Limitations on Growth and Net Gas Exchange of Diploid and Tetraploid Citrus Rootstock Cultivars Grown at Elevated CO2

Diploid (2x) and autotetraploid (4x) Citrus L. rootstock cultivars were grown at elevated CO 2 to obtain insights into limitations on growth and net gas exchange that have been associated with tetraploidy. Well-nourished 2x and 4x seedlings of `Volkamer' lemon (Volk, C. volkameriana Ten & Pasq.), `Troyer' citrange [Troy, C. sinensis (L.) Osbeck × Poncirus trifoliata (L.) Raf.] and `Cleopatra' mandarin (Cleo, C. reticulata Blanco.), were grown in greenhouses at either ambient or twice ambient CO 2 for 4 months. Plant growth, water relations, mineral nutrition, and net gas exchange characteristics of leaves were measured. Most 4x plants were smaller and had lower rates of whole plant transpiration but shorter fibrous roots than 2x plants. Fibrous roots of 4x were thicker than 2x roots as indicated by a lower specific root length (SRL) in 4x than in 2x roots. Root hydraulic conductivity was correlated to total plant growth but there were no effects of CO 2 or ploidy on root conductivity. Tetraploid leaves had lower N concentrations than 2x leaves when expressed on a dry weight basis but these differences disappeared when N concentration was expressed on an leaf area basis because 4x leaves had more leaf dry weight per area (LDW/a) than 2x leaves. Plant growth was greater and SRL was lower at elevated CO 2 than at ambient CO 2 . LDW concentrations of N, P, and K were lower at elevated CO 2 than at ambient apparently due to a growth dilution effect. LDW/a, net CO 2 assimilation (A CO2 ), and leaf water use efficiency were greater at elevated CO 2 than at ambient. Overall, there was no effect of ploidy on A CO2 but 4x Volk and Troy had lower rates of A CO2 than their 2x at elevated CO 2 . Net gas exchange of tetraploid leaves was less responsive to elevated CO 2 than 2x leaves. The low SRL of tetraploids was correlated with low whole plant transpiration rates and low leaf area-based N concentrations, which may be operative in determining the growth characteristics associated with tetraploidy.