- 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.