The Ecology of Photosynthetic Pathways

Because of the biochemical and morphological differences among the three photosynthetic pathways, each has its own unique set of advantages and disadvantages. These differences result in differential performance in different environments.

The C3 pathway is the oldest (originating around 2,800 million years ago) and is also the most widespread, both taxonomically and environmentally. C3 plants can be found in the warmest deserts and the coldest arctic habitats.

C4 photosynthesis has evolved independently approximately fifty times but is most widespread in grasses where about 50% of the species are C4 photosynthesizers (Edwards and Smith 2010). C4 photosynthesis occurs in 3% of vascular plant species, but accounts for 25% of terrestrial photosynthesis (Sage,2004). It is thus a relatively recent innovation, with the earliest appearances occurring during the low atmospheric CO2 levels that occurred in the mid-Oligocene (approximately thirty million years ago). C4 plants reach their peak of productivity in high-light and warm climates with summer rains (Figure 7), dominating tropical and subtropical grasslands and savannas whereas C3 grasses dominate cooler, temperate grasslands. Because C4 photosynthesis is saturated at lower CO2 concentrations and allows lower stomatal conductance for a given photosynthetic rate, C4 plants also perform well in saline and dry habitats. Edwards and Smith (2010) found that eighteen of twenty evolutionary origins of C4 grasses were correlated with reductions in mean annual precipitation, consistent with a shift out of tropical forests into open, high-light tropical savannas as the global climate became drier in the Oligocene. Australia is now one of the driest continents in the world, and a survey of over a thousand grass species found that C4 grasses dominate 80–85% of the continent (Hattersley 1983). But even within Australia there are differences in pathway distribution, with C4 species being most numerous where the summer is hot and wet while C3 species are most common where the spring is cool and wet (extreme southern Australia and Tasmania). Studies along environmental gradients of temperature and precipitation have often highlighted differences between C3 and C4 species. For example, Tieszen et al. (1979) used an altitudinal gradient to show that the percent of grass species with the C3 pathway in African savannas increased with altitude as precipitation increased and temperature decreased. Cabido et al. (1997) found a similar pattern in the temperate grasslands of Argentina (Table 1).

Figure 7: The percentage of grass flora made up of C4 plants in the grasslands of North America

© 2010 Adapted from Teeri & Stowe 1976. All rights reserved.

C4 plant species also have a potential advantage over C3 species in low-nutrient habitats. Rubisco can account for 25–30% of leaf nitrogen in C3 plants, but C4 plants contain three to six times less rubisco. The higher affinity of PEP carboxylase for CO2 means that C4 plants don't need as much enzyme, so overall leaf nitrogen is often less than half that found in C3 plants. However, this reduction in leaf nitrogen does not result in lower maximal photosynthetic rates; thus, C4 plants have higher photosynthetic-nitrogen-use efficiency (ratio of photosynthesis to leaf nitrogen) than C3 plants. This gives them an advantage in high-light, low-nutrient environments.

Altitude (m)
350
600 1,000
1,400 1,600
1,800
1,900
2,100
% of C3 species 2.3 10.0 35.8
39.2
60.6 65.6 69.2 80.0 % of C4 species
97.7 90.0 64.2 60.8 38.2 34.4
30.8
20.0
Table 1: The percentage of C3 and C4 grass species at different elevations in central Argentina
Source: Cabido et al. 1997

Despite the advantages outlined above for C4 species, there are several disadvantages to this form of photosynthesis. For example, the regeneration of PEP in the C4 pathway (Figure 4) leads to higher costs for ATP than in the C3 pathway. ATP for photosynthetic biochemistry is supplied by the light reactions. The extra cost of C4 biochemistry is a disadvantage where low light occurs, as found in inner-canopy competitive situations in cool, wet forests. It is rare to find C4 plants in such environments.

CAM photosynthesis is found in more than 7% of vascular plant species, and has evolved independently several times. Due to their stomata being open at night when the vapor pressure differences between the leaf and the surrounding air are lowest (reducing transpiration), CAM photosynthetic plants have higher transpiration efficiencies than either C3 or C4 plants. Distribution patterns for CAM plants reflect this and are dominated by habitat aridity. Water storing cacti are adapted to enduring long periods with no precipitation. This is because they close their stomata, even at night during prolonged periods of drought and can refix CO2 lost in respiration before it diffuses out of the leaf or stem. Consequently, some cactus species lose very little biomass over months without rain.

An interesting aspect of CAM distribution is the high number of CAM species found in tropical environments. Epiphytes are plants that do not have their roots in the ground but instead grow on other plants (primarily trees). Thus, they frequently lack a ready supply of liquid water. Many epiphytic species in tropical and subtropical regions, such as orchids and bromeliads, therefore exhibit the CAM pathway.

A disadvantage for CAM plants is that they often have low photosynthetic capacity, slow growth, and low competitive abilities because their photosynthetic rates are limited by vacuolar storage capacity and by greater ATP costs, similar to those for C4 species. However, there is great plasticity in the expression of CAM photosynthesis. Many CAM plants can function in a C3 mode with stomata open during the day when water is available, so low photosynthetic and growth rates are not always limiting factors.

An interesting example that illustrates another advantage for the nighttime activity of CAM photosynthesis is found in aquatic plants such as Isoetes howellii. This plant is found in temporary vernal ponds in the western United States in which CO2 is depleted on warm, sunny days by other submerged aquatic plants. Since CO2 diffuses very slowly in water and its solubility decreases with increasing temperature, these habitats can easily become CO2 depleted. Isoetes howellii, with its CAM photosynthesis, takes advantage of higher CO2 concentrations in the water at night when other plants are releasing CO2 through respiration. In a similar fashion, the advantage of C4 photosynthesis in low CO2 conditions is illustrated by some submerged aquatic plants, such as Hydrilla verticillata. Hydrilla uses C4 photosynthesis to cope with the reduced CO2 concentrations that occur in warm, high-light aquatic environments in the summer.

It could be argued that CAM plants are the most efficient because they create the most amount of energy from the least amount of water. However, these plants won't do very well in a very wet environment because they're used to opening their stomata for only small amounts of water. It really all depends on the environment, each plant adapts to be the most efficent for where they are!

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