Leaf characteristics and net gas exchange of diploid and autotetraploid citrus

The effect of tetraploidy on leaf characteristics and net gas exchange was studied in diploid (2x) and autotetraploid (4.x) 'Valencia' sweet orange (Citrus sinensis (L.) Osb.) and 'Femminello' lemon (Citrus limon (L.) Burm. f.) leaves. Comparisons between ploidy levels were made under high irradiance (I) in a growth chamber or low total I in a glasshouse. Tetraploids of both species had thicker leaves, larger mesophyll cell volume and lower light transmittance than diploids regardless of growth I. Mesophyll surface area per unit leaf area of 2x leaves was 5-15% greater than on 4.u leaves. Leaf thickness and mesophyll cell volume were greater in high I leaves than low I leaves. In high I, average leaf area was similar for 2x and 4x leaves, whereas in low I it was 30% greater in 4x than in 2x leaves. Nitrogen and chlorophyll concentration per cell increased with ploidy level in both growth conditions. The ratio of chlorophyll a:b was 25% greater in 2x than in 4x leaves. When net CO2 assimilation rate (A(CO2)) was based on leaf area, 4x orange leaves had 24-35% lower A(CO2) than their diploids. There were no significant differences in A(CO2) between 2x and 4x orange or lemon leaves when expressed on a per cell basis. Overall, lower A(CO2) per unit leaf area of tetraploids was related to increase in leaf thickness, larger mesophyll cell volume, the decrease in mesophyll area exposed to internal air spaces, and the lower ratio between cell surface to cell volume. Such changes probably increased the resistance to CO2 diffusion to the site of carboxylation in the chloroplasts. (C) 1997 Annals of Botany Company.