The System of Rice Intensifi cation (SRI) 97Similarly, Kordon (1974a, 1974b) reported that the primary and adventitious root
growth was prevented by less oxygen in the root media.
Ros et al (2003) also showed that increased competition for nutrients at higher
seed rates in the nursery bed reduced seedling vigour after transplanting. Based on
these two facts it may be assumed that the SRI practice of maintaining well-drained
soil in an SRI seed bed helps roots to grow at the very early seedling stage. At the
same time, less inter-plant competition due to reduced seedling density helps seed-
lings access more growth-promoting nutrients, leading to healthier seedlings. Thus
SRI seedlings are heavier and sturdier compared to seedlings grown in conven-
tional nursery beds (Stoop, 2005).
Transplanting rice seedlings at a younger stage has been supported by many
researchers (Ota, 1975; Yamamoto et al, 1995; Horie et al, 2005). This practice
captures the benefit of the early phyllochron^1 stages (less than four leaves) having
higher potential to produce more tillers/plant (Katayama, 1951). However, in the
SRI method, special attention has also been given to careful uprooting and then
gently transplanting seedlings immediately into the field with minimal root loss so
as to minimize transplanting trauma. This practice makes an important contribu-
tion to subsequent root vigour and activity and hence to overall yield performance,
compared to conventional management practices. In addition, SRI practice also
suggests that the seed sac along with the soil attached to the seedling should be
kept intact while uprooting to ensure that the endosperm is still attached, so that
there is maximum sustained nutrition of the young plant. In general, uprooting
causes stress to the seedling which could be minimized when the endosperm
remains attached (Sakai and Yoshida, 1957; Ota, 1975; Hoshikawa et al, 1995).
In conventional management practice, it has been reported that around 40–60
per cent of the root remains in the soil during pulling up from the nursery. Prun-
ing up to 60 per cent of the root during transplanting significantly decreased sub-
sequent root and shoot dry matter (Ros et al, 1998). This suggests that a
considerable loss of roots at the seedling stage affects seedling vigour and hence
yield. Therefore, it may be suggested that SRI practices lead to increased shoot and
root dry matter by protecting root systems during transplanting. SRI practices lead
farmers to give greater attention to the conditions of growing, uprooting, storing,
transporting and planting seedlings; and the effects that these practices have on
plant development and yield.
One important factor that influences plant morphogenesis, and which has a
profound influence on the formation of roots and shoots and their relative growth,
is the level of cytokinin in the plant. As a phytohormone that is mainly synthesized
in the roots, cytokinin has a significant effect on tiller bud formation (John et al,
1993; Bangerth et al, 2000); it mobilizes plant nutrients (Li et al, 1992), delays leaf
senescence, regulates chloroplast development, and determines sink–source rela-
tionships (Hutchinson and Kieber, 2002).
Therefore, it may be assumed that SRI plants, at the seedling stage, that have
more root quantity compared to conventionally grown rice seedlings, also have a
greater supply of cytokinins. The implication of this for tiller development is