Reproductive Effort in Plants. 1. Carbon Allocation to Reproduction

Abstract

Reproductive effort, or the proportion of an organism's resources allocated to reproduction, is a crucial aspect of an organism's life history; the optimal allocation of resources to reproduction in different environments has been the subject of much theorizing. Adequate tests of these theories have been hampered by the difficulties involved in assessing reproductive effort. In this paper, we address the problem of determining which structures and activities should be considered part of reproduction, using Agropyron repens as the experimental material. We approached the problem by first determining the structures and activities necessary for vegetative growth and then determining reproductive structures and activities by subtraction. Using carbon as the currency of allocation, we defined vegetative growth as those structures directly involved in the capture of carbon (i.e., leaves) plus all necessary support structures and activities. The necessary support structures and activities were determined by comparison with vegetative plants grown under similar conditions. Reproduction in A. repens involves not only the production of flowering and fruiting structures, but also the production of a substantial amount of stem material, the loss of carbon through respiration, and carbon gain through photosynthesis. Reproductive photosynthesis includes both direct photosynthesis by the reproductive structures and reproductive enhancement of leaf photosynthesis. Simple measures of reproductive effort based on biomass allocation to flowering and fruiting structures do not adequately reflect carbon allocation to reproduction since there is both genotypic and environmentally induced variation among plants in the amount of reproductive stem material and in the ability to compensate for the cost of reproduction through reproductive photosynthesis.