Soil Compaction Enhanced the Expression of Root Plasticity, Water and Nitrogen Uptake of Rice under Mild Drought with High N Fertilization

The functional roles of root plasticity in rice for adaptation to drought may be affected by the rates of nitrogen (N) application and soil compactions (SC). This study examined the expression of root plasticity and its function for water and N uptake and the maintenance in shoot dry matter production triggered by mild drought with variable rates of N and SC. A chromosome segment substitution line (CSSL50) and its recurrent parent, Nipponbare, were grown under continuously waterlogged (CWL, control) and water deficit (WD; 20% soil moisture content, SMC) conditions. Nitrogen treatments at the rate of 30, 60, 120, 240 and 360 mg N per box were used in Experiment 1 and at 30, 60 and 120 mg N per box in Experiment 2. The fertilizers were mixed well with air-dried sandy loam soil set either at bulk density of l.25 (light soil compaction) or 1.50 g cm-3 (heavy soil compaction) prior to sowing. Under CWL, shoot dry weight (SDW) and total root length (TRL) were not significantly different between CSSL50 and Nipponbare regardless of the rates of N and SC. Under WD, however, SDW and TRL were significantly greater in CSSL50 than in Nipponbare but the magnitude of differences varied with rates of N and SC. The genotypic differences in SDW between CSSL50 and Nipponbare were greater at high than at low N regardless of SC treatments. This result was partially attributed to the longer TRL of CSSL50 than that of Nipponbare at high N regardless of SC treatments, which consequently contributed to the greater water and N uptake of the former than the latter genotype. Longer TRL in CSSL50 than in Nipponbare was more pronounced at heavy than at light SC at 30, 60 and 120 mg N rates, due to the maintained TRL in the former but reduced TRL in the latter genotype under heavy SC. These results imply that the developmental plasticity of root system triggered by WD can be also expressed with greater magnitude under a more compacted soil with high N rate and contributed to the maintenance in dry matter production.