An investigation of aspects of the nitrogen physiology of Helianthus annuus L

Helianthus annuus L. plants were grown hydroponically in Long Ashton nutrient solutions containing either 2 mM ammonium, 2 mM nitrate or 1 mM ammonium+ 1 mM nitrate as the nitrogen supply to determine the effect of these nutrients upon physiological processes within the plant. Nitrate fed plants had a larger root mass than ammonium fed plants whereas ammonium fed plants had a larger shoot mass than those fed with nitrate. Ammonium+ nitrate fed plants produced both large shoots and roots. The fruit mass of plants fed nitrate was greater than plants fed ammonium. Ammonium+ nitrate fed plants produced a greater fruit mass than either of the other single nitrogen sources. Nutrient supplied at late stages of fruit filling was shown to be beneficial to the fruit. Sunflower plants supplied with ammonium+ nitrate made more efficient use of water than ammonium fed plants which produced 2.87g dry mass 1 - 1 water consumed while the mixed feed plants produced 4.15g 1- 1 H2 0 lost. Plants supplied with ammonium nutrient showed a lower net leaf photosynthetic rate than nitrate or ammonium+ nitrate fed plants. Root respiration expressed as CO2 exchange per gram fresh mass was significantly higher in both nitrate and ammonium+ nitrate fed plants than those fed with ammonium only. This indicated a higher requirement for ATP for the transport of nitrate into the root as well as the assimilation of part of that nitrate in the root in comparison to those fed ammonium only which would only require ATP for ammonium assimilation via glutamine synthetase. Analysis of the 15N free amino (+ amido) content of the xylem sap showed that the products of 15 NH 4 + contributed 80% to the xylem sap of plants fed ammonium+ nitrate while the products of 15N0 3 - constituted 20%. Of the total nitrogen content of the xylem sap, nitrate constituted 80% in the nitrate-only, 50% in the ammonium+ nitrate and 0% in the ammonium-only fed plants, whereas ammonium was no greater than 2% for any of the three nutrient sources. Glutamine was the principal free amino compound in the xylem sap for all three treatments. It would appear from these findings that sunflower plants fed ammonium+ nitrate, assimilate ammonium primarily in the roots while nitrate is assimilated in the leaves. Ammonium uptake was shown to be more rapid than that of nitrate, and it appeared to suppress nitrate uptake when the two nitrogen sources were combined. Nitrate appeared to aid in the translocation of the products of ammonium which was primarily assimilated in the root. The assimilation of organic compounds by mixed feed plants was greater than that for plants fed either nitrogen source alone. 15N0 3 - vacuum infiltration of mature leaves on 10 week old plants showed that the products of nitrate assimilation were transported mainly to the capitulum. Even at this late stage of fruit filling, the root was shown still to be processing nitrogen as 26% of the bound 15N in the plant was found in the root. Leaf nitrate reductase activity was 8 times that of the root while glutamine synthetase activity was 7 times, which indicated that the leaf was the major site of nitrate reduction. It was demonstrated that the presence of both casein and polyvinylpyrrolidone indicated that both proteases and phenolic compounds inhibit in vitro NRA and GSA in sunflower plants. Ammonium+ nitrate nutrient was shown to be more beneficial to the growth and fruit production of Helianthus annuus L. plants than either nitrogen source singly