Root length increased in a quadratic fashion with the advancement of
plant age (Figure 1). Maximum root length was achieved at 105 days after
sowing, which coincides with the date of flowering. After this date, the root
length was more or less constant. Klepper (1992) reported that the general
pattern of root development over the life of the crop shows a shift from
a heavy investment in roots during seedling establishment and early vegetative
growth in the first part of the growing season to heavy investment
in reproductive structures during the latter part of the season. This may explain roots reaching a plateau during the grain or spikelet filling growth
stage. A root system that extends the root zone to more fully extract available
soil water and mineral nutrition has the potential to increase yield under
drought (Mambani and Lal, 1983; Nguyen et al., 1997). Capacity for water
and nutrient uptakemay limit rice productivity even in flooded soils (Ingram
et al., 1994). The rice plant invests up to 60% of its carbon in the root system
(Nguyen et al., 1997). Ingram et al. (1994) and Yu et al. (1995) reported
that the ability of rice to reach deep soil moisture or to penetrate compacted
soil is linked with the capacity to develop a few thick (fibrous) and long root
axes. Thick roots persist longer and produce more and larger branch roots,thereby increasing root length density and water uptake capacity.