Graduate Thesis Or Dissertation
 

Relationships of ammonium and nitrate status with inflorescence necrosis in 'Pinot noir' grapevines

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  • Inflorescence necrosis (IN) is a recently described disorder in grapes characterized by partial or complete deterioration of the pedicel and rachis near bloom. Elevated levels of ammonium in the rachis are associated with the incidence of IN and hypothesized as the cause of the disorder. An incubation system with single-node cuttings was developed to test this hypothesis, to determine the presence of the primary ammonium assimilation pathways, and to investigate the source of ammonium accumulation. Shading and various sources of nitrogen were applied to grapevines to determine their effects on IN severity, tissue ammonium, and nitrate status. Symptoms of IN were induced by incubating the base of single-node 'Pinot noir' cuttings, with a single leaf and cluster, in solutions of 120 mM or higher NH₄⁺. The addition of α-keto-glutarate prevented IN and reduced the concentration of ammonium in flowers and fruit to that of the control. Incubation of cuttings with inhibitors of the glutamine synthetase/ glutamate synthase (GS/GOGAT) pathway, 10 mM methionine sulfoxinine (MSX) or 5 mM azaserine (AS), increased the ammonium concentration in the lamina, pedicel, and flower or fruit, suggesting the presence of this pathway. Since MSX and AS had no effect on the concentration of ammonium in the petiole and rachis, GS/GOGAT activity may be low or absent in these tissues. Studies with cluster tissues indicated that glutamate dehydrogenase (GDH) was not an important pathway for ammonium detoxification nor where nitrate reduction or photorespiration major sources of ammonium. Shading (60%) two-year-old vines increased the concentration of ammonium and nitrate in the lamina, petiole, and rachis and enhanced the severity of IN. At one week post-anthesis, vines fed either NH₄⁺ or NO₃ had higher levels of ammonium in the rachis, petiole, fruit, and tendril than vines receiving no additional nitrogen. Neither nitrogen treatment caused an increase in IN. At all samplings, new roots of NH₄⁺ -fed vines had higher ammonium levels than those fed NO₃, whereas levels in shoots were elevated only after anthesis. Nitrate concentration in all sampled tissues was proportional to soil NO₃ except in the rachis, flowers, or fruit. This study suggests that 1) ammonium is the causal agent of IN, 2) the rachis and pedicel have a low capacity for ammonium assimilation and detoxification, and 3) nitrate reduction, photorespiration, and soil supplied nitrogen are not major sources of the elevated ammonium responsible for IN.
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