Effect of reproduction on nitrogen allocation and carbon gain in Oenothera biennis

Reproduction in Oenothera biennis has been shown to decrease growth in young plants, whereas reproduction in older plants temporarily increases growth and has no negative effect on growth in the long term. The causes of these variable effects were investigated by examining the effect of reproduction upon photosynthetic rate, leaf area production, chlorophyll content and nitrogen allocation in young versus old plants grown at low versus high nutrient availability. Reproduction was controlled experimentally by gibberellic acid applications, and measurements were made at three developmental stages: bolting, flowering, and capsule maturation. At each stage, measurements were also made on corresponding vegetative plants of the same age. Reproduction decreased nitrogen allocation to roots and increased allocation to shoots. The decrease in root allocation was greater at low nutrient availability. Reproduction increased leaf area and, at bolting, the magnitude of this increase was greater in plants grown at high nutrient availability. Reproduction generally decreased photosynthetic rates, chlorophyll content and nitrogen content of leaves. The magnitude of the decreases was usually less for plants grown at high nutrient availability. Photosynthetic rate increased with reproduction for older plants grown at high nutrient availability in the latter part of the experiment. We suggest that differences among Oenothera biennis individuals in the effect of reproduction on carbon gain are related to differences in extent of nutrient reserves. Older plants and plants grown at high nutrient availability have greater nutrient reserves upon which to draw when reproduction is initiated. Reproduction in younger plants grown at lower nutrient availability will rapidly deplete nutrient reserves and nutrients which are part of the photosynthetic apparatus (e.g. the nitrogen within the chlorophyll molecule) will have to be mobilized to supply reproductive structures. Reproduction in this latter case will therefore have more of a detrimental effect on photosynthetic rate and leaf area production.