The System of Rice Intensifi cation (SRI) 101been recommended during the reproductive stage. This late flooding practice
allows NH 4 to dominate, hence favouring cytokinin production.
A third component that affects cytokinin content is the root mass. It has been
indicated that quantitative root growth is higher in high-yielding cultivars (Kim et
al, 1985; Harada et al, 1988). These cultivars transport larger amounts of cytoki-
nin to the above-ground parts of the plant during the ripening stage. This has the
effect of suppressing a decline in rubisco content (Ookawa et al, 2004), resulting
in a higher rate of grain filling and delayed senescence of the plant.
Since higher root mass is a greater supplier of cytokinin compared to fewer
roots, there is evidence to indicate that intermittent irrigation may increase root
mass during vegetative stages (Matsuki and Katsutani, 1940; Baba, 1977) and
stimulates more root activity, and hence more cytokinin content. This favourable
outcome can be achieved by maintaining higher rates of cytokinin production at a
later growth stage, first by following intermittent irrigation during vegetative stage
and then by maintaining shallow flooding during the reproductive phase. Clearly,
this is not possible under completely flooded conditions because of the lesser quan-
tity of roots and minimal root activity since, under continuously flooded condi-
tions, over 75 per cent of roots degenerate by the time plants are entering their
reproductive phase. (Kar et al, 1974).
This adaptive trait could be exploited to manage rice crops under limited water
conditions without compromising grain yield. This could also be why farmers
claim that they experience better yield with SRI practice compared to conven-
tional production methods under drought conditions.
Application of compost
The fourth management practice of SRI is compost application. In general, plant
root growth is greatly affected by the soil environment and, in this context, the
effect of organic manure on soil is well known. The incorporation of organic
manure into soil can bring beneficial effects to root growth by improving the phys-
ical and chemical environments in which roots grow (Sidiras et al, 2001; Yang et
al, 2004). These non-biological aspects of the soil create a productive biological
environment by stimulating root growth-promoting bacteria (Timmusk, 2003).
In addition, the effectiveness of organic manure in improving root growth and
nutrient uptake by rice plants is significantly different under different water
regimes. For example, continuous water logging significantly decreases root growth
(Sahrawat, 2000), whereas under intermittent irrigation, the incorporation of
organic matter improves root morphological characteristics and root activity of
rice plants. It has the effect of increasing root density, active absorption area, root
oxidation ability and nutrient uptake (Yang et al, 2004).
There are immense benefits from compost application in rice farming given
that compost improves the soil’s physical, chemical and biological properties. And,
therefore, it is not surprising that SRI recommends application of compost. How-
ever, the question of compost availability for large areas needs to be addressed.