PARENT SESSION

Symposium S8C C4 and CAM
Friday September 3rd, 2004 8:30 AM-10:30 AM Room 510B
Chair: Rowan Sage
Co-Chair: Howard Griffiths

Rubisco capacity and the photosynthetic performance of C₄ photosynthesis at sub-optimal temperatures. David Kubien^*,1, Rowan Sage², ¹ Institute of Molecular Biosciences, Palmerston North, New Zealand² Department of Botany, Toronto, ON, Canada

ABSTRACT- C₄ plants have several competitive advantages over C₃ vegetation, including greater resource use efficiency and higher rates of photosynthesis at warm (>25^oC) temperatures. However, C₄ plants are rare or absent in cool climate habitats, probably because their photosynthetic performance below 15-20^oC is inferior to that of C₃ species. A potential explanation for the poor performance of C₄ plants at low temperature is that their low Rubisco capacity limits CO₂ assimilation at sub-optimal temperatures. Used in this manner, capacity refers to the capability of fully-activated Rubisco to consume RuBP under a given set of environmental conditions, and as such largely reflects the amount of the enzyme. Several lines of evidence support this assertion. Recent work has shown that the maximum rate of carboxylation in vitro (Vcmax) and gross CO₂ uptake in vivo are equivalent below about 20^oC in a range of C₄ species. When anti-RbcS constructs are used to reduce Rubisco capacity in the C₄ dicot Flaveria bidentis, the equivalence of Vcmax and photosynthesis extends to warmer temperatures. When Rubisco capacity is reduced by 50% (relative to wild-type) in anti-RbcS F. bidentis, the rate of gross photosynthesis is reduced by 53% when measured at 10^oC, and by 22% at the thermal optimum(35^oC). Conversely, in anti-RbcS Nicotiana tabacum reducing Rubisco capacity leads to decreases in photosynthesis that are insensitive to measurement temperature. C₄ plants have considerably less Rubisco than C₃ species, and increasing the amount of the enzyme should enhance C₄ photosynthetic performance at cool temperatures. However, C₄ species rarely show this acclimation response, which may reflect biochemical or structural constraints that C₄ species are unable to overcome.

KEY WORDS: temperature, capacity, C₄ photosynthesis, Rubisco