Physiological and Molecular Basis for the Low Nitrate (NO3-) Assimilation in Blueberry

Blueberry (Vaccinium spp.) has recently emerged as a crop of significant economic value with multiple health benefits driving the increased demand. Blueberries require very specific soil conditions (pH < 5.5, high organic matter, and ammonium form of nitrogen) to maintain profitable fruit production. Increased acreage is forcing producers to plant blueberries in agricultural soils (pH > 6.0, low organic matter, and nitrate form of nitrogen), requiring costly soil amendments. In most woody perennials, nitrate is primarily assimilated in the shoot, yet blueberries have been shown to assimilate little to no nitrate in the shoots. A decreased ability to uptake and assimilate nitrate in the shoots may be a key factor for the strict soil requirements. Nitrate reduction to nitrite is the rate-limiting step requiring nitrate reductase (NR) to catalyze the process. Using different methods to supply nitrate to the shoots may induce NR activity indicating that the shoots can assimilate nitrate. 'Sweet Crisp' (V. corymboseum) and 'Alapaha' (V. ashei) were acclimated to only ammonium nitrogen using a deep-water culture hydroponic system. After three weeks, plants were supplied with nitrate nitrogen in one of the following three treatments: 5 mM nitrate supplied to the roots using the hydroponic system; 5 mM nitrate supplied directly through a cut stem; 5 mM nitrate supplied to the leave surface through a foliar application. Leaf samples were collected from the first treatment at 0, 5, 10, and 15 d after treatment.  Leaf samples from cut-stem and foliar applications were collected at 0, 24, 48, and 72 h after treatment. An in-vitro method using frozen leaf samples was used to quantify NR activity. Very low levels of NR activity were measured in the nitrate supplied to the roots, while the foliar applied nitrate significantly induced NR activity. These results indicate that blueberry shoots are capable of assimilating nitrate if nitrate is translocated to the shoots.