CO2 Fixation in Tropical Plants

Chloroplasts, like mitochondria, have their own DNA. Scientists speculate that these organelles were originally cyanobacteria that were engulfed by a eukaryotic cell through endosymbiosis (Section 1.8). Some interesting, even elaborate, interactions have evolved from the interplay of both nuclear and chloroplast genes. There are about 3000 chloroplast proteins, but 95% of them are encoded by nuclear genes. 

One of the interesting interactions involves rubisco, the principal enzyme of COg fixation. The large subunit of this enzyme is coded by a chloroplast gene, and the small subunit by a nuclear gene. Mechanisms not yet understood must coordinate the two syntheses to ensure equimolar production of both subunits. 

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What is different about CO2 fixation in tropical plants ... 

What is different about CO2 fixation in tropical plants In addition to the Galvin cycle, there is an alternative pathway for CO2 fixation in tropical plants. It is called the G4 pathway because it involves four carbon compounds. In this pathway, CO2 reacts in the outer (mesophyll) cells with phosphoenolpyruvate to produce oxaloacetate and Pj. Oxaloacetate in turn is reduced to malate. Malate is transported from mesophyll cells, where it is produced, to inner (bundle-sheath) cells, where it is ultimately passed to the Galvin cycle. Plants in which the C4 pathway operates grow more quickly and produce more biomass per unit of leaf area than G3 plants, in which only the Calvin cycle operates. 

In many plants that grow in the tropics (and in temperate-zone crop plants native to the tropics, such as maize, sugarcane, and sorghum) a mechanism has evolved to circumvent the problem of wasteful photorespiration. The step in which CO2 is fixed into a three-carbon product, 3-phosphoglycerate, is preceded by several steps, one of which is temporary fixation of CO2 into a four-carbon compound. Plants that use this process are referred to as C4 plants, and the assimilation process as C4 metabolism or the C4 pathway. Plants that use the carbon-assimilation method we have described thus far, in which the/ rs step is reaction of CO2 with ribulose... 

An additional phenomenon may also be important in the tropical forest C/P interaction. Humic molecules and organic acids actively compete with phosphorus for soil fixation sites. This means that increases in soil carbon density at higher [CO2] may serve to displace phosphate ions from sorption sites and into the soil solution, where they can then be utilized by plants. It is not inconceivable that this effect could give rise to a runaway positive feedback CO,-induced increases in tropical forest plant growth... 

In spite of the overall low productivity of high altitude habitats, mountain plants possess a highly efficient carbon fixation capacity in order to complete their life cycles in the short growing season of temperate climates. In fact, many alpine species are of the stress tolerant type (Jones Maberly, 2003) (see section 5.4). As reviewed above, the equivalent thermal restriction in the tropical high elevations would be the few hours of moderate temperature in each circadian cycle. Low temperature, high irradiance and low CO2 partial pressure (PCO2) -see below- all contribute to reduce carbon assimilation. 

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