Tobacco plants (Nicotiana tabacum L.) transformed with an inverted cDNA encoding ribulose 5-phosphate kinase (phosphoribulokinase,PRK; EC 2.7.1.19) were employed to study the in vivo relationship between photosynthetic electron transport and the partitioning of electron transport products to major carbon metabolism sinks under conditions of elevated ATP concentrations and limited ribulose 1,5-bisphosphate (RuBP) regeneration. Simultaneous measurements of room temperature chlorophyll fluorescence and CO2 gas exchange were conducted on intact leaves. Under ambient CO2 concentrations and light intensities above those at which the plants were grown, transformants with only 5% of PRK activity showed 'down-regulation' of PS II activity and electron transport in response to a decrease in net carbon assimilation when compared to wild-type. This was manifested as a decline in the efficiency of PS II electron transport (Β’PS 1]), an increase in dissipation of excess absorbed light in the antennae of PS II and a decline in : total linear electron transport (J1), electron transport dedicated to carbon assimilation (JA) and electron transport allocated to photorespiration (JL). The transformants showed no alteration in the Rubisco specificity factor measured in vitro and calculated in vivo but had a relatively smaller ratio of RuBP oxygenation to carboxylation rates (Vo/Vc), due to a higher CO2 concentration at the carboxylation site (Cc). The relationship between Ops n and Β’co2 was similar in transformants and wild-type under photorespiratory conditions demonstrating no change in the intrinsic relationship between PS II function and carbon assimilation, however, a novel result of this study is that this similar relationship occurred at different values of quantum flux, J1, JA, JL and Vo/Vc in the transformant. For both wild-type and transformants, an assessment was made of the possible presence of a third major sink for electron transport products, beside RuBP oxygenation and carboxylation, the data provided no evidence for such a sink.
Abbreviations: Co-CO2 concentration at the site of carboxylation; Ci-intercellular CO2 concentration; gmmesophyll conductance to CO2; J1 -total linear electron flow; JA-linear electron flow allocated to COz assimilation; Jc -linear electron flow supporting carbon reduction and oxidation cycles; JL -linear electron flow allocated to photorespiration (RuBP oxygenation and fixation of released photorespiratory CO2); PRK-phosphoribulokinase; qp, qN-coefficients for photochemical and non-photochemical quenching of fluorescence respectively; Rubisc o -ribulose 1,5-bisphosphate carboxylase-oxygenase; S -Rubisco specificity to CO2/O2; Vc, Vo -rates of RuBP carboxylation and RuBP oxygenation, respectively; ~co2-relative quantum yield of C O 2 assimilation; ~cmaximum Oco: under non-photorespiratory conditions; qbexc-the efficiency of excitation capture by open PS II centres; qbps n -relative quantum yield of PS II electron transport