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We grew three diploid (2X)  Citrus  rootstock seedlings and their autotetraploids (4X) at elevated CO 2  to obtain insights into limitations on growth and net gas exchange that have been associated with tetraploidy. Well-nourished Volkamer lemon (Volk), Troyer citrange (Troy), and Cleopatra mandarin (Cleo) were grown in greenhouses at ambient or twice ambient CO 2  for 3 months. We measured plant growth, water relations, mineral nutrition, and net gas exchange characteristics of leaves. Overall, tetraploid roots were thicker as 4X had lower root length: dry weight ratio or specific root length (SRL) than 2X roots. Tetraploid plants were smaller and had higher root/shoot ratios, shorter fibrous roots, and lower whole plant transpiration than 2X. Tetraploids also had lower leaf N and P concentrations on a dry weight basis. Since 4X leaves had thicker leaves (more dry weight per area) than 2X leaves, these nutrient differences disappeared when expressed on an leaf area basis. Elevated CO 2  increased plant growth but decreased leaf N, P, and K apparently by a growth dilution effect. Elevated CO 2  also increased fibrous root thickness, leaf thickness, and net assimilation of CO 2  (ACO 2 ) but decreased stomatal conductance and transpiration such that leaf water use efficiency increased. There was no effect of ploidy level on ACO 2  but 4X Volk and Troy had lower rates of ACO 2  than their diploids at elevated CO 2 . Hydraulic conductivity of intact root systems (measured in a pressure pot) was correlated to total plant growth but variability obscured effects of CO 2  or ploidy on root conductivity. The low SRL of tetraploids were correlated with lower rates of water use and lower leaf nutrient concentrations, which may be operative in determining the growth characteristics associated with tetraploidy.

590 Growth and Physiological Characteristics of Diploid and Tetraploid Citrus Rootstock Seedlings Grown at Elevated CO2