RESPONSE TO DIVERGENT SELECTION FOR EARLY VIGOR IN SUNFLOWER (HELIANTHUS-ANNUUS L.)

Early vigour (EV), biomass at the first development stages, may be a p ositive trait to improve sunflower yield in water-limited environments . Field experiments were carried out from 1992 to 1995 to define an in dex to quantify EV in sunflower plants, evaluate the response to diver gent selection for this trait, study EVxenvironment interaction, estim ate the heritability of EV, and study the association of EV with other traits. A sunflower population with a broad genetic base and 10 sunfl ower lines derived from it were used. Stem volume calculated from stem diameter and height measured approximately 425 degree-days after emer gence was correlated with above-ground plant biomass at the same date. In 1992, divergent selection for high and low stem volume was carried out on this population and plants from every group were recombined in dependently, obtaining 2 new populations. Selection response, using th ese and the initial population, was studied in 1993. Ten S2 lines, 5 w ith high-EV derived from the high-EV population and 5 with low-EV deri ved from the low EV population, were sown in 1995 to study the EVxenvi ronment interaction Differences among lines with different EV were sig nificant in all the environments studied. In 1993, a random sample fro m the initial population was selfed and broad sense heritability and h eritability by parent-offspring regression were estimated. Heritabilit y based on variance component estimates of S-1 progeny means was 0.61. Realised heritability (+/- s.e.) from a divergent selection for high and low stem volume were 0.40+/-0.10 and 0.52+/-0.15, respectively. He ritability by parent-offspring regression was 0.40+/-0.10. These value s and the high variability of stem volume observed in the new populati ons indicate that additional gain from selection should be possible.