Shoot and root developmental responses of contrasting rice genotypes to different intensities of soil moistures during progressive soil drying

Rainfed lowland rice is negatively affected by drought and thus, the sustainability of its production is critical in attaining rice self-sufficiency target of the government. Root plasticity is a key trait for plant adaptation under drought and an important consideration for breeding high yielding rice adapted to drought-prone environments. This study was conducted to: characterize shoot and plastic root development responses of the parents of doubled-haploid lines (DHLs) and, identify the drought intensity that can induce greater differences in root plasticity between parents which will be used for phenotyping their corresponding DHLs for the succeeding QTL analysis study. CT9993 (upland japonica) and IR62266 (lowland indica) were grown in boxes under greenhouse and subjected to continuously waterlogged (CWL) and transient waterlogged-to-droughted (W-D) conditions with different target soil moisture contents (SMC). Under W-D, CT9993 reduced biomass at 25% and maintained up to 10% before it further reduced at 5% target SMC. In contrast, IR62266 had progressive reductions in biomass with reductions in target SMCs. CT9993 had the ability to maintain stomatal conductance and transpiration while IR62266 showed a decrease. The photosynthesis was maintained in CT9993 while it was increased in IR62266 and thus, increased water use efficiency (WUE) of the latter under W-D of up to 10% target SMC. In terms of root system development, CT 9993 had higher ratio of nodal roots with longer than 40 cm in length, more equally distributed total root lengths among soil depths and greater promotion of L-type lateral roots at deeper soil layer than IR62266 under W-D with 10% target SMC. The results implied that root plasticity such as nodal root elongation, root length and branching in CT 9993 and shoot response such as increased WUE in IR 62266 are some of the key traits to be phenotyped using their DHLs under progressive drought stress at 10% target SMC.