Estimation of the whole‐plant CO2compensation point of tobacco (Nicotiana tabacumL.)

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AbstractThe whole‐plant CO2compensation point (Γplant) is the minimum atmospheric CO2level required for sustained growth. The minimum CO2requirement for growth is critical to understanding biosphere feedbacks on the carbon cycle during low CO2episodes; however, actual values of Γplantremain difficult to calculate. Here, we have estimated Γplantin tobacco by measuring the relative leaf expansion rate at several low levels of atmospheric CO2, and then extrapolating the leaf growth vs. CO2response to estimate CO2levels where no growth occurs. Plants were grown under three temperature treatments, 19/15, 25/20 and 30/25°C day/night, and at CO2levels of 100, 150, 190 and 270 μmol CO2 mol−1 air. Biomass declined with growth CO2such that Γplantwas estimated to be approximately 65 μmol mol−1for plants grown at 19/15 and 30/25°C. In the first 19 days after germination, plants grown at 100 μmol mol−1had low growth rates, such that most remained as tiny seedlings (canopy size <1 cm2). Most seedlings grown at 150 μmol mol−1and 30/25°C also failed to grow beyond the small seedling size by day 19. Plants in all other treatments grew beyond the small seedling size within 3 weeks of planting. Given sufficient time (16 weeks after planting) plants at 100 μmol mol−1eventually reached a robust size and produced an abundance of viable seed. Photosynthetic acclimation did not increase Rubisco content at low CO2. Instead, Rubisco levels were unchanged except at the 100 and 150 μmol mol−1where they declined. Chlorophyll content and leaf weight per area declined in the same proportion as Rubisco, indicating that leaves became less expensive to produce. From these results, we conclude that the effects of very low CO2are most severe during seedling establishment, in large part because CO2deficiency slows the emergence and expansion of new leaves. Once sufficient leaf area is produced, plants enter the exponential growth phase and acquire sufficient carbon to complete their life cycle, even under warm conditions (30/25°C) and CO2levels as low as 100 μmol mol−1.