Nitrogen-source preference in blueberry (Vaccinium sp.): Enhanced shoot nitrogen assimilation in response to direct...

Blueberry (Vaccinium sp.) is thought to display a preference for the ammonium (NH₄⁺) form over the nitrate (NO₃^-) form of inorganic nitrogen (N). This N-source preference has been associated with a generally low capacity to assimilate the NO₃^- form of N, especially within the shoot tissues. Nitrate assimilation is mediated by nitrate reductase (NR), a rate limiting enzyme that converts NO₃^- to nitrite (NO₂^-). We investigated potential limitations of NO₃^- assimilation in two blueberry species, rabbiteye (Vaccinium ashei) and southern highbush (Vaccinium corymbosum) by supplying NO₃^- to the roots, leaf surface, or through the cut stem. Both species displayed relatively low but similar root uptake rates for both forms of inorganic N. Nitrate uptake through the roots transiently increased NR activity by up to 3.3-fold and root NR gene expression by up to 4-fold. However, supplying NO₃^- to the roots did not increase its transport in the xylem, nor did it increase NR activity in the leaves, indicating that the acquired N was largely assimilated or stored within the roots. Foliar application of NO₃^- increased leaf NR activity by up to 3.5-fold, but did not alter NO₃^- metabolism-related gene expression, suggesting that blueberries are capable of post translational regulation of NR activity in the shoots. Additionally, supplying NO₃^- to the cut ends of stems resulted in around a 5-fold increase in NR activity, a 10-fold increase in NR transcript accumulation, and up to a 195-fold increase in transcript accumulation of NITRITE REDUCTASE (NiR1) which codes for the enzyme catalyzing the conversion of NO₂^- to NH₄⁺. These data indicate that blueberry shoots are capable of assimilating NO₃^- when it is directly supplied to these tissues. Together, these data suggest that limitations in the uptake and translocation of NO₃^- to the shoots may limit overall NO₃^- assimilation capacity in blueberry.