Yoshida - 1981 - Fundamentals of Rice Crop Science

138 FUNDAMENTALS OF RICE CROP SCIENCE
Plants
Plant
parts
concn in
culture
solution
(ppm)
Table 3.16. Free ammonia content in tissues of rice and
cucumber 0–7 days after treatment with ammonium nitrogen in
culture solution. a NH 4 -N
Free ammonia
(mg N/g fresh wt)
0 day 3 days 5 days 7 days
Rice
Cucumber
Roots 20 0.03 0.04 0.03 0.04
200 0.03 0.14 0.18 0.28
Leaves 20 0.02 0.05 0.03 0.04
200 0.02 0.14 0.10 0.17
Roots 20 0.00 0.01 0.04 0.02
200 0.00 0.08 0.15 0.11
Leaves 20 0.02 0.04 0.01 0.02
200 0.02 0.29 0.35 0.53
a Oji and lzawa (1974).
On the other hand, rice tissues accumulate nitrate when the nitrate concentration
of the solution is high, whereas cucumber tissues do not (Table 3.17). That
suggests that rice has a lower capacity than cucumber to reduce nitrate to
ammonia. This capacity may limit the over-all rate of nitrogen assimilation and,
hence, growth in rice.
In bioenergetics, the assimilation of nitrate requires more energy than that of
ammonia because nitrate must be reduced to ammonia. Stoichiometric relationships
for protein synthesis from both ammonia and nitrate can be given by the
following equations (Penning de Vries and van Laar 1977):
(3.19)
(3.20)
In higher plants, including rice, most of the nitrate reduction occurs in green
leaves under light. At high light intensities, where the rate of carbon dioxide
diffusion limits the over-all rate of photosynthesis, the energy required for the
nitrate reduction can be supplied by surplus energy produced by the photochemical
reaction in photosynthesis (see Chapter 5; Penning de Vries 1975b). In this case,
the nitrate reduction proceeds free of cost, without consuming assimilates to
generate the required energy. At low light intensities, however, the CO 2 reduction
(dark reaction) and nitrate reduction are probably competitive (Penning de Vries