Low-temperature photosynthetic performance of a C4 grass and a co-occurring C3 grass native to high latitudes

The photosynthetic performance of C-4 plants is generally inferior to that of C-3 species at low temperatures, but the reasons for this are unclear. The present study investigated the hypothesis that the capacity of Rubisco, which largely reflects Rubisco content, limits C-4 photosynthesis at suboptimal temperatures. Photosynthetic gas exchange, chlorophyll a fluorescence, and the in vitro activity of Rubisco between 5 and 35 degreesC were measured to examine the nature of the low-temperature photosynthetic performance of the co-occurring high latitude grasses, Muhlenbergia glomerata (C-4) and Calamogrostis canadensis (C-3). Plants were grown under cool (14/10 degreesC) and warm (26/22 degreesC) temperature regimes to examine whether acclimation to cool temperature alters patterns of photosynthetic limitation. Low-temperature acclimation reduced photosynthetic rates in both species. The catalytic site concentration of Rubisco was approximately 5.0 and 20 mumol m(-2) in M. glomerata and C. canadensis, respectively, regardless of growth temperature. In both species, in vivo electron transport rates below the thermal optimum exceeded what was necessary to support photosynthesis. In warm-grown C. canadensis, the photosynthesis rate below 15 degreesC was unaffected by a 90% reduction in O-2 content, indicating photosynthetic capacity was limited by the capacity of P-i-regeneration. By contrast, the rate of photosynthesis in C. canadensis plants grown at the cooler temperatures was stimulated 20-30% by O-2 reduction, indicating the P-i-regeneration limitation was removed during low-temperature acclimation. In M. glomerata, in vitro Rubisco activity and gross CO2 assimilation rate were equivalent below 25 degreesC, indicating that the capacity of the enzyme is a major rate limiting step during C-4 photosynthesis at cool temperatures.