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 gro wn at elevated CO2 to obtain insights into limitations on growth and set ga s exchange that have been associated with tetraploidy, Well-nourished 2x an d 4x seedlings of 'Volkamer' lemon (Volk, C. volkameriana Ten & Pasq,), 'Tr oyer' citrange [Troy, C,sinensis (L,) Osbeck x Poncirus trifoliata (L,) Raf .] and 'Cleopatra' mandarin (Cleo, C. reticulata Blanco.), were grown In gr eenhouses at either ambient or twice ambient CO2 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. Fibrou s roots of 4x were thicker than 2x roots as indicated by a lower specific r oot length (SRL) in 4x than in 2x roots, Root hydraulic conductivity was co rrelated to total plant growth but there were no effects of CO2 or ploidy o n root conductivity. Tetraploid leaves had lower N concentrations than 2x l eaves when expressed on a dry weight basis but these differences disappeare d when N concentration was expressed on an leaf area basis because 4x leave s had more leaf dry weight per area (LDW/a) than 2x leaves, Plant growth wa s greater and SRL was lower at elevated CO2 than at ambient CO2. LDW concen trations of N, P, and K were lower at elevated CO2 than at ambient apparent ly due to a growth dilution effect. LDW/a, net CO2 assimilation (A(CO2)), a nd leaf water use efficiency were greater at elevated CO2 than at ambient. Overall, there was no effect of ploidy on A(CO2) but 4x Volk and Troy had l ower rates of A(CO2) than their 2x at elevated CO2. Net gas exchange of tet raploid leaves was less responsive to elevated CO2 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 determi ning the growth characteristics associated with tetraploidy.