Yoshida - 1981 - Fundamentals of Rice Crop Science

208 FUNDAMENTALS OF RICE CROP SCIENCE
When a plant is placed in darkness for 48 hours, it uses up its substrate and
eventually stops growing. At this point, the respiration is due to maintenance.
The equation 5.18 also indicates that when photosynthates are used to produce
new tissues for growth, the maximum conversion efficiency would be 75%. This
75% efficiency is very close to the 70% growth efficiency when only growth
respiration is considered.
When a plant is young and growing actively, growth respiration is the major
component of total respiration. With a mature plant, however, maintenance
becomes a substantial fraction of total respiration. It has not been determined yet if
all of the maintenance respiration is really needed or if it includes a wasteful
consumption of photosynthetic products. Growth respiration is certainly useful
respiration and cannot be reduced unless the efficiency of biochemical conversion
is increased.
5.2.4. Growth and maintenance respiration — biochemical approach
Growth is a process during which a substrate is converted into new chemical
compounds that constitute new tissues. The substrate must be converted into
carbon-skeletons appropriate for the biosynthesis of new compounds; it must be
consumed to provide energy (ATP) and hydrogen (NADH 2) for biosynthesis.
These considerations led Penning de Vries and his associates (1974) to define
production value as:
wt of the end-product
Production value =
(g/g). (5.19)
wt of substrate required for C-
skeletons and energy production
Production value indicates the weight (g) of new organs (growth) that can be
obtained from 1 g of substrate. Note that this definition is identical with growth
efficiency discussed earlier (see equation 5.14). If the substrate’s chemical composition
and the end product are known, and if the biochemical reactions involved
in synthesizing end product constituents are known, it is possible to calculate the
production value for a given system (Penning de Vries et al 1974). One useful set
of data on production values is shown in Table 5.5. It must be noted that these
results are independent of temperature and plant species, and are determined only
by the composition of the substrate and end product.
The data indicate:
• 1.00 g glucose produces 0.83 g carbohydrates but only 0.33 g lipids.
• The production value for nitrogenous compounds varies with nitrogen
source; 1.00 g glucose produces 0.62 g nitrogenous compounds when nitrogen
is supplied from ammonia but the same mount of glucose produces
only 0.40 g nitrogenous compounds when the nitrogen source is nitrate.
The chemical composition of the plant varies with plant part (leaf or seed) and
with species. When the chemical composition of a given plant part is known, it is