ system has a high light compensation point and is inefficient at low light intensities; C₄ species are therefore ineffective as shade plants. Moreover, C₄ plants have higher temperature optima for growth than C₃ species: most C₄ plants are found in arid regions or the tropics. The pathway is widely distributed amongst plant families but is most prominent in grasses, where many of the attempts to account for the distributions of C₃ and C₄ species have been focused.

The most common approach to understanding the proportion of C₃ and C₄ plants in any region goes back to Collatz et al. (1998). It involves the identification of a climatological crossover temperature, above and below which C₄ and C₃ plants, respectively, are favoured – that is, they have a carbon gain advantage – and also a level of precipitation sufficient for plants of both types to grow. Collatz et al. estimated these for grasses to be a mean daytime temperature of 22°C and precipitation of at least 25 mm per month. Then, for example, the number of months in the year typically favouring C₄ growth may be used to account, statistically, for the proportion of C₄ grasses in a local flora. Subsequent refinements of the approach have re‐estimated those growth criteria or acknowledged the importance of factors beyond temperature and precipitation. Thus, for instance, Griffith et al. (2015) explored a range of mean, minimum and maximum monthly temperatures for grasses in the USA and then found that the best fitting model was based on exceeding a monthly maximum temperature of 27°C, not a mean of 22°C (but still a mean monthly precipitation ≥25 mm; Figure 3.17). However, while this combination of temperature and precipitation thresholds was powerful in accounting for the distribution of C₄ grasses, in a number of regions, further factors were also important. In the Eastern Temperate Forest region, for example, there was a strong negative effect of tree cover on the proportion of C₄ grasses, since their shade promotes the cooler growing conditions more favourable to C₃ grasses; while in the Temperate US Sierras, there was a strong negative effect of mean annual precipitation, though whether this is favourable to C₃ grasses, unfavourable to C₄ grasses, or favourable to other plants that increase shading is uncertain.

Figure 3.17 Effects of temperature and precipitation on the proportional contributions of C₃and C₄grasses to the floras of various regions of the USA, as indicated. Data were collected from sampling plots within each region, with their location marked as dots on the map, and these data are shown as symmetrical ‘density curves’, associated with the number of months at each location where conditions exceeded the estimated temperature‐precipitation threshold (to the nearest month). The solid line is the predicted median proportion derived from a ‘quantile regression’ based on the temperature–precipitation threshold.

Source: After Griffith et al. (2015).

the CAM pathway

Plants with a CAM pathway also use PEP carboxylase with its strong power of concentrating CO₂. (The system is now known in a wide variety of families, not just the Crassulaceae.) In contrast to C₃ and C₄ plants, though, CAM plants open their stomata and fix CO₂ at night (as malic acid). During the daytime the stomata are closed and the CO₂ is released within the leaf and fixed by RuBisCO. However, because the CO₂ is then at a high concentration within the leaf, photorespiration is prevented, just as it is in plants using the C₄ pathway. Plants using the CAM photosynthetic pathway have obvious advantages when water is in short supply, because their stomata are closed during the daytime when evaporative forces are strongest. This appears to be a highly effective means of water conservation – water use efficiency for CAM plants is estimated to be around three times greater than for C₄ plants and more than six times greater than for C₃ plants (Borland et al., 2009) – but CAM species have not come to inherit the earth. One cost to CAM plants is the problem of storing the malic acid that is formed at night: most CAM plants are succulents with extensive water‐storage tissues that cope with this problem. In general, CAM plants are found in arid environments where strict stomatal control of daytime water is vital for survival (desert succulents), and in habitats where CO₂ is in short supply during the daytime, for example in some submerged aquatic plants, and in photosynthetic organs that lack stomata (e.g. the aerial photosynthetic roots of orchids).