Impact of three lateral root types identified in pearl millet on water uptake

Pearl millet plays an important role for food security in arid regions. Improving its root system toward increased water uptake efficiency could improve pearl millet tolerance to drought and lead to a significant production rise under water-limiting conditions. The objective of this work was to provide an integrated description of root system development at early stages and to assess the consequences of these findings for water uptake. A precise description of pearl millet root system development evidenced large variability among lateral root growth profiles. To further analyze this diversity, the growth rates of many lateral roots were measured daily and a statistical model was designed to classify these roots on the basis of their growth profiles. Three categories of lateral roots were identified in this way: (i) roots with high growth rate that could keep on growing after the end of the experiment, (ii) roots with intermediate growth rates and (iii) roots with low growth rates that quickly stop growing. The different lateral root types were randomly distributed along the primary root and there was no influence of root types on the intervals between successive lateral roots. The proportions of these different types were variable between plants of the same genotype, suggesting that small environmental variations could control these proportions. Water movements between soil and roots is modeled with R-SWMS to assess (i) the benefits of the three-type structuring, (ii) the relative contribution of each type to water uptake and (iii) the impact of specific proportions on global water uptake.