100 The Global Food System
water level go down gradually until the soil surface appeared, the field surface was
exposed to air for three to five days, and then flooded again.
These findings indicate that intermittent drying in the vegetative stage may
not only induce root growth into deeper soil layers but could also help the plant to
develop xeromorphic characteristics. Intermittent drying also improves soil, stim-
ulates tiller development and alters sink–source relationships. These findings are
very illuminating for current rice production situations where various efforts are
focusing on producing more food with less water.
In relation to root activity, it has been found that the three yield-improving
components, that is, tiller development, soil condition and sink–source relations,
have a close relation with cytokinin production which is regulated by root activity.
It is greatly regulated by environmental conditions such as nitrogen availability,
soil moisture condition, root mass, etc. Another phytohormone which is closely
linked to nitrogen status of soil and may vary with the intermittent irrigation is
auxin. Evidence exists that indicates that auxin induces lateral root formation while
cytokinin helps in their elongation (Debi et al, 2005).
The hypothesis that cytokinin concentration is regulated by N (Kuiper et al,
1988) is supported by the positive relationship between N supply and cytokinin
production (Mercier et al, 1997). It has also been indicated that production of
cytokinin, as well as biomass, is stimulated by a mixed N source (NO 3 – : NH 4 +
ratio 1:1) (Wang and Below, 1996; Briones et al, 2003) rather than any single
source. This statement is in agreement with the finding, reported by Kronzucker
et al, (1999), that rice yields can be 40–60 per cent higher for any given amount
of nitrogen taken up if the N is equally divided in forms between ammonium
(NH 4 ) and nitrate (NO 3 ), rather than being entirely absorbed as ammonium. This
synergistic mode of uptake is still poorly understood, but has been reported repeat-
edly in the literature (Ta and Ohira, 1981; Ta et al, 1981; Smiciklas and Below,
1992). Interestingly, hybrid rice cultivars, which tend to have higher root activity
and quantity compared to traditional cultivars, are also more responsive to mixed
sources of nitrogen (Luo et al, 1993).
A second important factor which influences cytokinin is soil nitrogen. The
status and availability of soil N are dependent on the soil’s aerobic/anaerobic envi-
ronment. This influences the rate of N mineralization resulting from microbial
decomposition of organic matter. In this context, it is reported that microbial
decomposition responds to soil water content and to intermittent drying. The lat-
ter kills off a large portion of soil microbes, thereby releasing N for plant uptake
during rewetting cycles (Birch, 1958; Bottner, 1985; Kieft et al, 1987). These find-
ings indicate that soil nitrogen status may be enhanced, in terms of maintaining a
balanced ratio of NO 3 – : NH 4 + nutrition to enhance root-available nitrogen. These,
in turn, regulate cytokinin, and therefore, root activity. NO 3 – increases the
auxin:cytokinin balance required for tiller and biomass production, while urea and
NH 4 + shift this ratio in favour of cytokinin (Mercier et al, 1997), probably delaying
root senescence at later growth stages. This is important in the context of SRI
management practices where, after intermittent irrigation, shallow flooding has