Root plasticity and its functional roles were triggered by water deficit but not by the resulting changes in the forms of soil N in rice

The functional roles of root plasticity in rice adaptation to drought conditions may vary with soil nitrogen (N) conditions. To examine if: promoted root system plastic development triggered by mild drought stress and enhanced by N application would contribute to the increase in soil water uptake, and if expression of root system plasticity would be affected by different forms of N applied into the soil. Chromosome segment substitution line (CSSL) 50 and Nipponbare genotypes were grown under continuously waterlogged (CWL) and water deficit (WD) conditions. In rootbox (25 cm x 40 cm x 2 cm) experiment, three fertilizer N levels; (30 (low), 60 (standard) and 120 mg N (high) per rootbox) were used while in pot (5 L) experiment, six N forms (NH4 (+)-N alone, NO3 (-)-N alone, combined NH4 (+)-N and NO3 (-)-N with and without dicyandiamide (nitrification inhibitor) were used at the rate of 360 mg N per pot. In both experiments, CSSL50 and Nipponbare had no significant differences in shoot and root growth regardless of N levels and N forms under CWL conditions. However, under WD conditions, CSSL50 had significantly greater dry matter production (DMP) than Nipponbare due to the greater ability of the former for maintaining soil water uptake and photosynthesis. The observed higher water uptake and photosynthesis in CSSL50 under WD was closely related to its promoted root system development due to plasticity, which were significantly greater at high N than at low N level. The extent of promotion in root system development based total root length was not significantly different among N forms. The root system plasticity of CSSL50 in response to WD was expressed at a greater degree with high level of N applied and the functional roles of root plasticity for greater soil water uptake and DMP were due to WD regardless of N forms.