Heritability of height, width, resin, rubber, and latex in guayule (Parthenium argentatum)

Plant breeding in guayule, a rubber and latex producing plant, has not been as effective as predicted or desired. A surprisingly large amount of variability has been reported in this apomictic crop for traits such as plant height, width, resin, rubber, and latex contents. This study was designed to calculate the proportion of the total measured variability due to the environment and the proportion due to genetic influences within and between three-released germplasm lines. Plant heights were measured at 1, 2 and 3-years-of-age; plant width, and latex contents at 2 and 3-years-of-age; and resin and rubber content at 2-years-of-age. Broad-sense-heritabilities were estimated for each trait and year by dividing the genotypic by the total variance. To estimate the genetic component of the measured variance for each trait, the environment effects (variance from clonally propagated plants) were subtracted from the total variance (variance from open-pollinated (OP) seed propagated plants). In general, the variances of the means for the measured traits were lower in the clonally propagated plants compared with the apomictic OP seed propagated plants. The heritability estimates calculated for each trait differed from year to year. For instance, heritability for plant height was estimated in line AZ-2 to be 0.84 at 1 year of age; 0.47 at 2 years of age; and 0.0 at 3 years of age. These values imply that a large portion of the observed variation in this line is attributed to genetic effects in the first 2 years of growth. As the plant grows over several seasons, the environment effects compound, masking the genetic effects, making effective selection choices more difficult. Heritability for latex content for the same line was estimated to be 0.97 for the second year and 0.55 for the third year. Selections for the measured traits in this study appear to be most effective during the first and second years of growth, with effectiveness diminishing during the third year. Most selections previous to this study were performed between 3 and 5 years of growth, thus suggesting one reason for the lack of significant progress in most breeding programs.