High pH Stress Affects Root Morphology and Nutritional Status of Hydroponically Grown Rhododendron (Rhododendron spp.)

Rhododendrons (Rhododendron spp.) are ornamental plants in the family Ericaceae that thrive in acidic soils and are challenged by neutral or alkaline soils. This soil requirement limits the locations where rhododendrons can be grown and causes chlorosis, diminished growth, and low survival when rhododendrons are grown in high pH soils. While growth and survival impacts are widely documented, little is known about how high pH soils cause these symptoms in rhododendrons. We hypothesized that high pH stress impacts root form and function, leading to nutrient deficiencies that limit plant growth. We tested this hypothesis in a hydroponic experiment. "Mardi Gras" rhododendron liners were grown in a complete nutrient solution at pH 5.5 (optimum pH) or pH 6.5 (high pH) for 49 days. Biomass accumulation, nutrient uptake and concentration, and root stress were assessed. High pH nutrient solutions diminished leaf and root growth. Plants grown in high pH nutrient solutions developed clusters of short, highly branched roots. Plants grown in optimum pH did not exhibit this morphology. High pH affected the uptake and translocation of most essential nutrients. S and Mn deficiencies likely limited plant growth. High pH had a nuanced effect on root oxidative status. These results suggest that rhododendron root morphology and nutrient uptake are directly affected by high pH and that aboveground symptoms might be a consequence of impaired root function.