Quantitative Trait Correlations In a Vitex Breeding Program

Breeders generally attempt to improve a number of traits simultaneously.  If these traits are positively correlated, the response to selection will be more rapid than for characteristics selected separately. Trait incorporation may even be delayed for many generations if several important traits are negatively correlated. Furthermore, environmental differences can alter trait correlations, so evaluating traits and calculating correlations in more than one environment may be a useful aid in selection.  The objectives of this study were to assess trait correlations in two environments to determine whether each trait is independent of the other traits, and whether trait correlations will be the same in each environment.  Segregating populations of Vitex and their parents were cloned and grown in containers or in the field.  Traits evaluated included Cercospora leaf spot resistance, first flower date, last flower date, flowering duration (time from date of first flower to last flower), total weeks of flowering, average inflorescence number, average inflorescence length, average flower rating, plant height, and plant width. Flowering traits were measured weekly from the date of first flower until the date of first frost. Plant height and width were measured three times over the course of the study at three, 19, and 33 weeks after planting. Pearson correlation coefficients between traits were estimated for all populations. Values for trait correlations were designated as low (r≤0.49), moderate (0.50≤ r ≤ 0.69), and high (r≥0.70).  Most trait correlations were low to moderate in both environments. Correlations between width measurements taken 19 weeks after planting and width measurements taken 33 weeks after planting were high and positive in both environments.  Last flower date and total weeks of flowering had high negative correlations in both environments. Total weeks of flowering and average inflorescence number had high positive correlations in both environments. Average inflorescence number and last flower date were highly negatively correlated in the field with moderate negative correlation found in containers, while correlation between total weeks flowering and flowering duration was highly positive in containers and moderately positive in the field. Though degree of correlation varied between the container and field environments, significance and direction of correlation were similar in both environments.  Traits that show significant correlation may affect selection progress in Vitex.