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

Abstract The meaning of variation in 15 N/ 14 N 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 15 N abundances were correlated to agronomic and physiological traits. At the level of means, at high N-input there appeared no difference in 15 N partitioning according to plant organs, with the same abundances for blades, stalks + sheaths and kernels. However, at low N-input blades and kernels were 15 N-enriched, whereas stalks were significantly 15 N-depleted with an abundance close to that observed for high N-input. 15 N 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 15 N abundance and correlations involving 15 N abundance were always lower at high N-input than at low N-input. 15 N abundances of blades and stalks + sheaths were negatively related to silking date whatever the stage (silking or maturity) and N-fertilization whereas kernel 15 N abundance was not affected by silking date. At low N-input, whole-plant 15 N abundance at maturity was positively correlated to whole-plant and kernel protein content whereas at high N-input such correlation disappeared. Whole-plant 15 N 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 15 N 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 15 N 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.