A Need for Speed: High Throughput Phenotyping for Rhododendron pH Adaptability

The genus Rhododendron (Ericaceae), comprising over 1,000 species, has long been valued for its showy ornamental characteristics.  Unfortunately, most cultivars lack any tolerance to neutral or high pH soils common in many parts of North America.  Genetic variation for pH tolerance among cultivars or wild populations could improve the adaptability of this genus to alkaline soils where it is traditionally not well suited.   We present a novel, colorimetric method using a MATLAB script (MathWorks, Inc.) to measure rhizosphere acidification, a candidate trait for pH tolerance, among rhododendron full and half-sib families.  Half-sib progeny were obtained from an open pollinated R. catawbiense (elevated pH intolerant) plant, while full sib progeny resulted from a cross between ‘Haaga’ (elevated pH intolerant) and INKARHO® (reportedly elevated pH tolerant).  All seedlings were grown on half-strength woody plant medium for three months as root systems developed. Mature seedlings were then subcultured on media with an elevated pH (7.75) and screened over a two-week period. The pH indicator phenolsulfonphthalein was incorporated into the screening media with rhizosphere acidification quantified by measuring the change in hue.  Measurements were taken on a weekly basis using a digital camera and the MATLAB script mentioned above.  Root mass was weakly correlated with increased rhizosphere acidification among the ‘Haaga’ and INKARHO® full-sibs (R²=0.37), while more strongly correlated among the R. catawbiense open pollinated half-sibs (R²=0.72). Using linear models from one population to predict mean acidification in the other based on root mass data alone, predicted values were significantly different from those observed (P-values of 0.04 and 0.002 for full and half sibs, respectively). The significant differences between these two progeny sets for mean rhizosphere acidification and differing variation explained by root mass indicate a potential genetic basis for this trait, which could be used to maximize the adaptability of this valuable genus.