The System of Rice Intensifi cation (SRI) 107senescence and increased the rate of photosynthesis at later growth stages (San-oh
et al, 2006).
Further Sheehy et al (2004) reported fewer panicles per square metre in SRI
than with conventional production methods. This accords with our claim that hill
spacing needs to be decided according to varietal characteristics, most notably til-
lering capacity. This factor may be one of the reasons for yield variation observed
under SRI practices either at the same or different locations.
The second major component that many farmers can adopt is water manage-
ment according to SRI principles. It is evident from the literature that AWD, or
intermittent irrigation, is already a component of water management practices of
Asian rice-growing countries such as China, Japan and Korea. AWD is currently
being evaluated and promoted among rice farmers producing in water-limited
environments (Bouman and Tuong, 2001). But, in general, traditional practices
focus on intermittent irrigation at later growth stages (Matsushima, 1973; Sheehy
et al, 2004), while SRI recommends AWD at the vegetative stage. Certainly, in the
latter case, due to a temporary drying effect at early growth stages, plants would
develop with osmotic adjustment and develop more roots at a deeper soil layer.
This root induction practice at the early vegetative stage may be useful for plants
grown under limited water conditions such as aerobic rice cultivation where water
stress has been found to be a more limiting factor than any other stress. It has been
found that under these situations, water stress reduces crop nitrogen demand
(Belder et al, 2005) and hence yield is significantly reduced. However, temporary
drying through intermittent irrigation also affects soil nitrogen status by changing
nitrification–denitrification and/or ammonia volatilization processes. This can
lead to more nitrogen loss compared to completely flooded conditions, especially
in soils with low organic matter.
Research opportunities lie in the measurement of redox potential at increased
depths in the soil. This information may explain the pathway of nitrogen transfor-
mation processes in situations where deep root systems might get an advantage of
transformed nitrogen (ammonium form) in the deeper soil layer, i.e. under a
reduced soil environment. Moreover, if the AWD/intermittent irrigation practice
at the vegetative stage is integrated with the split dose of nitrogen at crucial growth
stages, this may overcome many limitations of aerobic rice culture and intermit-
tent irrigation practice.
Apart from this, changing the soil status from aerobic to anaerobic by practising
AFD (alternate flooding and drying naturally) three–four times during the vegetative
stage can improve plant type, light-receiving ability and resistance to lodging, along
with root health. But again it is important to consider when and how long the inter-
mittent irrigation should be applied to save water as well as increase yield since rice
root–shoot responses and their physiology also differ at the genetic level along with
environmental factors. Certainly, these scientific considerations of SRI principles
and related research questions offer opportunities for further studies.
Reported cases where SRI plants performed no better than best management
practices, as claimed by critics of SRI, cannot be ignored (Sheehy et al, 2004; Latif