Root plasticity expression in hardpan penetration is triggered by rewatering rather than by progressive drought during soil moisture

Rainfed lowland rice productivity is low because of soil moisture fluctuations and the presence of hardpan, both of which affect root system development. Here, the authors clarify the plasticity expression in root hardpan penetration under soil moisture fluctuations (SMF) and different hardpan strengths and its ability to access soil water below the hardpan and contribute to shoot dry matter production, using IR64 and its introgression line, YTH304. Under SMF, both the SMC and hardpan penetration resistance fluctuated and inversely interacted between episodes of drought and well-watered conditions. Both genotypes had similar root and shoot growth, stomatal conductance and root water uptake below the hardpan under well-watered conditions but under SMF, YTH304 had greater values for those traits than IR64 especially with hardpan at 1.5 g/cum bulk density (BD). The greater plasticity is nodal root hardpan penetration in YTH304 than in IR64 under SMF occurred when soil was wet as a result of rewatering and relieved from drought rather than during progressive drought. This allows deep root development in YTH304 and effectively accessed the soil water below the hardpan when the shallow soil layer experienced drought and consequently, contributed to the maintenance of shoot dry matter production. Furthermore, the plastic advantage of YTH304 over that of IR64 under SMF was inhibited by higher magnitude of hardpan strength (1.7 g/cum BD). The results provide valuable insights on the proper phenotyping environments suitable for screening rice lines custom-tailored to the kind of hardpan present in a characterized rainfed lowland rice ecosystem. Since YTH304 has the major genetic background of IR64 containing few introgression segments from the donor parents, future work can be possibly done to pinpoint the genetic control of the plasticity in nodal root hardpan penetration as well as deep root system development in rainfed lowland rice.