Drought-induced changes in rooting patterns and assimilate partitioning between root and shoot in upland rice
Drought is a major stress affecting rainfed rice systems. Root characteristics such as root length density, root thickness, and rooting depth and distribution have been established as constituting factors of drought resistance. Deep rooting cultivars are more resistant to drought than those with shallow root systems. The present study sought to quantify the effects of different levels of drought on dry matter partitioning and root development of three rice cultivars CG14 (Oryza glaberrima), WAB56-104 (O. sativa tropical japonica, improved) and WAB450-24-3-2-P18-HB (CG14 × WAB56-104 hybrid). Two experiments on assimilate partitioning under different levels of drought stress were conducted under rain shelters at the West Africa Rice Development Association, Mbe, Ivory Coast. PVC tubes (diameter = 0.2 m, height 0.6 m) containing about 25 kg of sandy loam were used for the drought stress experiments. For rooting depth and root distribution studies, the tubes were subdivided into four compartments of 0.15 m each. In the first trial, tubes with WAB56-104 were gradually droughted to five levels of soil moisture content that were kept constant thereafter. In the second trial, plants of all cultivars were subjected to three drought treatments: (1) constant soil moisture content at field capacity (about 22% moisture content), (2) constant soil moisture content of 14% (about −0.5 MPa soil matrix potential) and (3) constant soil moisture content of 9% (about −1 MPa soil matrix potential). Rice reacted to drought stress with reductions in height, leaf area and biomass production, tiller abortion, changes in root dry matter and rooting depth and a delay in reproductive development. Assimilate partitioning between root and shoot, determined from changes in dry matter, was not affected by drought when the plants were gradually stressed. In no case, additional biomass was partitioned to the roots; on the contrary, dry matter partitioning to the root completely ceased under severe stress. Due to the irrigation technique used, vertical soil moisture distribution varied little, but roots grew deeper under drought stress. This was particularly the case for the upland adapted WAB56-104. Implication for modeling of drought responses in upland rice systems is discussed.