Case study 3: Water-saving rice agro-ecosystems for West Africa
Rice is the most consumed cereal in West-Africa and its demand is
strongly increasing mostly due to population growth and changes in
consumption pattern linked to increased urbanization
(Elbehri,
Kaminski, Koroma, Iafrate & Benali 2013; ECOWAS 2019). Currently, local
production covers only roughly 60% of the demand, the rest of the
populations’ needs being met through imports that strongly impact the
region’s economy and make it vulnerable to changes in prices on the
global commodity market
(ECOWAS 2019).
Since a majority of West Africans are projected to live in urban areas
in the future, demand for rice is expected to increase further by 73.5
percent from 2011 to 2025
(Elbehri et
al. 2013; ECOWAS 2019). Several programs have been launched to increase
local production to meet this future demand and guarantee food
sovereignty.
However, irrigated rice cultivation requires a large amount of
freshwater. It was estimated that, in a dry environment with an
evaporation rate higher than the precipitation rate found in large parts
of West Africa, traditional irrigated rice cultivation requires between
700 to 1,500 mm of water to produce 1 kg of rice
(Bhuiyan 1992).
Furthermore, climate change is trending towards hotter and dryer
atmosphere prevailing in the region, which will increase evaporative
demand. With increased competition from industries and city growth for
freshwater and increased uncertainty in precipitation patterns resulting
in reduced water availability in the region, it will not be possible to
meet the growing demand with local rice production using the
conventional irrigated rice cultivation system
(Nie et
al. 2012). Several water-saving alternatives have been suggested
including alternate wetting and drying (AWD) or aerobic cultivation in
order to reduce water consumption and increase cultivated surfaces.
While these practices can save up to 50% of the water used for rice
production, they often incur a yield penalty with the current varieties
that have been selected for irrigated agro-ecosystems
(Bouman, Peng,
Castañeda & Visperas 2005; Peng et al. 2006; Kreye et al.2009; Sasaki et al. 2010).
Aerobic rice cultivation aims to maximize crop water use efficiency by
growing plants in soil without flooding or puddling
(Matsunami,
Matsunami & Kokubun 2009; Matsuo & Mochizuki 2009). It allows greater
water savings and can be deployed in regions without access to
irrigation water but has a high yield penalty and is associated with
increased weed management and risks of nematodes. In this system,
periodic drought stress may reduce yield stability and yield potential
(Sandhu et
al. 2019). For these reasons, aerobic rice has not become popular among
farmers in irrigated areas
(Meena, Bhusal,
Kumar, Jain & Jain 2019). On the other hand, AWD is a simple practice
where, instead of keeping the fields permanently flooded, irrigation is
periodically stopped until the soil water table reaches a certain depth,
easily measured using a pipe set in the soil, and then re-started until
the field is flooded again
(Bouman & Tuong
2001). AWD cycles are repeated either during the vegetative or flowering
stage or throughout the rice cultivation cycle, although keeping paddies
flooded in hot environments during the flowering stage help avoid the
problems linked to heat sensitivity though the cooling effect of
transpiration
(Jagadish, Murty
& Quick 2015). Two types of AWD have been described: moderate AWD when
field water level is allowed to drop down to 15 cm below the soil
surface, and severe AWD when soils are allowed to dry beyond −20 kPa
(Carrijo et
al. 2017). AWD can be easily adopted in these areas as it does not
change the cultivation practices, is not associated with increased
labour needs and can contribute to a reduction in water consumption of 5
to 30%, depending on the season and soil, as well as reduce methane
emissions and grain arsenic levels
(Linquistet al. 2015; Carrijo, Lundy & Linquist 2017).In general, AWD has
a limited yield penalty (5.4% in a meta-analysis of 56 studies) but the
yield decrease is more important in severe AWD or if AWD is maintained
throughout the crop cycle
(Carrijo et
al. 2017). The impact on yield is also very dependent on the genotype
and most of the currently used high-yield varieties show yield reduction
in AWD (Carrijoet al. 2017; Sandhu et al. 2017). Thus, there is a clear
need to develop new varieties to optimize yield in AWD rice
agroecosystems.
The use of root traits has been little explored in such agroecosystems,
but could have a large impact to optimize Water Use Efficiency (WUE), P
use efficiency (PUE) and N use efficiency (NUE) and thus reducing inputs
globally in AWD systems. Indeed, AWD results in periodic changes in
water content in the topsoil but also changes the dynamics of nutrient
availability and in particular of N and P availability
(Wang et
al. 2016; Acosta-Motos et al. 2020). For instance, AWD increases
topsoil P availability and has been linked with changes in the soil
microbiota and the stimulation of aerobic P-solubilizing bacteria in the
aerobic topsoil compartment
(Li et al.2018). Conversely, N availability seemingly decreases upon AWD due to
increased denitrification, volatilisation and leaching although these
losses can be avoided by timely N application
(Tan et
al. 2013; Djaman et al. 2018). Water and N signalling are known
to interact in ways that affect root phenes for synergistic or
antagonistic resource uptake (reviewed in
Araus, Swift,
Alvarez, Henry & Coruzzi 2020). Hence, varieties for AWD
agro-ecosystems need to be adapted to fluctuations in soil water and
nutrient content in the topsoil. Root phenes could be mobilized to that
end as a large genetic diversity for these phenes is available in rice
(Ahmadi et
al. 2014). Interestingly, AWD seems to have a positive effect on root
development as indicated by increased root biomass and maximum root
length (Wanget al. 2016; Acosta-Motos et al. 2020).
Root phenes that increase the volume of topsoil exploration and promote
P uptake should be prioritized to improve rice yields in AWD systems.
Phenes that merit investigation include crown root number, lateral root
density (there are two types of lateral roots in rice, thin and
determinate short lateral roots and indeterminate long lateral roots;
Rebouillatet al. 2009), and root hair length and density. A recent
large-scale study by
Sandhu et
al. (2017) is consistent with nodal root number and lateral root
density positively impacting yield in AWD. Evaluation of new rice
varieties derived from crosses between popular varieties and
drought-tolerant accessions was performed in fully-irrigated and AWD
systems in seven sites across Asia
(Sandhu et
al. 2017). Out of 82 lines tested in at least three sites, lines with
stable and high yield in AWD conditions compared to irrigated conditions
were identified. Comparison of the root phenotype of a subset of these
stable high-yielding lines and the control line IR64 (high yield variety
that shows a reduction in yield in AWD) showed that higher nodal root
number and root dry weight at 10–20 cm depth played an important role
in maintaining grain yield under AWD
(Sandhu et
al. 2017). Interestingly, quantitative changes in these root phenes
induced after initiation of AWD cycle, i.e. plasticity response of the
root system, were shown to be important for yield stability
(Sandhu et
al. 2017). These observations are in agreement with previous analyses
describing the important role of root system plasticity for drought and
low P tolerance in field and controlled conditions
(Sandhu et
al. 2016). In the best performing lines under AWD (initially selected
as drought tolerant breeding lines), the number of nodal roots below 20
cm and deep root length at the flowering stage appear to have a positive
effect on yield
(Sandhu et
al. 2017). Therefore, these lines may have had the ability to grow
roots in deeper soil layers for improved water and nitrogen acquisition
in the dry down period (particularly at flowering stage in severe AWD
scenario and less so when the water table is maintained above 15-30 cm),
while AWD-induced root branching plasticity in the topsoil improved P
uptake (Fig. 2C). Moreover, a study performed using a combination of pot
experiments and functional-structural plant model indicated that, for P
uptake, root system efficiency is improved by increased root branching
both in irrigated and water stress conditions
(De Bauw et
al. 2020). In AWD, phosphate acquisition was shown to occur mainly at
the root tip and led to phosphorus depletion along the root due to the
low mobility of P in the soil
(De Bauw et
al. 2020). Root interactions with soil symbiotic microbes such as
arbuscular mycorrhizal fungi (AMF), which is inhibited by flooding,
might also improve water and nutrient acquisition in the topsoil
(Vallino,
Fiorilli & Bonfante 2014; Mbodj et al. 2018). Furthermore, AMF
were shown to confer drought tolerance in rice
(Chareesri, De
Deyn, Sergeeva, Polthanee & Kuyper 2020). Interestingly, AMF
colonization was also shown to reduce P loss from paddy fields thus
improving PUE and reducing environmental impacts
(Zhang et
al. 2020). Thus, root phenes that promote AMF infection in topsoil such
as increased number of crown roots and large lateral roots with more
cortex cells and less aerenchyma to accommodate intracellular fungal
structures may lead to improved water and nutrient uptake. Moreover,
rice response to AMF infection is dependent on the plant genotype and
could be targeted for plant breeding
(Diedhiouet al. 2016; Huang et al. 2020; Lefebvre 2020).
Overall, improving root phenes in rice for AWD systems could lead to
improved WUE but also PUE
(Acosta-Motoset al. 2020) and NUE
(Wang et
al. 2016) through synergistic interactions between water and nutrients,
thus leading to more sustainable rice agro-ecosystems with reduced water
and fertilizer consumption. Lines adapted to aerobic conditions that
show root plasticity in response to AWD in topsoil (increased branching)
could be interesting materials to test. Rhizosphere phenes could also be
targeted to improve water and nutrient acquisition efficiency in this
system.