Photosynthesis and carbon partitioning in transgenic tobacco plants deficient in leaf cytosolic pyruvate kinase

Whole-plant diurnal C exchange analysis provided a noninvasive estimation of daily net C gain in transgenic tobacco (Nicotiana tabacum L.) plants deficient in leaf cytosolic pyruvate kinase (PKc-). PKc- plants cultivated under a low light intensity (100 mu mol m(-2) s(-1)) were previously shown to exhibit markedly reduced root growth, as well as delayed shoot and flower development when compared with plants having wild-type levels of PKc (PKc+). PKc- and PKc+ source leaves showed a similar net C gain, photosynthesis over a range of light intensities, and a capacity to export newly fixed (CO2)-C-14 during photosynthesis. However, during growth under low light the nighttime, export of previously fixed (CO2)-C-14 by fully expanded PKc- leaves was 40% lower, whereas concurrent respiratory (CO2)-C-14 evolution was 40% higher than that of PKc+ leaves. This provides a rationale for the reduced root growth of the PKc- plants grown at low irradiance. Leaf photosynthetic and export characteristics in PKc- and PKc+ plants raised in a greenhouse during winter months resembled those of plants grown in chambers at low irradiance. The data suggest that PKc in source leaves has a critical role in regulating nighttime respiration particularly when the available pool of photoassimilates for export and leaf respiratory processes are low.