Conclusions
Root plasticity under alternative water regimes creates
opportunities to grow ‘more rice with less water.’ The
contribution that root systems’ structure and function,
reflected in greater root length density and more physiological
activity, can make toward higher yield, especially
with supportive water management, has been assessed in
quantitative terms in the preceding sections. Our research
has shown that managing soil–plant–water relationships to
exploit rice plants’ adaptive root traits can achieve ‘more
crop per drop,’ achieving more satisfactory crops with
reduced water applications.
Greater root length density increases the storage
capacity of the root zone, and a deeper root system is
associated with more water uptake from the soil and with
better crop performance under drought conditions. Deep
and healthy root systems are not only correlated with better
water uptake, but they also influence yield physiology, e.g.,
by regulating cytokinin production (Faiss et al. 1997).
Phytohormones are regulated at least in part by conditions
in the rhizosphere, such as nitrogen availability, soil
moisture condition, root mass, and root length density.
Among all these parameters, root length density and root
mass are important variables for characterizing temporal
trends in the water relations of rice, especially when water
supply is scarce. The amount of water available to the plant
depends on the relative root length density and on the
ability of roots to absorb water from the soil (Mishra and
Salokhe 2008b).
The simplest way to increase rooting depth and the root
distribution of crops is to increase the duration of the
vegetative period. This may be achieved by sowing earlier
or by delaying flowering. SRI practice recommends transplanting
younger seedlings (B15 day-old) with wider
spacing. This seedling age helps the plant to enjoy a prolonged
vegetative period along with better canopy growth,
while an optimization of spacing enhances canopy photosynthesis
by avoiding shading effects. In this regard, a
hypothetical model of SRI plant performance has been
discussed in Mishra et al. (2006), and finds confirmation in
Thakur et al. (2010).
If seedlings are raised in seedbeds with SRI techniques
and are subsequently transplanted with optimum spacing,
and with intermittent irrigation or preferably with just moist
soil condition during the vegetative stage, this further helps
plants to grow more roots with higher root length density at
deeper soil layers to increase the storage size and to utilize
water more efficiently. SRI principles should, therefore, be
helpful to develop location-specific management practices
that optimize water and other input use without compromising
grain yield, and indeed enhancing it.
Root plasticity under intermittent irrigation also opens
up opportunities for mitigation of methane production in
rice fields. Up to 90% of the CH4 emitted in rice paddies is
released through transport within rice plants (Conrad
2007), while between 19 and 90% of this CH4 produced is
oxidized. This means that up to 75% of CH4 oxidation
takes place in the rhizosphere (Frenzel 2000). Accordingly,
strategies to lower net CH4 emission from rice fields should
include reduction of CH4 production, increasing CH4 oxidation,
and lowering CH4 transport through the plant.
Among the CH4 emission-mitigation strategies that do
not compromise rice productivity, introduction of drainage
periods during the crop cycle appears to be the most efficient
(Neue 1993; Yan et al. 2009). It has been estimated
that introducing periods of soil drainage by adopting
intermittent irrigation in poorly drained rice fields could
reduce agricultural CH4 emissions by 10% (Kern et al.
1997). A higher rate of root activity for a longer duration,
which was seen in these research findings (Fig. 6), would
further enhance CH4 oxidation in the rhizosphere. These
benefits are more relevant for the current scenario where
rice production needs to be increased with reduced water
application and with reduced ‘climate-forcing’ practices.
SRI should thus be seen as an opportunity to develop more
eco-friendly management practices in the rice sector.
In conclusion, it can be said that SRI is opening up
various possibilities for better understanding of rice plants’
growth response and their root plasticity under varied soil
environments that could be exploited and manipulated to
enhance crop production through enhanced root and rhizosphere
activity. Since SRI agronomic crop management
practices have been seen to increase root growth and yield
from practically any variety, they should be explored in
more detail to gain a better understanding of roots and
rhizosphere functioning and the interdependence between
below- and above-ground plant development. Such investigation
would be useful to develop alternative crop management
practices that will buffer or mitigate the adverse
effects of climate change and provide more and better
ecosystem services.