Relationship between genotype means of 13C isotope discrimination (soluble sugars (Δ¹³C_sugars), starch (Δ¹³C_starch) and bulk matter (Δ¹³C_bulk) and W_i for the phenotypes “Δ”and “N”. Open circles represent genotypes with the phenotype “low Δ”, open squares “low N”, closed circles “high Δ”, closed squares “high N”. The dash line represents the theoretical relationships (Sect. 2.2.)