Energy sorghum hybrids: Functional dynamics of high nitrogen use efficiency

The nitrogen use efficiency (NUE) of high biomass energy sorghum hybrid plants increased during 180 days of growth to a maximum of 370 g DW g−1 N−1. Shoot N uptake was biphasic and continued for 120 days. Leaf N accumulation was rapid until day 60. Specific leaf nitrogen (SLN) varied from 0.9 to 1.7 g N m−2 green leaf area, a typical range for C4 grass canopies. Stem N increased to a maximum at day 120. NUE increased during development in parallel with increasing stem to leaf biomass ratio and as stems decreased from 0.7% to 0.2% N. At the end of the season, green leaves were ∼1% N, represented 17% of total shoot biomass and accounted for 50% of N present in shoots (above ground biomass) while stems were ∼0.2% N, comprised 83% of shoot biomass and accounted for 50% of shoot N. High NUE was due in part to N-remobilization from lower leaves and stem nodes/internodes to upper portions of the canopy. Up to 70% of dry weight and 90% of N was remobilized during senescence of lower leaves and 70% of N was remobilized from lower stem nodes/internodes. The NUE of energy sorghum was similar to Saccharum officinarum and Miscanthus x giganteus, and higher than grain Sorghum bicolor, Zea mays, and Panicum virgatum. High NUE of energy S. bicolor is due to long duration of vegetative growth, high stem to leaf biomass ratio, and very efficient N-remobilization from lower leaves and stem internodes during development.