Quantitative Genetics of Mass Allocation and the Allometry of Reproduction in Amaranthus albus: Relation to Soil Nutrients

Mass allocation and the size dependency of reproductive allocation (RA) are primary components of life history that depend on resource availability and that affect microevolution. For example, tight genetic covariance between life history traits in one environment could limit the ability of selection to modify separate components of reproductive fitness. Here, the quantitative genetics of mass allocation and the allometry of reproduction were examined in relation to soil nutrients in the annual Amaranthus albus L. Questions addressed were (1) How do mass components and allocation change in response to increased soil nutrients? (2) Is there variation among families and inbred lines and among genetic by environment interactions? (3) Are there phenotypic and genetic correlations between vegetative mass (VM) components and total seed mass or RA? and (4) Are there genetic trade‐offs between allocation to VM components and relative fitness? A first generation of plants was reared from seeds of seven field‐collected maternal genotypes (inbred lines). Seed families from four first‐generation plants per inbred line were used to rear a second generation in a greenhouse for 10 wk. For each of 28 families, nine plants were regularly fertilized, while nine others were unfertilized. Dry masses of roots, stems, leaves, and seeds were determined and were used to calculate allocation; relative fitness was based on seed numbers. Stem allocation increased but root and seed allocation decreased under high soil nutrients. Path analysis showed that stem mass was positively correlated with RA, while root and leaf masses were negatively correlated. Positive genetic correlations occurred between total seed mass and VM in unfertilized and fertilized plants, but RA was independent of VM. Genetically based trade‐offs were detected between allocation to roots or leaves and relative fitness. Because seeds continually mature along indeterminate branches, increased reproduction of larger plants is mostly caused by greater number, length, and mass of stems. Trade‐offs among allocation components and fitness may constrain selection for increased seed production when soil nutrients are readily available.