Agriculture Reference
In-Depth Information
in the root cell membrane [103,104]. Nitrates are then reduced to nitrite
through the reaction catalysed by the enzyme nitrate reductase (NR; EC
1.6.6.1), [105] followed by the reduction of nitrite to ammonia catalysed
by the enzyme nitrite reductase (NiR; EC 1.7.7.1), [106]. Under particular
environments, root ammonia transporters [107] can allow a direct uptake
of ammonia when available in the soil, in rice paddy fields or in acidic
forest habitats [101,108]. Ammonia can be generated inside the plant by a
variety of metabolic pathways such as photorespiration, phenylpropanoid
metabolism, utilization of N transport compounds and amino acids catabo-
lism. Symbiotically fixed N is also an important source of ammonia read-
ily available to herbaceous plants or woody species that are able to form a
symbiotic relationship with N fixing microorganisms [87,109]; (Figure 2).
Several studies have shown that a wide variety of plant species are
able to take up organic N compounds, especially under low N conditions
[10,102,110-113]. However, the importance of this N source and the meth-
ods used to evaluate its contribution to plant N requirements has been
questioned. A few studies have been done on the uptake of organic N by
commercial crops: e.g., corn [114], agricultural grasses including species
of clover [112] and wheat [96]. Despite these limited studies, they demon-
strate the ability of plants to directly take up organic N, but have not es-
tablished the importance and signifi cance of organic N as a source of crop
N, for example when they are grown under organic farming conditions.
In line with the fi nding that plants can take organic N up directly, there is
also an interesting report in which it has been shown that herbaceous species
can use protein as a N source without the assistance of other organisms. This
indicates that the spectrum of N compounds that can be taken up by the roots
is quite diverse, indicating that the relationships existing between the soil fau-
na and the plant for N capture is more complex than originally thought [115].
Urea is a low molecular weight organic molecule containing N that exists
in natural systems and is also applied as a synthetic fertilizer in conventional
agriculture. It is well known that urea is absorbed as an intact molecule by
plant leaves and roots [116] by means of specifi c root transporters [117,118].
Although the use of urea is mainly as a source of N fertilizer, the contribu-
tion of plant urea uptake and metabolism in a physiological and agricultural
context is still not investigated. However, plants possess urea transporters,
and can hydrolyse and use urea very effi ciently [119].
