Is the root system the key to higher nitrogen uptake in rapeseed? The case of a composite hybrid line

The recently commercialised rapeseed CHL (composite hybrid lines) are reported to give an average increase of about 15% in grain yield compared with 'old' varieties in normal growing conditions in France. In Italy, this observation has not be confirmed, since CHL Synergy and a good-yielding variety Ceres generally have similar yields (average of 12 field trials). However, several differences in morphological and physiological characters have been identified in the two genotypes, especially at root level. Both shoot biomass and the amount of shoot nitrogen in Synergy were slightly higher compared with Ceres for most of the growing season, until the end of winter. As regards the root system, Synergy shows evident hybrid vigour, having faster and, especially in early stages, greater weight and total length than Ceres, together with higher specific root length. These traits are a prerequisite for the uptake of greater amounts of nitrogen, which may eventually be leached. Although an increase in plant density led to higher root length density in the first soil layer (0-30 cm) for both varieties, nitrate leaching of Synergy exceeded that of Ceres at the high plant density, thus indicating that this hybrid is more sensitive to competition for nitrogen uptake. The increase in nitrate uptake rate per unit root length (Vmax) found for the hybrid compared with the 'old' variety was accompanied by an unfavourable increment of affinity (1/Km), which may be responsible of a loss of efficiency at high plant density. The morphological and physiological modifications of the hybrid seem to be more useful for reducing nitrogen leaching rather than for increasing grain yield.