94 The Global Food System
accumulation of carbon dioxide around the roots which speeds up the root senes-
cence.
Impact of Root Activity on Plant Physiology and Yield
The details of how a healthy root system influences the physiology of the ‘visible’
plant, especially how it affects yield, are discussed here. For the purposes of dis-
cussion, indicators for root health are defined as root mass and respiration rate.
In general, factors listed as responsible for increasing the physiological efficiency
of rice plants include: canopy architecture (Takeda et al, 1984); single leaf photo-
synthetic activity (Park and Ota, 1969; Hayami, 1983); root activity and root
quantity (Lee, 1980; Jiang et al, 1985); and plant adaptability to pure stands or
monoculture (Donald, 1968, 1981; Ishi et al, 1986). All these factors are closely
dependent on root activity since higher α′-naphthylamine-oxidizing activity of
roots is correlated with higher chlorophyll content of the leaves, slower leaf
senescence (Ota and Lee, 1970; Youn and Ota, 1973), and more erect standing
leaves.
These important findings reflect the causality in the relationship between
above-ground and under-ground plant parts. Similarly, Jiang et al (1985) sug-
gested that if root activity is kept high during the ripening period, leaf blades will
maintain their upright posture; and they will also age more slowly, delaying both
withering and declining photosynthetic activity. In addition, they also found that
when root activity is high, the photosynthetic rate does not decline so much in the
afternoon. Therefore, these authors concluded that keeping root activity high is
very important to maintain the functioning of the source, i.e. higher photosyn-
thetic efficiency of the leaves during the ripening period. The conundrum was how
to do just that since HYT was based on the supposition that higher yields were
dependent on higher plant densities, leaving little scope to reduce the root activity-
inhibiting effects of shading. A partial solution was suggested by Matsushima
(1973) – intermittent irrigation at later growth stages to stimulate root activity.
Based on Matsushima’s suggestion, Tsuno and Wang (1988) concluded that
drained field conditions could induce higher root activity by enhancing root respi-
ration and root revitalization, resulting in greater leaf area, higher photosynthesis
activity and higher yield. Recently, these finding have been complemented by
another two major findings, i.e. high root activity contributes to a higher photo-
synthetic rate (Osaki et al, 1997), and the growth of shoots is very much depend-
ent on root growth (Nikolaos et al, 2000).
Another valuable clue regarding the importance of root growth derives from
the suggestion of Xuan et al (1989) that root quantity, as well as root activity, are
both required for raising yield. This suggestion came after the discovery by
Terashima et al (1988) that a super high-yielding cultivar has larger root systems
compared to other indigenous cultivars.