Root trait response to drought, recovery, and P deficiency of upland rice

Abstract

Phosphorus deficit and intermittent drought can cause high-yield reductions in rice (Oryza sativa L.) production. Large differences in tolerance of low P availability and drought resilience were found in the upland rice, but the underlying mechanisms remain poorly understood, especially the belowground processes. To reveal whether and what root plasticity traits are associated with the tolerance to drought recovery, phosphorus deficit, and their interaction in upland rice, across-scale experiments with bibliometric analysis, greenhouse experiments, field experiments, and mathematic simulations were carried out. <br /> In Chapter 2, I first review the entire landscape of root plasticity during drought and recovery reported in the literature. The knowledge gaps that should be focused on in the future, like regrowth during the recovery phase, root anatomy plasticity, and nutrient homeostasis are pointed out. These contexts are specially addressed with upland rice in my thesis. <br /> Chapters 3 and 4 present the phenotypic dynamics of shoot and root plasticity in phosphorus contrasting genotypes (P-efficient genotype DJ123 and P-sensitive genotype Nerica4) during periods of drought and recovery. These two upland rice genotypes were grown in a greenhouse for six weeks with contrasting water and phosphorus levels. In conclusion, the plant recovery rate after drought is significantly influenced by its phosphorus homeostasis. The P efficient genotype, DJ123, with a better P homeostasis had a better recovery and drought resistance than P sensitive genotype Nerica4. Responses in xylem number and the cortical cell file number of DJ123 explain higher biomass and P accumulation than Nerica4 under phosphorus deficit and intermittent drought conditions. Higher phosphorus acquisition and specific root anatomical plasticity (like the xylem number of nodal root) of DJ123 under drought and phosphorus deficit conditions were associated with its better drought resilience. <br /> To further investigate how root plasticity induces phosphorus efficient acquisition in DJ123 than in Nerica4, a combination of greenhouse, field, and model simulation experiments were carried out in Chapter 5 to test if rhizosphere pH change improves P uptake from phosphorus-deficient soils. In the greenhouse and field experiment, DJ123 had greater P uptake, in total and per unit root length (uptake efficiency), than Nerica4 under low P but not under high P. Rhizosphere pH was increased due to an excess uptake of anions over cations in the DJ123, which contributed to increased phosphorus availability and uptake. In combination with root morphology traits, model simulation with pH-P model can explain the higher uptake of DJ123 compared to Nerica4 by the change in rhizosphere pH. 0.5-unit rhizosphere pH change matters for an efficient P uptake in low-pH phosphorus fixing soil. <br /> Past publications suggested that root dimorphism is important in co-optimizing the acquisition of multiple soil resources. Although these responses are complex, this dissertation demonstrates root trait trade-offs are not always true for optimizing phosphorus and water acquisition. Rhizosphere pH and certain root anatomical plasticity (like the xylem number of nodal root) can be targeted in breeding to increase crop yield under phosphorus deficiency and intermittent drought conditions such as low-input agronomic systems.