LEAF PHOTOSYNTHESIS AND RESPIRATION OF HIGH CO2-GROWN TOBACCO PLANTS SELECTED FOR SURVIVAL UNDER CO2 COMPENSATION POINT CONDITIONS

Four self-pollinated, doubled-haploid tobacco, (Nicotiana tabacum L.) lines (SP422, SP432, SP435, and SP451), selected as haploids by surviv al in a low CO2 atmosphere, and the parental cv Wisconsin-38 were grow n from seed in a growth room kept at high CO2 levels (600-700 parts pe r million). The selected plants were much larger (especially SP422, SP 432, and SP451) than Wisconsin-38 nine weeks after planting. The speci fic leaf dry weight and the carbon (but not nitrogen and sulfur) conte nt per unit area were also higher in the selected plants. However, the chlorophyll, carotenoid, and alkaloid contents and the chlorophyll a/ b ratio varied little. The net CO2 assimilation rate per unit area mea sured in the growth room at high CO2 was not higher in the selected pl ants. The CO2 assimilation rate versus intercellular CO2 curve and the CO2 compensation point showed no substantial differences among the di fferent lines, even though these plants were selected for survival und er CO2 compensation point conditions. Adult leaf respiration rates wer e similar when expressed per unit area but were lower in the selected lines when expressed per unit dry weight. Leaf respiration rates were negatively correlated with specific leaf dry weight and with the carbo n content per unit area and were positively correlated with nitrogen a nd sulfur content of the dry matter. The alternative pathway was not i nvolved in respiration in the dark in these leaves. The better carbon economy of tobacco lines selected for low CO2 survival was not apparen tly related to an improvement of photosynthesis rate but could be rela ted, at least partially, to a significantly reduced respiration (mainl y cytochrome pathway) rate per unit carbon.