LeafPhotosynthesis andRespiration ofHighCO2-Grown TobaccoPlants Selected forSurvival underCO2 Compensation Point Conditions1

Fourself-pollinated, doubled-haploid tobacco, (Nicotiana tabacumL.)lines (SP422, SP432, SP435, andSP451), selected as haploids bysurvival inalowCO2atmosphere, andtheparental cvWisconsin-38 weregrownfromseedinagrowth roomkeptat highCO2levels (600-700 parts permillion). Theselected plants weremuchlarger (especially SP422, SP432, andSP451) than Wisconsin-38 nineweeksafter planting. Thespecific leafdry weight andthecarbon (but notnitrogen andsulfur) content per unit areawerealsohigher intheselected plants. However, the chlorophyll, carotenoid, andalkaloid contents andthechlorophyll a/bratio varied little. ThenetCO2assimilation rateperunit area measured inthegrowth roomathighCO2wasnothigher inthe selected plants. TheCO2assimilation rateversus intercellular CO2 curveandtheCO2compensation point showednosubstantial differences amongthedifferent lines, eventhough theseplants wereselected forsurvival underCO2compensation point conditions. Adult leaf respiration rates weresimilar whenexpressed perunit areabutwerelower intheselected lines whenexpressed perunit dryweight. Leafrespiration rates werenegatively correlated withspecific leafdryweight andwiththecarbon content perunit areaandwerepositively correlated withnitrogen and sulfur content ofthedrymatter. Thealtemative pathway wasnot involved inrespiration inthedarkintheseleaves. Thebetter carbon economyoftobacco lines selected forlowCO2survival wasnotapparently related toanimprovement ofphotosynthesis ratebutcouldberelated, atleast partially, toa significantly reduced respiration (mainly cytochrome pathway) rateperunit carbon.