Relationship among nitrogen fertilization, root plastic developmental responses and yield of rice under rainfed lowland conditions

Root plasticity is an important crop adaptation for maintaining soil water uptake and crop productivity under drought. The root plastic developmental responses can be maximized only with enough nutrition such as nitrogen (N) fertilization. This study was conducted to determine the effect of nitrogen (N) fertilization on root plastic developmental responses and yield of rice grown under rainfed lowland rice condition. Seven genotypes consisting of IR64 parent and its introgression lines (IR64 INLs) were grown under rainfed lowland conditions either without N (ON) and with N (90 kg N/ha) applied in 2 splits at 45 kg N/ha each at the early tillering and panicle initiation stages when the soil was covered by a shallow water layer. Phosphorus and potassium were applied at basal in all plot at 40 kg/ha. Soil moisture fluctuations were apparent throughout the growth duration with 0-10 cm soil depth showing greater reduction during drought periods. There was a significant interaction between genotypes and N treatments indicating that N application did not enhance the yield of all genotypes under drought. In both N treatments, genotypic variations in yield were strongly and negatively correlated with days to heading. With limited N available, genotypic variations in grain yield was correlated with harvest index and 1000 grain-weight only. However, with enough N available, genotypic variations in grain yield under drought were strongly correlated with the harvest index, number of spikelets per panicle, percent filled grains and the weight of 1000 grain-weight. The result may imply that N application maximized root plastic developmental responses to drought to sustain soil water uptake as well as increased sink sized and spikelet fertility, and higher dry matter allocation during grain filling. Root scanning and measurements are still on going to quantitatively analyze the relationship between N fertilization and root plasticity responses.