Sweet pearl millet (Pennisetum glaucum [L.] R. BR.) (SPM) and sweet sorghum (Sorghum bicolor [L.] Moench) (SS) are annual C4 grasses that could be interesting alternatives to corn (Zea mays [L.]) for ethanol production in Eastern Canada. The nitrogen (N) requirements of SPM and SS are considerably lower than for corn, suggesting a greater efficiency in soil- and fertilizer-N uptake. Since it is known that fine roots are the main pathway for plant nutrient uptake, differences in root morphology could partly explain this variable N uptake efficiency.

In a randomized block design, we compared the root systems of SPM (hybrid CSSPM7), SS (hybrid CSSH45), and corn (hybrid DKC 43-27) during two growing seasons on a well-drained sandy loam near Quebec City, Canada. Composite soil cores (8 cm diameter, 30 cm depth) were taken in each plot on the row and on the inter-row. Roots were separated from soil using a hydropneumatic elutriation system, red dyed, and scanned. Image analysis was performed with the software WinRhizo (Regent Instrument Inc., Quebec, Canada).

The ash-free root biomass was greater for corn and SS (5043 kg ha^-1 and 4632 kg ha^-1) than for SPM (2728 kg ha^-1) (P<0.001). However, the total root length showed an opposite trend with 72 km m^-3 for SPM, and only 47 km m^-3 and 41 km m^-3 for SS and corn respectively (P<0.001). The three crops also differed in the proportion of total root surface accounted for by fine roots of diameter < 0.2 mm. This proportion was larger for SPM (28% of total root surface) than for SS (24%) and corn (19%) (P<0.001). Other hypotheses to explain differences in N uptake must be investigated. Nevertheless, an improved knowledge of root systems of bioenergy crops is essential, in particular in evaluating their contribution to soil organic carbon.