Question

C4 plants show poor rate od photosyn at ow temp due to

Answer 1

ABSTRACT
The photosynthetic performance of C4 plants is generally
inferior to that of C3 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 C4 photosynthesis at subop-
timal temperatures. Photosynthetic gas exchange, chloro-
phyll a fluorescence, and the in vitro activity of Rubisco
between 5 and 35

∞C were measured to examine the nature
of the low-temperature photosynthetic performance of the
co-occurring high latitude grasses, Muhlenbergia glomer-
ata (C4) and Calamogrostis canadensis (C3). Plants were
grown under cool (14/10

∞C) and warm (26/22

∞C) temper-
ature 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

mmol m-2 in M. glomerata
and C. canadensis, respectively, regardless of growth tem-
perature. 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

∞C was unaffected by a
90% reduction in O2 content, indicating photosynthetic
capacity was limited by the capacity of Pi-regeneration. By
contrast, the rate of photosynthesis in C. canadensis plants
grown at the cooler temperatures was stimulated 20–30%
by O2 reduction, indicating the Pi-regeneration limitation
was removed during low-temperature acclimation. In M.
glomerata, in vitro Rubisco activity and gross CO2 assimi-
lation rate were equivalent below 25

∞C, indicating that the
capacity of the enzyme is a major rate limiting step during
C4 photosynthesis at cool temperatures.

Answer 2

Explanation:

C4 plants are rare in the cool climates characteristic of high latitudes and elevations, but the reasons for this are unclear. We tested the hypothesis that CO2 fixation by Rubisco is the rate-limiting step during C4 photosynthesis at cool temperatures. We measured photosynthesis and chlorophyll fluorescence from 6°C to 40°C, and in vitro Rubisco and phosphoenolpyruvate carboxylase activity from 0°C to 42°C, in Flaveria bidentis modified by an antisense construct (targeted to the nuclear-encoded small subunit of Rubisco, anti-RbcS) to have 49% and 32% of the wild-type Rubisco content. Photosynthesis was reduced at all temperatures in the anti-Rbcs plants, but the thermal optimum for photosynthesis (35°C) did not differ. The in vitro turnover rate (kcat) of fully carbamylated Rubisco was 3.8 mol mol–1 s–1 at 24°C, regardless of genotype. The in vitro kcat (Rubisco Vcmax per catalytic site) and in vivo kcat (gross photosynthesis per Rubisco catalytic site) were the same below 20°C, but at warmer temperatures, the in vitro capacity of the enzyme exceeded the realized rate of photosynthesis. The quantum requirement of CO2 assimilation increased below 25°C in all genotypes, suggesting greater leakage of CO2 from the bundle sheath. The Rubisco flux control coefficient was 0.68 at the thermal optimum and increased to 0.99 at 6°C. Our results thus demonstrate that Rubisco capacity is a principle control over the rate of C4