The interplay between limiting processes in C-3 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 inte rplay between rate-limiting processes of C-3 photosynthesis in wild-type to bacco (Nicotiana tabacum L. cv. W38) and transgenic tobaccos with antisense DNAs directed against the Rubisco small subunit (anti-SSu plants) or the c hloroplast glyceraldehyde-3-phosphate dehydrogenase (anti-GAPDH plants). Hi gh ribulosebisphosphate (RuBP) pools were generated in leaves by exposing t hem briefly to very low CO2, after which they were transferred to varying C O2 concentrations, and transient CO2 assimilation rates were measured withi n 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 pre ssures (C-i), and that, in anti-GAPDH plants, the transition from RuBP-satu ration 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 ge nerally a good agreement between RuBP pools extracted from leaves after rap id freeze-clamping and estimates made from post-illumination CO2 uptake. Ho wever, in the anti-SSu plants, the post-illumination CO2 uptake underestima ted the actual RuBP content and the discrepancy became larger as the Rubisc o content decreased. Possible explanations for this are discussed.