2018 ASHS Annual Conference

Soil pH is a major factor affecting horticultural productivity. Site selection and soil amendments are routinely used to provide optimum pH for cultivation. Nevertheless, some plants are capable of changing the pH in their rhizospheres. Plants acidify their rhizosphere by means of H+-ATPase-mediated proton extrusion (direct rhizosphere acidification) and/or as a consequence of nutrient uptake (nutrient uptake-dependent acidification). N uptake -in particular- can have large effects in rhizosphere pH. Nitrate uptake leads to rhizosphere alkalization, whereas ammonium uptake leads to rhizosphere acidification. Acid-loving plants (family Ericaceae) thrive in acidic soils where ammonium is the most prevalent form of N. Thus, these plants could exhibit both direct and nutrient uptake-dependent acidification. The relative importance of both of these processes for acid-loving plants is unknown. We hypothesized that ammonium uptake plays a greater role than H+-ATPase activity in the rhizosphere acidification of acid-loving plants. We grew one-year-old rooted cuttings of southern highbush blueberry ‘Emerald’ (SHB, Vaccinium corymbosum interspecific hybrids) in a split-root hydroponic system where each half of the root system was in a different reservoir. One reservoir was supplied with a buffered, complete nutrient solution containing 2.5 mM ammonium. The other reservoir was supplied with a non-buffered nutrient solution containing either 2.5 mM ammonium or no nitrogen. Plants were arranged in a completely randomized design with two treatments (NH4+ buffered/NH4+ non-buffered and NH4+ buffered/none). We measured rhizosphere pH, N uptake from solution, tissue N concentration, and root electrolyte leakage (REL). Additionally, we measured the expression of genes that encode glutamine synthethase (GS) and plasma membrane-bound H+-ATPases. We found that N uptake and tissue N concentrations were not different between plants in both treatments. Additionally, roots in all reservoirs exhibited similar REL, suggesting the split-root hydroponic system did not stress the root systems. Nutrient solution pH measurements indicated that SHB exhibits both direct and nutrient uptake-dependent rhizosphere acidification. Gene expression data supported this notion. Furthermore, ammonium uptake acidified the rhizosphere at a significantly higher rate than H+-ATPase activity. Altogether, these results suggest that nutrient uptake-related rhizosphere acidification plays a pivotal role in promoting adequate soil pH for cultivation of SHB and other acid-loving plants.