Water supply and not nitrate concentration determines primary root growth in Arabidopsis.

Understanding how root system architecture (RSA) adapts to changing nitrogen and water availability is important for improving acquisition. A sand rhizotron system was developed to study RSA in a porous substrate under tightly regulated nutrient supply. The RSA of Arabidopsis seedlings under differing nitrate (NO₃⁻) and water supplies in agar and sand was described. The hydraulic conductivity of the root environment was manipulated by using altered sand particle size and matric potentials. Ion-selective microelectrodes were used to quantify NO₃⁻ at the surface of growing primary roots in sands of different particle sizes. Differences in RSA were observed between seedlings grown on agar and sand, and the influence of NO₃⁻ (0.1-10.0 mm) and water on RSA was determined. Primary root length (PRL) was a function of water flux and independent of NO₃⁻. The percentage of roots with laterals correlated with water flux, whereas NO₃⁻ supply was important for basal root (BR) growth. In agar and sand, the NO₃⁻ activities at the root surface were higher than those supplied in the nutrient solution. The sand rhizotron system is a useful tool for the study of RSA, providing a porous growth environment that can be used to simulate the effects of hydraulic conductivity on growth.