The System of Rice Intensifi cation (SRI) 93Roots supply nutrients and water, synthesize plant hormones, anchor plants
and interact with soil biota. The physiological activities of roots contribute to the
reinforcement of their oxidation activity. Oxidation activity in roots is measured
by a phenolic redox pigment, particularly α′-napthylamine, which is widely
employed to assess root activity. Rice root respiration has a proportional relation-
ship with α′-napthylamine-oxidizing activity (Yamada and Ota, 1958). The oxida-
tion rate is greater in new roots compared to old ones (Aimi and Fuzimaki, 1959).
Therefore, by root activity, we are mainly focusing on root respiration.
Rice roots follow an aerobic pathway like other plant parts, although the root
environment under flooded cultivation is hypoxic. For this purpose, lysigenous
intercellular spaces, called aerenchyma, operate as an oxygen supply route to the
roots (Yamada and Iyama, 1953). It was shown that the development of lysigenous
intercellular space varied greatly according to the growing period and environmen-
tal conditions (Yamada et al, 1954; Arikado, 1955). Initially, it was believed that
root activity increases with increased number of aerenchyma (to the extent that
oxygen is a limiting factor in respiration) (Arashi and Nitta, 1955); and the number
of aerenchyma not only varies with the growing environment, but also varies with
the rice growth stages itself. In general, it has been found that flooded rice has its
peak root activity from the beginning to the middle heading period, whereas
upland rice has a wider peak compared to submerged rice (Mitsui and Tensho,
1952). It was argued that by increasing the number of aerenchyma, root activity
can be enhanced. But later it was found that upland cultivars with fewer aeren-
chyma had higher root activity compared to lowland cultivars (Puard et al, 1986).
Further, it was discovered that in addition to the supply of oxygen provided mainly
by aerenchyma, sugar content of roots also has a close relationship with root respi-
ration rate and activity (Tsuno and Yamaguchi, 1987).
The supply of photosynthates or sugar to the roots is mostly done by the lower
leaves (Tanaka, 1958). Therefore it was suggested that, to sustain high root activ-
ity, it is necessary that the rice plant’s configuration allows sunlight to reach the
lower leaves as much as possible. Thus, the suggestion emerged to keep the upper
leaves erect for this purpose. In addition, it was also found that oxygen supply to
the roots is also easier from lower leaves than from upper ones and that withered
lowered leaves, especially at later growth stages, lose their ability to take up and
pass on oxygen (Arikado, 1975). Consequently, it was considered essential to keep
lower leaves alive as long as possible so as to provide roots with both photosynt-
hates and oxygen (Arikado, 1975). Thus a healthy root function depends on the
rate of respiration, sugar content and oxygen supply.
Therefore, when the rice plant, especially the upland cultivars that have fewer
aerenchyma compared to lowland cultivars, is grown under continuously flooded
conditions with dense planting pattern, it retards the function of lower leaves and
so the root activity, resulting in >75 per cent root degeneration at the time when
flooded rice plants commence flowering (Kar et al, 1974), i.e. at a time when peak
root activity is required by plants to achieve higher yield. Also, the lower oxygen
concentration in the rhizosphere and continuous soil submergence results in more