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Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Content,
Assimilatory Charge, and Mesophyll Conductance in Leaves1
Hillar Eichelmann and
Agu Laisk*
Tartu Ülikooli Molekulaar-ja Rakubioloogia Instituut, Riia tn
23, Tartu, 51010, Estonia
The content of
ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)
(Et; EC 4.1.1.39) measured in different-aged
leaves of sunflower (Helianthus annuus) and other plants
grown under different light intensities, varied from 2 to 75 µmol
active sites m 2. Mesophyll conductance (µ) was measured
under 1.5% O2, as well as postillumination CO2
uptake (assimilatory charge, a gas-exchange measure of the
ribulose-1,5-bisphosphate pool). The dependence of µ on
Et saturated at
Et = 30 µmol active sites m2
and µ = 11 mm s1 in high-light-grown leaves. In
low-light-grown leaves the dependence tended toward saturation at
similar Et but reached a µ of only 6 to 8 mm s1. µ was proportional to the assimilatory charge,
with the proportionality constant (specific carboxylation efficiency)
between 0.04 and 0.075 µM1
s1. Our data show that the saturation of the relationship
between Et and µ is caused by three
limiting components: (a) the physical diffusion resistance (a minor
limitation), (b) less than full activation of Rubisco (related to
Rubisco activase and the slower diffusibility of Rubisco at high
protein concentrations in the stroma), and (c) chloroplast metabolites,
especially 3-phosphoglyceric acid and free inorganic phosphate, which
control the reaction kinetics of ribulose-1,5-bisphosphate
carboxylation by competitive binding to active sites.
1
This work was supported by the Estonian Science
Foundation (grant no. 1808).
*
Corresponding author; e-mail alaisk{at}ut.ee; fax
372-7-477-250.
Plant Physiol. (1999) 119: 179-190
Copyright Clearance Center: 0032-0889/99/119//12
© 1999 American Society of Plant Physiologists
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