Genotypic variation in relative growth rate and carbon isotope discrimination in sunflower is related to photosynthetic capacity

A previous study showed that carbon isotope discrimination (Delta) was negatively related to transpiration efficiency (W, the ratio of dry matter accumulation to transpiration) and biomass accumulation amongst sunflower (Helianthus annuus L.) genotypes. Three experiments which explore the physiological basis of relationships between Delta, relative growth rate (RGR) and leaf gas exchange characteristics are reported. Growth was analysed on seven genotypes during the early vegetative stage (up to 31 days after emergence). Carbon isotope discrimination, Delta, W, and photosynthetic CO2 assimilation rate per unit leaf area (n) at ambient concentration of CO2 were measured on plants that made up the final harvest. Six of the seven genotypes were also grown under a low nitrogen (N) regime and harvested at 30 days after emergence. Carbon isotope discrimination was negatively related to relative growth rate (RGR), net assimilation rate (NAR) and photosynthetic rate per unit area, which in turn, were all positively correlated with each other. Genotypic variation in A (51%) was greater than that in leaf conductance, g (32%) and there was no relationship between g and Delta. Under the low N regime, A and g declined by 33 and 12%, respectively, across all genotypes producing a significant rise in Delta of 1.1 parts per thousand. In a second experiment, growth analysis on 13 cultivated forms of H. annuus, a wild accession and a H. argophyllus accession, also showed that there was a positive relationship between RGR and NAR, and that both growth indices were negatively related to Delta. In a further experiment, using 14 genotypes (including the wild accession) there was a highly significant negative correlation (r = -0.85, P < 0.001) between photosynthetic capacity, measured using: an oxygen electrode, and Delta. These results indicate that variation in photosynthetic capacity can account for genotypic variation in both Delta and RGR during vegetative growth in sunflower. Hence, faster growing sunflower genotypes have higher rates of photosynthesis and use water more efficiently.