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).
APPLICATION 3.2 Turning to CAM crops
The high water use efficiency of CAM plants makes them excellent candidates for cultivation over areas where rainfall is too little or evapotranspiration too high for the cultivation of C3 or C4 crops. Such areas may well expand in future. Examples include the production of pineapple, Ananas comosus, for consumption, of Opuntia spp. for animal fodder, of sisal, Agave sisalana, for fibre, and of Agave tequilana for the alcoholic beverages tequila and mescal (Borland et al., 2009). In addition, species of Agave
