Yoshida - 1981 - Fundamentals of Rice Crop Science

124 FUNDAMENTALS OF RICE CROP SCIENCE
root medium — an indication of respiration dependence. Rice is highly adaptive to
anaerobic environments. Although anaerobiosis inhibits ion uptake of adapted rice
plants, this inhibited uptake is at least equal to uptake by nonadapted plants with
continuous aeration (John et al 1974).
d. Metabolic inhibitors. Various metabolic inhibitors such as fluoride, iodoacetate,
cyanide, and DNP decrease ion absorption.
DNP permits respiratory electron flow to proceed but blocks formation of ATP
from ADP. Since ATP is probably the direct source of energy for ion absorption,
the fact that DNP decreases the rate of ion absorption indicates that energy is
required for active ion absorption.
e. Carbohydrate. Barley roots low in sugar content absorb more potassium
from a solution to which sugar has been added than from a control solution lacking
sugar.
f. Light. Accumulation of chloride, bromide, and nitrate by the green algae
Nitella clavata depends on light as an energy source.
3.4.2. Energetics of active ion absorption
The amount of energy required for active ion absorption can be estimated by the
following formula (Epstein 1972):
(3.7)
At a temperature of 20°C and an accumulation ratio of 10,000, we get
(3.8)
This is comparable to the amount of energy released by 1 mol ATP when it
hydrolizes to inorganic phosphate and ADP, i.e., about 7,000 cal/mo1 standard-free
energy at pH 7.0 and 25°C.
3.4.3. Effects of hydrogen sulfide and temperature on nutrient absorption
a. Hydrogen sulfide. Hydrogen sulfide, one of the common toxic substances
produced in highly reductive soils, is a well-known inhibitor of the iron- and
copper-carrying redox enzyme.
When rice roots are in contact with hydrogen sulfide in culture solution, leaves
wilt depending on the concentration and the duration of contact. If the contact is