Yoshida - 1981 - Fundamentals of Rice Crop Science

210 FUNDAMENTALS OF RICE CROP SCIENCE
Our knowledge of maintenance respiration is much less certain than our knowledge
of growth respiration. The most potential maintenance processes in plants
are (Penning de Vries 1975a):
• Protein turnover.
• Active transport processes to maintain certain ion concentrations in the cells.
Proteins in living organisms undergo constant renewal by a process referred to
as protein turnover. The average turnover rate of leaf proteins may be about 100
mg protein/g protein per day at normal temperatures in leaves assimilating at
moderate light intensities. In other words, about 10% of leaf protein are resynthesized
everyday. This process consumes 28–53 mg glucose/g protein per day,
which corresponds to 7–13 mg glucose/g dry weight per day in leaves. Such a
consumption implies that the complete removal of protein turnover would reduce
maintenance costs by about 10 mg glucose/g dry weight per day or by about 10 kg
glucose/day for every 1 t dry weight. The cost of maintaining the ion concentration
in leaves is estimated at about 6–10 mg glucose/g dry weight per day. In addition,
more energy is used to break down and resynthesize the lipids that constitute cell
membranes. The cost of membrane maintenance is estimated at 1.7 mg glucose/g
dry matter per day. Adding these figures gives a maintenance respiration cost of
15–25 mg glucose/g dry matter per day.
This estimated maintenance respiration cost agrees with reported values (compare
with equation 5.18), but is lower than other measured values for plants grown
under high light intensities. For example, values as high as 50–150 mg glucose/g
dry weight per day have been reported for the leaves of Hordeum sp. and Triticum
sp. (Penning de Vries 1975a). There is also a clear difference in the costs of
maintenance respiration between species: the maintenance respiration of sorghum
is about one-third that of white clover (McCree 1974). The existence of uncoupled
or idling respiration in plants has been suggested to explain unexpected high rates
(Beevers 1970) and low yields (Tanaka 1972b). It is also possible that part of the
protein turnover represents a process of little use.
5.2.5. Bioenergetics of crop yields
Crop yields can be compared agronomically in several ways: yield per crop, yield
per day, and yield per year.
From a physiologist’s point of view, a more meaningful comparison of crop
yields can be made in terms of the amount of glucose or energy from which a
harvested organ is produced. To simplify the pathway of yield formation, consider
that glucose is the product of photosynthesis, different compounds such as lipids,
proteins, and carbohydrates are synthesized from glucose, and harvested organs of
different crops are composed of different proportions of these three.
When glucose is converted into proteins, lipids, and carbohydrates, the conversion
efficiency can be defined as:
grams of a compound produced
Conversion efficiency = (5.21)
1 gram of glucose