Leaf Photosynthesis and Respiration of High CO2-Grown Tobacco Plants Selected for Survival under CO2 Compensation Point Conditions

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Abstract
Four self-pollinated, doubled-haploid tobacco, (Nicotiana tabacum L.) lines (SP422, SP432, SP435, and SP451), selected as haploids by survival in a low CO(2) atmosphere, and the parental cv Wisconsin-38 were grown from seed in a growth room kept at high CO(2) levels (600-700 parts per million). The selected plants were much larger (especially SP422, SP432, and SP451) than Wisconsin-38 nine weeks after planting. The specific leaf dry weight and the carbon (but not nitrogen and sulfur) content 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 CO(2) assimilation rate per unit area measured in the growth room at high CO(2) was not higher in the selected plants. The CO(2) assimilation rate versus intercellular CO(2) curve and the CO(2) compensation point showed no substantial differences among the different lines, even though these plants were selected for survival under CO(2) compensation point conditions. Adult leaf respiration rates were 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 carbon content per unit area and were positively correlated with nitrogen and sulfur content of the dry matter. The alternative pathway was not involved in respiration in the dark in these leaves. The better carbon economy of tobacco lines selected for low CO(2) survival was not apparently related to an improvement of photosynthesis rate but could be related, at least partially, to a significantly reduced respiration (mainly cytochrome pathway) rate per unit carbon.