137 Nitrate Assimilation and Carbon-14 Partitioning between Shoots and Roots of Tall Fescue

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Efficient use of nitrogen by turfgrasses depends on the ability of roots to absorb and assimilate nitrate. If a larger amount of nitrate is assimilated in the roots than in the shoots and organic N is transported to shoots as needed, nitrogen loss through clipping removal would be reduced. However, the ability of roots to assimilate nitrate depends on carbohydrate supply from the shoots. Our study examined the relationship between nitrate assimilation and photosynthate partitioning between shoots and roots of tall fescue grown in nutrient solution. To alter the pattern of nitrate reduction and photosynthate partitioning, we treated the plants as follows: 1) nutrient solution was aerated and nitrate was supplied to the roots, 2) nutrient solution was not aerated and nitrate was supplied to the roots, 3) nutrient solution was aerated and nitrogen was supplied to the leaves as nitrate, and 4) nutrient solution was aerated, and nitrogen was supplied to the leaves as urea. Photosynthate partitioning was assessed using carbon-14 as a tracer. Nitrate and nitrite reductase activities were determined by in vivo methods. Fortyeight hours after the grass leaves were exposed to carbon-14, >60% of the fixed carbon was translocated to stems and >15% to roots. Foliar application of urea resulted in less export of fixed carbon from leaves and lower leaf nitrite reductase activity than when nitrate was supplied to leaves. Less than 5% of the plant total nitrate reduction was attributed to root based activity. Root aeration decreased root nitrate reductase activity. Our results suggest that root-zone aeration and foliar N application could affect total nitrate assimilation and photosynthate partitioning to roots.