How important are environmental maternal effects in plants? A study with Centaurea maculosa

1 Previous studies have reported significant influences of maternal environment on offspring fitness in plants. We investigated the early plant development from seeds of Centaurea maculosa plants grown in four environments of increasing severity: (1) control; (2) herbivory; (3) herbivory + nutrient shortage; and (4) herbivory + nutrient shortage + grass competition. 2 Although these treatments had huge effects on the size of the mother plants and the number of seeds they produced, there was no evidence that the treatments affected the weight of the seeds they produced. There was, however, significant variation in seed weight among maternal plants within treatments. 3 We grew individually weighed seeds from 33 of these maternal plants in three competitive regimes (individually, with one conspecific neighbour, with three Festuca pratensis neighbours) to test if maternal environment and seed weight influenced first-year growth, and if these effects were more pronounced in the presence of competition. 4 There were a few weak but significant environmental maternal effects on offspring performance. Seed weight was positively correlated with initial growth, but its influence decreased over time and disappeared after 8 weeks. The presence of one conspecific neighbour or three Festuca pratensis neighbours did not influence growth during the first few weeks, but strongly suppressed growth after 9 weeks. Competition did not accentuate the influence of seed weight on offspring performance. 5 Differences among individual maternal plants were a major source of variation in seed weight and early offspring growth. Some of the evidence that has been cited in support of environmental maternal effects in plants may be the result of confounding maternal identity and maternal environment. 6 Our results support the generalization that seed size is one of the least plastic of plant characters. Plants express great plasticity in reproductive output, but this occurs primarily in terms of the number of seeds produced, and only secondarily, if at all, in terms of seed size or quality. 7 Effects of a plant's maternal environment on its performance can be detected, but they appear to be small compared to other factors that influence a plant's fitness, such as its genotype and the environment in which it grows.