Genetic variation for carbon isotope discrimination in sunflower: association with transpiration efficiency and evidence for cytoplasmic inheritance

Plants accumulate isotopes of carbon at different rates because of discrimination against [sup.13]C relative to [sup.12]C. In plants that fix carbon by the [C.sub.3] pathway, the amount of discrimination correlates negatively with transpiration efficiency (TE) where TE is the amount of dry matter accumulated per unit water transpired. Therefore, carbon isotope discrimination (Δ) has become a useful tool for selecting genotypes with improved TE and performance in dry environments. Surveys of 161 sunflower (Helianthus spp.) genotypes of diverse origin revealed a large and unprecedented range of genetic variation for A (19.5-23.8%). A strong negative genetic correlation ([r.sub.g]) between TE and Δ ([r.sub.g] = 0.87, P < 0.001) was observed in glasshouse studies. Gas exchange measurements of field grown plants indicated that Δ was strongly correlated with stomatal conductance to water vapor (g), ([r.sub.g] - 0.64, P < 0.01), and the ratio of net assimilation rate (Δ) to g,([r.sub.g]= 0.86, P < 0.001), an instantaneous measure of TE. Genotype CMSHA89MAX1 had the lowest TE (and highest Δ) of all genotypes tested in these studies and Inw yields in hybrid combination. Backcrossing studies showed that the TE of this genotype was due to an adverse effect of the MAX1 cytoplasm, which was inherited from the diploid perennial H. maximiliani Schradcr. Overall, these studies suggested that there is an excellent opportunity for breeders to develop sunflower germplasm with improved TE. This can be achieved, in part, by avoiding cytoplasms such as the MAX1 cytoplasm.
IN AUSTRALIA, the majority of sunflower is grown in the summer in dry subtropical environments ( [1] WUE = Tea/(1 + [EsT.sup.-1]) where TEa is the transpiration efficiency (above ground [...]