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

Abstract

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 (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.