Yoshida - 1981 - Fundamentals of Rice Crop Science

140 FUNDAMENTALS OF RICE CROP SCIENCE
3.7.3. Asparagine and glutamine
A wide range of free amino acids and amides is found in rice tissues. The kinds and
quantities of these free amino acids and amides depend on plant parts, growth
stages, nitrogen sources, climatic environments, and assay techniques. Two kinds
of free amide are normally present in rice leaf tissues: glutamine can be found
regardless of nitrogen nutrition while asparagine can be found only when the rice
plant is provided with ample nitrogen. The chemical formulas are:
COOH
CH(NH2)
COOH
CH(NH 2 )
CH 2 CH 2
CONH 2 CH 2
Asparagine CONH 2
Glutamine
Asparagine appears to serve for the temporary storage of excess nitrogen when
excessive ammonia has been absorbed. Hence, the presence or absence of
asparagine in leaf tissues may indicate the need for nitrogen. An asparagine test
using simplified paper chromatography has been proposed to determine the need
for nitrogen topdressing at panicle initiation (Ozaki 1955).
3.7.4. Nitrogen nutrition and carbohydrate metabolism
When absorbed into the roots, ammonia is usually combined with a -oxoglutarate
to produce glutamate. Glutamic dehydrogenase catalyses this reaction and contains
zinc. Once glutamate is formed, the amino group in glutamate can be
transferred to other oxoacids (keto-acids) by transamination.
Photosynthesis produces carbohydrates, some of which are translocated to the
roots. In rice, the amount of sucrose translocated per day is about 5–10% of the
root dry weight (Yoshida, T. 1968). This sucrose eventually provides
a -oxoglutarate and other oxoacids which, in turn, are combined with ammonia or
amino groups to form various amino acids. In this way nitrogen nutrition and
carbohydrate metabolism are interrelated.
When more ammonia is absorbed, more carbohydrate is consumed to
provide a -oxoglutarate. As a consequence, less carbohydrate is accumulated in
the plant. When ammonia absorption is decreased, surplus photosynthates
accumulate as starch and sugars, mainly in the leaf sheath and culm. Thus, a low
starch or carbohydrate content in the leaf sheath plus culm is associated with a high
nitrogen content and a high weight proportion of leaf blade to leaf sheath plus
culm. An example of this relationship is shown in Figure 3.12. When the rice plant
absorbs nitrogen actively at early growth stages, photosynthetic products are
preferentially used for protein synthesis and production of leaf blades. As a result,
the carbohydrate content of the leaf sheath plus culm tends to be lower. When