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Keywords:

  • Abutilon theophrasti;
  • allometry;
  • families;
  • plant size;
  • resource heterogeneity

1. Resource availability, competition and genetic difference are among the main factors causing size variation in plant populations. These factors also have great effects on the variation in reproductive allocation. We examined the effects of soil nutrient availability, competition and genetic identity on the size dependency of reproductive allocation in Abutilon theophrasti families grown under different conditions of soil nutrient and conspecific competition.

2. Soil nutrients, competition and genetic identity brought about a strong dependency of reproductive allocation on size. However, these factors differed in functional forms and signs of the size dependency.

3. An allometric (log–log) regression between seed mass and vegetative mass fit the data better than a linear regression. The slopes of allometric regressions for each nutrient treatment were lower than 1·0. Therefore, there was a negative relationship between plant size and reproductive allocation. However, the slopes calculated over different nutrient treatments were greater than 1·0 (i.e. positive relationship between plant size and reproductive allocation). We suggest that these two reproductive allometries indicate two different biological relationships: the former represents a physiological trade-off between resource acquisition and reproductive allocation within a plant and the latter represents an allometric response to soil nutrients.

4. Genetic identity showed intrinsic effects on size dependency of reproductive allocation owing to trade-off and not as a result of the allometric response to soil nutrients.

5. Nutrient availability changed the slopes of the allometric regressions. An increase in nutrient availability alleviated the negative relationship between plant size and reproductive allocation.

6. The slopes of the allometric regressions within nutrient treatments were significantly higher in competition treatments than in competition-free treatments, although the slopes of the regressions over different nutrient levels did not differ between the two treatments. These results suggest that the effects of competition on reproductive allocation are owing to indirect effects of size difference on the allometric relationship rather than intrinsic effects.