Genetic variation and QTLs for 15N natural abundance in a set of maize recombinant inbred lines

The meaning of variation in N-15/N-14 isotope ratio in plants grown in the field is better known when variation is due to environment than when it is due to plant genotype. To study the physiological and genetic meaning of variation of such a ratio, a set of 99 recombinant inbred lines of maize were evaluated at low and high N-input and organ N-15 abundances were correlated to agronomic and physiological traits. At the level of means, at high N-input there appeared no difference in N-15 partitioning according to plant organs, with the same abundances for blades, stalks + sheaths and kernels. However, at low N-input blades and kernels were N-15-enriched, whereas stalks were significantly N-15-depleted with an abundance close to that observed for high N-input. N-15 abundance of whole-plant and organs showed significant genotypic effects and genotype by nitrogen input interaction, varying according to the organ and the stage, silking and grain maturity. Genetic variation for N-15 abundance and correlations involving N-15 abundance were always lower at high N-input than at low N-input. N-15 abundances of blades and stalks + sheaths were negatively related to silking date whatever the stage (silking or maturity) and N-fertilization whereas kernel N-15 abundance was not affected by silking date. At low N-input, whole-plant N-15 abundance at maturity was positively correlated to whole-plant and kernel protein content whereas at high N-input such correlation disappeared. Whole-plant N-15 abundance at silking was negatively related to root fresh weight and to glutamine synthetase activity measured in young plants grown in hydroponics. Twelve QTLs for N-15 abundance were detected, mainly at high N-input; among them, 10 coincided with QTLs involved in nitrogen use efficiency (grain yield, N-uptake and N remobilization) and the root system. Interpretation of all results leads to the conclusion that two mechanisms could explain genetic variation in N-15 discrimination ability: morpho-physiological differences, in particular in the root system, and activities of the first enzymes of nitrogen metabolism, with a positive relationship between enzyme activity and discrimination abilities. (c) 2005 Published by Elsevier B.V.