The System of Rice Intensifi cation (SRI) 95In general terms, a larger root surface area is associated with higher cation
exchange capacity, and thus more nutrient uptake becomes possible. Further,
greater root quantity also correlates with drought tolerance. Several research groups
have shown that, in the case of drying soil, water deficits first appear near the sur-
face and then progress to the deeper layers; increased depth of rooting is common
under these conditions (Proffitt et al, 1985; Newell and Wilhelm, 1987). Thus,
root systems can be enhanced (deepened) by lowering soil moisture status, during
the vegetative stage, providing a buffer against drought conditions.
This finding is in agreement with earlier findings (e.g., O’Toole and Chang,
1979; Yoshida and Hasegawa, 1982) that those cultivars which are characterized
by higher ratios of deep root weight to the weight of the above-ground plant parts
have a higher drought-tolerance score. The force required to pull up rice seedlings
from the paddy soil is correlated with root weight, root branching, and thick root
number (O’Toole and Soemartono, 1981). Based on the relationship between root
abundance and drought tolerance, Ros et al (2003) recently proposed seedling
selection with higher shoot/root ratios as a means of increasing drought tolerance
and yield.
Thus there is abundant evidence to confirm the crucial role of the ‘hidden
half ’ plant parts in co-determining grain yield; this becomes all the more impor-
tant when the crop needs to be managed under limited soil moisture conditions.
However, despite these supporting facts, the importance of root activity in relation
to yield has been given limited attention in rice research. One of the reasons for
this oversight might be the dramatic success of the ‘Green Revolution’ which
eclipsed, by and large, almost all traditional cultural practices in Asia, including
HYT-based techniques. Perhaps it was also a case of out of sight, out of mind!
Only recently the ‘half told’ story of rice roots resurfaced when SRI practice
became more widely known, when rice plants showed more vigorous root growth
irrespective of cultivar species under SRI management. This indicates that the
cultural practices recommended in SRI have an influence on root quantity and
root activity and so provide the basis for reviewing basic management practices of
SRI which might have an influence on root activity and quantity.
SRI Management and its Relation to Root Quantity and
Activity
Growing seedlings in well-drained nursery beds with fewer
seeds, and transplanting at the 2–3 leaf stage (germination
and early seedling growth)
Although rice plants are ‘semi-aquatic’ and seeds germinate well in water (unlike
true aquatic plants, the germination of rice is not inhibited by lack of oxygen), this
does not mean that rice is an aquatic plant that grows better under submerged