Botany 4400/5400

Lecture 17

20 February 2006

Reading: Chapter 8, Taiz and Zeiger's Plant Physiology


II. Metabolism

C. Photosynthesis

5. C4 photosynthesis

f. Evolution of C4 photosynthesis and geographic distribution of C4 plants
C4 photosynthesis appears to be a recent evolutionary innovation in land plants, possibly driven by climate changes. The distribution of C4 plants on Earth of the present is also instructive about the adaptive value of C4 metabolism.

i. Evolution of C4

Fossil evidence indicates that flowering plants first appeared during the Cretaceous period, 65 to 144 million years ago. At that time, the CO2 concentration of the atmosphere is believed to have been about 0.3%, roughly 10 times the current level. At such high [CO2], the oxygenation reaction of RUBISCO would not occur very often. Plants with large bundle sheath cells, like modern C4, first appeared in the fossil record between 60 and 20 million years ago. From 60 to 20 million years ago, the [CO2] of the atmosphere is believed to have dropped to 0.025%, the level of 200 years ago. Photorespiration causes C3 photosynthesis to be inefficient at such low CO2 concentrations and this may have favored early C4 plants. Other evidence indicating the appearance of C4 photosynthesis is provided by stable isotopes of carbon. Many enzymes discriminate between 12C and 13C, enriching their products and compounds derived from their products in 12C. PEPCase discriminates against 13C less than RUBISCO does so compounds derived from C4 plant material can be distinguished from compounds derived from C3 plant material by their higher ratio of 13C to 12C. Applying this analysis to the enamel of fossil horse teeth from North America and central Asia, it is possible to see a distinct increase in the ratio of 13C to 12C about 7 million years ago, suggesting that C4 grasses became dominant at that time in those areas. (See Web topic 9.5 associated with your text for more information). A short overview of C4 evolution can be found in Edwards et al. 2001.

At present, roughly 1% of all plant species but 1/2 of all grass species are C4. C4 plants are present in 19 different plant families, 3 moncot and 16 dicot. No plant family has exclusively C4 plants, however. This implies that C4 photosynthesis evolved independently as often as 19 times over the past 50 million years. If so, such recurring evolution of the C4 path suggests two things:

1) The selective advantage of C4 photosynthesis is high.

2) The number of mutations necessary to convert a C3 plant into a C4 plant is relatively few and they can occur without compromising the survival of the mutant.

There is actually great interest in the topic of C4 evolution because it may provide insights useful for genetically modifying a C3 crop plant such, as rice, to be C4, with increased productivity and less need for water, increasing the sustainability of rice agriculture.s

ii. Present distribution of C4

C4 plants are more common in warm, dry environments. This is reasonable considering the facts that photorespiration increases in C3 plants as temperature increases and that C4 plants are more efficient in water use.

 As an example of this distribution, the percentage of grass species that are C4 in central Canada is 12% while the percentage of grass species in Arizona and central Florida is about 80%.

g. Crassulacean Acid Metabolism (CAM)

Some plants use a variation of C4 photosynthesis to help them conserve water. These plants open their stomata at night, when it is cool and moist, and fix CO2 into malate using PEPCase. The malate is stored in the vacuole until morning. During the day, these plants close their stomata to conserve water and decarboxylate the stored malate as a source of CO2 for the Calvin cycle. These "CAM" plants grow slowly because the capacity of the vacuole to store malate is limited. They can grow in very dry environments, however, because of their extremely high water use efficiency. Most cacti and succulents are CAM plants.

Water use efficiencies for C3, C4, ad CAM plants:

C3 = 400 to 500 g water lost / gram CO2 fixed

C4= 250 to 300 g water lost / gram CO2 fixed.

CAM = 50 to 100 g water lost / gram CO2 fixed.

II. Metabolism

D. Respiration

The dark reactions of photosynthesis use ATP and reductant from the light reactions to build The dark reactions of photosynthesis use ATP and reductant from the light reactions to build CO2 into organic molecules. Respiration is the opposite process. Respiration is the oxidation of organic molecules back into CO2 for the purpose of generating reductant and ATP. In addition to making reductant and ATP, respiration generates carbon compounds that are needed for syntheses of amino acids, nucleic acids, and secondary compounds.

Respiration occurs in the cytosol and mitochondria of all plant cells. It is often referred to as "dark respiration" to distinguish it from photorespiration, which is a totally different process. In the case of non-photosynthetic cells of plants, such as those in the roots, the carbohydrates oxidized by respiration come from photosynthetic tissues via the phloem. In the case of photosynthetic tissues, the carbohydrates oxidized by glycolysis come directly from the chloroplasts.

Respiration consists of three parts:
1) Glycolysis. This part of respiration occurs in the cytosol. It converts sucrose, glucose, or triose phosphates into pyruvate and generates some ATP and reductant (NADH). It requires no oxygen. The ATP generated by glycolysis is by "substrate-level phosphorylation", which is a reaction that phosphorylates ADP but does not involve the ATP synthase or chemiosmosis.

2) The TCA cycle. Also known as the Krebs cycle, this part of respiration occurs in the matrix of the mitochondrion. It oxidizes the pyruvate from glycolysis to CO2 and generates reductant (NADH). The TCA cycle also genertes a small amount of ATP by substrate-level phosphorylation. TCA stand for "tricarboxylic acid cycle", since three carboxylic acids are part of the cycle.

3) Respiratory electron transport (RET). This is the third part of respiration. It uses the reductant (NADH) from the Krebs cycle as a source of electrons. RET oxidizes NADH, passes the electrons between protein complexes in the inner mitochondrial membrane, and ultimately reduces O2, yielding water. Respiratory electron transport makes ATP by chemiosmosis, generating a proton gradient that is used by ATP synthases to phosphorylate ATP.