effect of reproduction on canopy structure, allocation and growth in Oenothera biennis

(1) Time of reproduction in Oenothera biennis was controlled experimentally by gibberellic acid applications in a glasshouse experiment. Performance of vegetative plants and plants reproducing at various ages was compared by means of growth analysis. (2) Regardless of age, reproduction increased stem allocation and decreased root and leaf allocation. Early reproduction reduced growth relative to vegetative plants, while late reproduction had no effect on growth in the long term and enhanced growth in the short term. Seed output increased with age at reproduction up to a point after which there was little change. (3) The reduced growth of plants reproducing at an early age was due to a decrease in leaf area ratio, and the enhanced growth of older plants to a temporary increase. Specific leaf area in older plants increased with reproduction, temporarily over-compensating for the decrease in leaf allocation. There was a temporary increase in unit leaf rate in plants reproducing at a young age but not in plants reproducing at a late age. (4) Changes in specific leaf area were the result of a switch in leaf morphology between vegetative and reproductive plants. Vegetative plants produced leaves with petioles which increased in length with age. This morphology minimized self-shading in the vegetative rosettes. Leaves produced after reproduction had short petioles and, consequently, a higher specific leaf area. Plants reproducing at an early age produced few new leaves and exhibited little change in specific leaf area. (5) It is suggested that the beneficial effects of reproduction on canopy structure and photosynthesis that are observed in many species can compensate for the cost of reproduction, providing the accumulated nutrient reserves are sufficient to supply the reproductive structures.