PLANT PHYSIOLOGY , Vol 103, Issue 4 1183-1188, Copyright © 1993 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Estimation of Bundle Sheath Cell Conductance in C4 Species and O2 Insensitivity of Photosynthesis
R. H. Brown and G. T. Byrd
Department of Crop and Soil Sciences, University of Georgia, Athens, Georgia 30602-7272
Low conductance to CO2 of bundle sheath cells is required in C4
photosynthesis to maintain high [CO2] at the site of
ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Elevated [CO2]
allows high CO2 assimilation rates by this enzyme and prevents Rubisco
oxygenase activity and O2 inhibition of carboxylation. Bundle sheath
conductance to CO2 was estimated by chemically inhibiting
phosphoenolpyruvate carboxylase and calculating the slope of the linear
response of leaf CO2 uptake to [CO2]. The inhibitor
3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate was supplied to
detached leaves of Panicum maximum, Panicum miliaceum, and Sorghum bicolor
at 4 mM. Uptake of CO2 was measured at 210 mL L-1 O2 over the CO2
concentration range of 0.34 to 28 mL L-1. Without the inhibitor, CO2 uptake
increased steeply at low [CO2] and saturated at about 1 mL L-1. After
inhibition, CO2 uptake was a linear function of [CO2] over much of the
range tested. The slope of this CO2 response, taken as bundle sheath
conductance, was 2.35, 1.96, and 1.13 mmol m-2 s-1 for P. maximum, P.
miliaceum, and S. bicolor, respectively, on a leaf area basis. Conductance
based on bundle sheath area was 0.76, 0.93, and 0.54 mmol m-2 s-1,
respectively. Uptake of CO2 by leaves of P. maximum supplied with the
inhibitor was not affected by reduction of [O2] from 210 to 20 mL L-1 over
the range of [CO2] used. Because [CO2] in bundle sheath cells of inhibited
leaves is likely to be much lower than ambient, the lack of O2 sensitivity
of CO2 uptake cannot be ascribed to lack of O2 reaction with ribulose
bisphosphate and is probably due to the low conductance of bundle sheath
cells, especially at low ambient [CO2]. The likely result of reducing [O2]
from 210 to 20 mL L-1 is to stimulate carboxylation of ribulose
bisphosphate, thus further reducing [CO2] in bundle sheath cells and
increasing CO2 diffusion to these cells from the mesophyll. However, the
increase in diffusion is greatly limited by low conductance of the bundle
sheath cell walls. Calculations based on estimated bundle sheath
conductance show that changes in bundle sheath [CO2] of 0.085 to 0.5 mL
L-1, which might be associated with reduced [O2], would have a negligible
effect on CO2 uptake.
