Timing of reproduction is a key life-history trait, and has been the focus of substantial attention. Delayed reproduction in low phosphorus availability soils is common among annual plants, in contrast to the early reproduction typically observed in other low-nutrient environments. This paradoxical response has received little attention. We hypothesize that this response from the high marginal value of additional allocation to root growth caused by the low mobility of phosphorus in soils. In order to better understand the benefits and costs of such delayed reproduction, we present a two-resource dynamic allocation model of plant growth and reproduction. Our model incorporates growth, respiration, and carbon and phosphorus acquisition of both root and shoot organs, and considers the reallocation of resources from senescent leaves. We parameterize the model with data from Arabidopsis, and explore the optimal reproductive phenology in several environments.
Results/Conclusions

The model predicts delayed reproduction in low-phosphorus environments, in contrast to other models of optimum phenology. Reproductive timing in low-phosphorus environments is sensitive both to mortality risks and phosphorus mobility. Simulated yields exhibited greatest sensitivity to the maximum assimilation rate and the timing of mortality risk. Simulated phenology in low-phosphorus was sensitive to a greater number of parameters than in high-phosphorus. The model predicts that delayed reproduction in response to low phosphorus conditions may be reduced in plants adapted to environments where mortality risk is encountered earlier in the growing season.