Agriculture Reference
In-Depth Information
In soils low in organic nitrogen trees commonly respond to fertilizer N.
Many such soils are found in areas of low rainfall and the N required is often
supplied in the water delivered through drip or trickle irrigation systems.
Uptake and transformations in the roots
Movement of nitrogen to the root surface is thought to be mainly by mass flow
together with water, with diffusion playing a smaller part (Neilsen and Neilsen,
). The uptake of both nitrate and ammonium is continuous throughout
the year in mild climates, with a relatively high peak during the summer (Titus
and Kang,
). This pattern, associated with leaf development, may indicate
a dependence of N uptake on transpiration. Organic N compounds such as
urea are also readily absorbed by apple roots.
Some nitrate, especially under conditions of high nitrate supply, may be
transported as such to the aerial tissues of apple and nitrate reductase has been
found in apple and pear leaves. Most nitrate is, however, normally reduced to
ammonium in the roots, with the greatest concentration of nitrogen reductase
in the fine roots. The processes of reduction of nitrate to nitrite and nitrite to
ammonium require energy from the respiration of carbohydrates.
Ammonium ions, whether produced by nitrate reduction or absorbed di-
rectly from ammonium fertilizers, are then metabolized to synthesize amino
acids. It is thought likely that, as in other higher plants, glutamine is the first
product of NH +
assimilation, catalysed by glutamine synthase (GS). Gluta-
mate is then formed from glutamine in the presence of glutamate synthase
(GOGAT) using
-oxoglutarate. Aspartate synthesis from glutamate and ox-
aloacetate is catalysed by glutamate-oxaloacetate transaminase (EC
).
Asparagine is produced by the transfer of amide N of glutamine to aspartate.
Arginine is synthesized from glutamate. These steps in synthesis are considered
in detail by Titus and Kang (
.
.
.
).
Upward movement and accumulation in leaves
Upward movement of nitrogenous compounds is apparently mainly in the
xylem. The nitrogen in the xylem sap consists of a range of amino acids,
with aspartate and glutamate and their amides making up to
% of total N
(Bollard,
,
; Hill-Cottingham and Bollard,
; Tromp and Ovaa,
). There is some evidence of radial movement of N compounds from the
xylem to the phloem but this is slower than the upward movement.
The leaves act as an active sink for nitrogenous compounds. The greater the
N supply the higher the proportion found in the leaves. Millard and Neilsen
(
% of the N content of unfertilized apple rootstocks to
be in the leaves, whereas
) found only
% were found in plants given progres-
sively more N. The proportion in roots showed the reverse pattern. Amino
% and
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