On the performance of C4 photosynthesis at low temperatures and its relationship to the ecology of C4 in cool climates

The C4 pathway of photosynthesis occurs in fewer than 4% of plant species, but is often dominant in ecosystems that cover more than 35% of the earth's land area. C4 plants perform poorly under cool, non-freezing conditions, and these species are rare in cool climate habitats. This thesis examines hypotheses regarding the mechanistic basis for this feature of C4 plant ecology. C4 plants have been suggested to be more susceptible to photoinhibition at low temperatures than C3 species. Experiments with the high-latitude C4 grass Muhlenbergia glomerata and the co-occurring C3 species Calamogrostis canadensis indicate that there is no difference in the occurrence of photodamage between these cool-climate species. When grown at low temperatures the C4 plant increases its non-photosynthetic dissipation of light energy, and this dynamic photoinhibition is more detrimental to the carbon gain of M. glomerata than to the C3 species. A fundamental benefit of C4 photosynthesis is that high CO 2 assimilation rates are achieved with less Rubisco than in C3 plants. The in vitro capacity of Rubisco is equivalent to photosynthesis in M. glomerata below approximately 20°C, but exceeds photosynthesis above 5°C in C. canadensis. Using the C4 dicot Flaveria bidentis modified with an antisense construct to have reduced Rubisco (anti-RbcS), it is shown that the enzyme exerts nearly total control of C4 photosynthesis below 15°C, and that this control extends to approximately 25°C if the amount of the enzyme is reduced by 60%. The poor performance of C 4 photosynthesis at low temperatures is due to the lower amount of Rubisco in C4 versus C3 plants. This explanation is consistent with the need for increased photoprotection in response to low growth temperatures. Although their low Rubisco content impairs C4 photosynthesis at low temperatures, some C4 species are able to exploit cool climates. Contrary to expectations, Muhlenbergia glomerata is not restricted to the driest microsites in the fens of boreal Canada, where C4 photosynthesis should be most advantageous. This species is favoured only when flooding reduces the otherwise abundant C3 shrubs, indicating that factors affecting the ecology of cool-climate C4 plants are similar to those occurring in warm habitats.

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