Physiological and molecular basis for low nitrate (NO3-) assimilation in blueberry
Alt, Douglas Steven
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Blueberry (Vaccinium sp.) has recently emerged as a crop of significant economic value. The low ability of blueberry plants to assimilate the NO3- form of nitrogen appears to limit its production in non-native soils. To investigate the physiological and molecular limitations of NO3- assimilation, three different experiments were conducted using two types of blueberry, V. corymbosum ‘Sweetcrisp’ and V. ashei ‘Alapaha’: 1. Supplying NO3- to the roots via a hydroponic solution; 2. Supplying NO3- to the leaves via a foliar application; 3. Supplying NO3-to the stem by placing a cut stem in a NO3- solution. An ‘in situ’ nitrate reductase (NR) assay method was developed and used to quantify NO3- assimilation in previously frozen root and leaf samples. NO3- supplied to the roots induced a 3.1- and 1.8-fold (V. ashei and V. corymbosum, respectively) increase in root NR activity, but no additional NO3- was transported in the xylem, which resulted in no significant changes in leaf NR activity. Foliar applied and stem supplied NO3-, induced NR activity in the leaves by 2- to 4-fold over the controls. Expression of NO3- metabolism-related genes increased (up to10-fold for Nitrate REDUCTASE and 40-fold for NITRITE REDUCTASE) when NO3- was supplied to the cut end of the stem. These data indicate that NR activity is limited by transcript accumulation as well as by post-translational mechanisms. Overall, the data indicate that blueberries are capable of assimilating NO3- in the shoots albeit at relatively low levels. Root NO3- uptake and its partitioning to the shoots may be significant limitations to nitrate assimilation in blueberries.