From Fluorescence to Fitness: Variation in Photosynthetic Rate Affects Fecundity and Survivorship

Genetic variation in photosynthetic traits within populations provides the potential for evolution, but few studies relate phenotypic variation in these traits to variation in fitness. We tested the prediction that a lower photosynthetic rate reduces fecundity and survivorship by comparing wild-type (WT) Amaranthus hybridus family lines to those having a single-gene mutation that confers resistance to atrazine (R) and lowers the rate of photosynthetic carbon assimilation. Wild-type and R family lines with nearly uniform nuclear genomes were used to minimize the confounding effects of other loci. We established experimental populations in agricultural and one-year-old field plots and measured chlorophyll fluorescence, gas exchange, and the fecundity and survivorship of WT and R genotypes for two generations. The R genotype had a lower efficiency of electron transport through photosystem II, which translated into a 20–30% decrease in photosynthetic rate at light levels above 400 μmol·m−2·s−1. Compared to the WT, the R genotype also had lower water-use efficiency, higher specific leaf area, and greater leaf nitrogen concentration on a mass, but not area, basis. In five of six replicate populations, the R genotype had lower fecundity than the WT in the first generation. Survivorship of seed over winter was similar for the two genotypes, but survivorship of R seedlings during early establishment was lower than the WT in the agricultural field. The consistent pattern of selection against the R genotype during vegetative growth stages suggests that a lower photosynthetic rate reduces fitness. This selection, paired with heritable variation for photosynthetic traits within populations, provides a more complete scenario for the evolution of photosynthetic traits.