Summary • The phenotype of offspring depends on the abiotic and biotic environment in which the parents developed. However, the direct effects of competition experienced by parent plants on single-seed traits are poorly documented despite their impact on plant fitness. • We hypothesize that single-seed traits can differentially respond to the resource deficiencies of parent plants due to competition: seed quality may decrease as seed number does, magnifying the negative effects of competition for offspring (‘passive response’ hypothesis), or increase and then enhance offspring fitness to offset the reduction in offspring number (‘adaptive response’ hypothesis). Here we tested these hypotheses for four single-seed traits. We assessed the sensibility of their responses to changes in competition intensity due to species with different competitive effects and to contrasting soil nitrogen conditions. • In a common-garden experiment, four single-seed traits related to fitness – seed mass, seed nitrogen concentration (SNC), germinability and the timing of germination – were measured on a phytometer species transplanted in 14 different neighbours grown in monoculture with and without soil nitrogen limitation. • Under nitrogen-limiting conditions, the responses of SNC and of the timing of germination were passive and mainly related to the effects of neighbours on soil nitrogen availability, as shown by the increase in SNC with N-fixing neighbours. Within-individual seed mass variability decreased with increasing competition intensity, as an adaptive response to counterbalance the reduction in seed production. With nitrogen supplementation, competitors had no detectable effect on single-seed traits despite an overall increase in SNC and germination rate, confirming their nitrogen-dependent passive responses to competition. Germinability did not change among treatments. • The impact of competition on single-seed traits depends on both phytometer trait identity and resource modulation by neighbours. The passive response of seed chemical composition to competitors may magnify the competitive effects on offspring. By contrast, the adaptive response of seed size variability may offset these competitive effects. As a consequence, experiments looking at the fitness consequences of competition should not only consider the effects on fitness parameters of a target plant but also on the offspring.