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productivity ameliorates yield grains while reducing losses of irrigation water applied (Playa ́n and Mateos, 2006).
This study recorded heavier grain weight in some VarMix combinations due to WUE. Yield differences was due to the inherent ability of VarMix to produce higher shoot dry mass and yield per unit amount of water taken up from the drying soil during reproductive stage. WUE is also more important than WU because the varieties used in this study were bred for transplanted irrigated lowland conditions except NSIC Rc 298, which was developed for direct- seeded irrigated lowland system. Most of the irrigated lowland varieties are bred based on their high yield potential, pest and disease resistance, and grain quality (Peng and Khush, 2003). Further, BI ability (length of LR per unit length of NR) generally increased under CAWD, and also had significant relationship with WUE.
VarMix, with proper identification of most suitable combination, can help optimize resources better than monocropping systems, which would lead to improve grain yield and harvest index under water- limited condition (Li et al., 2018). In water-limited environments, mixtures tend to conserve soil water before stem elongation, ensuring the availability and efficient utilization of resources that translates into greater water-use efficiency and higher yield than in pure stands. VarMix reduces the impacts of abiotic stresses by buffering yield through more efficient resource use including soil moisture that are particularly evident when mixtures comprise complementary physiological traits that influence WUE (Gyamfi et al., 2015).
Results indicated that NSIC Rc 298, a direct seeded irrigated lowland rice variety, showed root plastic developmental response (increase in trait value under CAWD relative CWL) based on TRL in response to CAWD. This was partially attributed to its plasticity in TLRL under the condition indicating the key role of the degree in branching of the lateral root as key response to water stress conditions (Suralta et al., 2018). Thus, root system development and physiology are closely associated with above-ground plant biomass (Suralta et al., 2018). NSIC Rc 298 also produced the greatest water use and shoot dry weight in monoculture under CAWD.
VarMix characteristic underground mechanism may partially intensify the root uptake area by using available assimilates during drought. In this study, the presence of NSIC Rc 298 in some of the VarMix triggered the increase in mean root plasticity
in response to CAWD, although it did not increase total water use. This result indicates that combined compensation (Yadav et al., 2009) and facilitation (Garcia-Barrios, 2002) mechanisms resulted in high yield gained by VarMix.
NSIC Rc 298 as monoculture produced high grain yield, TRL, and water use under CAWD. This variety when combined with appropriate varieties in VarMix may have worked through these steps. First, it expressed high root plasticity in response to CAWD, which may have resulted in the increase of its root water uptake from deeper soil layer through hydraulic lift (Doussan et al., 2006; Sekiya et al., 2011). Second, the water taken up by the roots of NSIC Rc 298 may have been redistributed in the upper drier soil layer (Neumann and Cardon, 2012) and initially take up by neighboring roots of other varieties to continue their shoot growth (Izumi et al., 2018). And third, part of the redistributed water may have been used by the other varieties to improve their root system as evidenced by the marked increase in the average BI particularly in the two VarMix such as five (NSIC Rc 298, Rc 216, PSB Rc 82, NSIC Rc 238, and Rc 300) and six (NSIC Rc 298, Rc 214, Rc 216, PSB Rc 82, NSIC Rc 238, and Rc 300) combinations. Ultimately, this led to their balanced water use to maintain above ground dry matter production and yield via increase in WUE. The targeted combination of physiological traits, which are the main drivers for yield under water stress through VarMix, could be further explored as a strategy to improve yield and stability of yield in water deficit environments (Gyamfi et al., 2015).
The above mechanisms will be subjected for further studies contrasting VarMix and monocultures.
Conclusion
VarMix, given right combination of varieties, can collectively increase branching index and can maintain optimal water use but high WUE during water deficit conditions. Increase in WUE under CAWD significantly increased the grain weight in some VarMix combinations. The high performing VarMix may express their full potential over monocultures when grown under real field conditions where the varieties are not only exposed to SMF but also to pest and disease pressures.
Acknowledgment
This research is part of the project, “Varietal Mixture of Rice to Enhance Yield and Mitigate Effects of Climate Change in Stress-Prone Areas” funded by Department of Agriculture - Bureau of Agricultural Research (DA-BAR).
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