The interplay between limiting processes in C3 photosynthesis studied by rapid-response gas exchange using transgenic tobacco impaired in photosynthesis

A gas-exchange system with a rapid response time was used to study the interplay between rate-limiting processes of C-3 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 ribulosebisphosphate (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 (C-i), and that, in anti-GAPDH plants, the transition from RuBP-saturation to RuBP-limitation occurred at lower assimilation rates and lower C-i 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.