Leaf thickness and carbon isotope composition in the Crassulaceae

Measurements of leaf thickness and δ¹³C value were obtained for twenty species and three intergeneric hybrids of the Crassulaceae. The data include plants growing in their native habitats and also in greenhouse cultivation. There is a strong relationship between leaf thickness and leaf δ¹³C values. The plants with the thickest leaves of ca. 7 to 11 mm had δ¹³C values ranging from -11.5‰ to -13.8‰. Plants with leaves that were thinner than 2.0 mm all had δ¹³C values that were more negative than -23‰. Plants having intermediate leaf thickness possessed intermediate δ¹³C values. The leaf tissue of four genotypes spanning the range of leaf thicknesses all exhibited a two-fold or greater nocturnal increase in titratable acidity. It appears that the differences in leaf thickness and δ¹³C values among the tested species are genetically determined.