BotanyA plant in your garden avoids photorespiratory losses, has improved water use efficiency, shows high rates of photosynthesis at high temperatures and has improved efficiency of nitrogen utilisation. In which of the following physiological groups would you assign this plant?

A plant in your garden avoids photorespiratory losses, has improved water use efficiency, shows high rates of photosynthesis at high temperatures and has improved efficiency of nitrogen utilisation. In which of the following physiological groups would you assign this plant?

  1. A
    CAM
  2. B
    Nitrogen fixer
  3. C
    C3
  4. D
    C4

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    Solution:

    The C4 plants avoids photorespiratory losses, has improved water use efficiency, shows high rates of photosynthesis at high temperatures and has improved efficiency of nitrogen utilization.

    The principal effect of the C4 cycle is that it concentrates CO2 in the bundle-sheath cells where the enzymes of the PCR cycle are located. By shuttling the CO2 in the form of organic acids it is possible to build much higher CO2 concentrations in the bundle-sheath cells than would be possible relying on the diffusion of CO2 alone. Higher CO2 concentrations would suppress photorespiration and support higher rates of photosynthesis. Under optimal conditions, C4 crop species can assimilate CO2 at rates two to three times that of C3 species.

    In addition to the virtual absence of photorespiration, most C4 plants tend to have a higher temperature optimum (30–45ºC) than C3 plants (20–25ºC). This difference is due primarily to the differential stability of the photorespiration relative to photosynthesis.

    Photosynthesis in most situations is limited by available CO2 and water. In C3 plants, even moderate water stress will initiate closure of the stomata and reduce the available supply of CO2. The low CO2 compensation point of C4 plants means that they can maintain higher rates of photosynthesis at lower CO2 levels. Thus, C4 plants gain an advantage over C3 plants when the stomata are partially closed to conserve water during a period of water stress.

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