The System of Rice Intensifi cation (SRI) 103square metre, and hence in turn the greatest number of grains per square metre. To
achieve this, he suggested that the plant should absorb as much as nitrogen as pos-
sible without compromising plant health. Since lodging – and thus reduced photo-
synthesis capacity – at a later stage was the major constraint, he also added that
improvement should also be made in the light-receiving efficiency of the rice crop.
For this, he suggested revitalizing the roots by intermittent irrigation at later
growth stages to increase root longevity. The idea was that this would prevent lodg-
ing and improve the physical condition of the plants by restricting their nitrogen
absorption during the middle growth period while it will also help to maintain
root function at later growth stages.
However, it proved extremely difficult for the growers to properly manage high
plant density rice crops without lodging. And hence, just as there were a number
of successful cases, there were various studies reporting failure also. Failures in
securing sufficient number of grains per unit area explained most of the disap-
pointing results (Kumura, 1989). This occurred mainly because of the unfavour-
able growth of the crops during the early growth period where plants were not able
to absorb as much nitrogen as is required for achieving sufficient grains per unit
area. It was based on the fact that the number of grains per square metre is in direct
proportion to the amount of nitrogen absorbed by the rice plants per square metre
during the late spikelet differentiation stage, and so a higher number of grains
could not be achieved unless the plant is made to absorb much nitrogen. There-
fore, without efficient promotion of root growth in early growth periods, high
yields could not be realized. The latter provides a basis to support the SRI view
that seedling vigour with healthy roots is important, and why sparse planting is
needed along with intermittent irrigation during the vegetative period in order to
realize the high yield potential of the rice plant.
The above presented review on root activity and quantity provides enough
evidence to suggest that the physiological efficiency of the plant is improved by
enhancing cytokinin activity, first by increasing root quantity and activity; second,
by increasing root longevity; and third, by regulating available soil nitrogen sta-
tus.
An Integrated Model of High-yielding Rice Plants Under
SRI Management Practices
The above review and interpretation allow us to propose the following integrated
SRI model of the rice plant with appropriate emphasis on optimal root activity
(Figure 4.2).