106 The Global Food System
Higher root growth and activity increase physiological
efficiency of the rice plant
Prolonged photosynthetic efficiency of the lower leaves could be maintained by
avoiding shading between plants and by keeping root activity high during later
growth stages. In turn, the photosynthetically active lower leaves would provide
sugar as well as oxygen to maintain high activity of the roots during the later
growth period.
Discussion
There is sufficient documented evidence to suggest that integration of the four key
management practices of SRI could produce better on-farm yields compared to
conventional production methods through improving the physiological efficiency
of the rice plant. Therefore, one can challenge the contrary argument of McDon-
ald et al (2006) that there is no correlation between these four practices and the
physiological efficiency of the rice plant. Further, a careful review of the existing
agronomic literature confirms that SRI practices have a sound scientific base
(Horie et al, 2005).
High-yielding varieties (HYVs) and hybrid cultivars which are highly respon-
sive to chemical fertilizers can have a clear place in SRI. Anthofer (2004) suggested
that modern varieties also respond positively to SRI practices. SRI-grown plants
show higher root growth (Stoop, 2005), but, interestingly both HYVs and hybrid
rice do have higher root growth and activity compared to traditional varieties
(Harada et al, 1988; Ookawa et al, 2004). These issues need to be addressed and
investigated by systematic research focusing on varietal responses to SRI practices,
particularly on root growth and its relation to yield.
Further, it seems reasonable to infer from a wide body of relevant research that
all four SRI management practices have synergistic effects on root growth. How-
ever, a greater opportunity for productivity gains exists by using just the two major
components that have attracted the greatest interest of thousands of farmers in
Cambodia (Anthofer, 2004), i.e. using less seed and less water by planting single
seedlings per hill with wider spacing and intermittent irrigation. In this context,
giving emphasis to growing and transplanting vigorous seedlings singly is more
pertinent than simply recommending planting single seedlings. Seedling vigour
with minimal root disturbance needs to be taken into consideration along with
seedling age in order to realize higher yield potential. Little differences in yield
were found between SRI and conventional management practices in the three
Chinese trials that Sheehy et al (2004) reported. This is an important finding since
in both practices single seedlings were transplanted. Fortunately, there is now evi-
dence that a single seedling per hill increased yield compared to three seedlings
per hill. This effect was due to higher root growth and root activity which delayed