Abstract

<jats:p>
A gas-exchange system with a rapid response time was used to study the
interplay between rate-limiting processes of C3
photosynthesis in wild-type tobacco (Nicotiana tabacum
L. cv. W38) and transgenic tobaccos with antisense DNAs directed against the
Rubisco small subunit (anti-SSu plants) or the chloroplast
glyceraldehyde-3-phosphate dehydrogenase (anti-GAPDH plants). High
ribulosebisphos-phate (RuBP) pools were generated in leaves by exposing them
briefly to very low CO2, after which they were
transferred to varying CO2 concentrations, and transient
CO2 assimilation rates were measured within the first
2–3 s. Comparison of the transient (RuBP-saturated) and steady-state
rates confirmed that the CO2 assimilation rate in
anti-SSu plants was RuBP-saturated (i.e. Rubisco limited) at all intercellular
CO2 partial pressures
(Ci), and that, in anti-GAPDH
plants, the transition from RuBP-saturation to RuBP-limitation occurred at
lower assimilation rates and lower
Ci as GAPDH activity was
decreased. In addition, we investigated whether the integrated
post-illumination CO2 uptake could be used as a
non-destructive means of estimating RuBP pools in leaves. In wild-type plants
there was generally a good agreement between RuBP pools extracted from leaves
after rapid freeze-clamping and estimates made from post-illumination
CO2 uptake. However, in the anti-SSu plants, the post-
illumination CO2 uptake underestimated the actual RuBP
content and the discrepancy became larger as the Rubisco content decreased.
Possible explanations for this are discussed.</jats:p>