Analysis of the Other Half: Root Morphology of a Segregating Backcross Population between Vaccinium corymbosum and V. arboreum

The blueberry industry is based in the asexual propagation of outstanding genotypes by rooting of vegetative parts.  However, rooting percentage is often reduced when wild material has been introgressed.   In blueberry, high rooting percentage of softwood cuttings will ensure that a potential selection fits the standard propagation protocols necessary for commercialization. Vaccinium arboreum, commonly known as sparkleberry, is widely adapted to sandy Floridian soils and the root morphology as well as plant architecture are traits of interest in breeding with cultivated blueberry. Unfortunately, V. arboreum has had a very low rooting percentage using standard softwood cuttings for propagation.  Thus we established two experiments to evaluate rooting percentage and root morphology of a segregating backcross population between ‘Southern Belle’, a cultivated blueberry (V. corymbosum) and selection FL08-467, a hybrid between ‘Primadonna’ and V. arboreum selection FL06-753. ‘Southern Belle’ showed 90% rooting of softwood cuttings after eight weeks of establishment. FL06-753 and FL08-467 had 0% and 25% rooting, respectively, while 87 genotypes of the backcross population ranged from 0 to 100%. There was a positive correlation between leaf retention and rooting (R²=0.72). Root morphology analysis included root exploration depth and width 10 weeks after rooting of softwood cuttings, as well as root analysis by image acquisition and analysis using Winrhizo® software.  Attributes measured included root surface area, volume, length, and number of tips on different diameter categories, as well as number of links and average link length. Among seventy six genotypes of the backcross population, 17 % presented higher root density, length, surface area, and volume than the cultivated parent, while almost half of the population presented equal or higher values than the hybrid parent.  The data suggested that low softwood cutting rooting percentages as a result of using V. arboreum can be increased in just one generation of backcrossing.