Genotypic Variation in Transpiration Efficiency, Carbon-Isotype Discrimination and Carbon Allocation During Early Growth in Sunflower

Transpiration efficiency of dry matter production (W), carbon-isotope discrimination () and dry matter partitioning were measured on six sunflower (Helianthus annuus L.) genotypes grown for 32 days in a glasshouse. Two watering regimes, one well watered (HW) and the other delivering half the water used by the HW plants (LW), were imposed. Four major results emerged from this study. (1) There was significant genotypic variation in W in sunflower and this was closely reflected in Δ for both watering treatments. (2) The low watering regime caused a decrease in Δ but no change in W; nonetheless the genotypic ranking for either Δ or W was not significantly altered by water stress. (3) A positive correlation between W and biomass accumulation occurred among genotypes of HW plants. (4) Q, the ratio of total plant carbon content to leaf area, was positively correlated with W and negatively correlated with Δ. These results are discussed with reference to the connection between transpiration efficiency and plant growth. In short, Δ can be used to select for W among young vegetative sunflower plants. However, selection for W may be accompanied by changes in other important plant growth characteristics such as Q.