Contribution of photosynthetic rate to growth and reproduction in Amaranthus hybridus

A. Michele Arntz, Evan H. DeLucia, Nicholas Jordan

Research output: Contribution to journalArticlepeer-review

48 Scopus citations


While it is known that genetic variation for photosynthetic and growth traits exists in natural populations, the functional significance of this variation remains unclear, particularly for photosynthetic traits. To test the hypothesis that photosynthetic rate has direct effects on reproduction as well as contributing indirectly to reproduction through effects on growth, we compared wild-type Amaranthus hybridus families to those with a single gene mutation that confers a lower photosynthetic rate. Wild-type and photosynthetic-mutant families were grown in competitive and non-competitive environments and we compared size, biomass allocation, architecture, and reproduction at three developmental stages. To assess the contributions of individual growth traits to reproduction, we calculated covariances between standardized traits and relative fitness (selection differentials), and compared selection between the two biotypes. Finally, we used path analysis to calculate the indirect effects of photosynthetic rate on fitness through growth. The size, allocation, and architecture of photosynthetic mutants did not differ from those of the wild type in either the competitive or non-competitive environment, with the exception that they were taller by the last developmental stage. However, the reproductive biomass of the photosynthetic mutants was significantly reduced compared to the wild type. In the competitive environment, the wild type achieved greater fitness because, while similar in size to the mutants, at any given size it produced more reproductive biomass. This suggests that photosynthetic rate affected the linkage between plant size and reproduction and is evidence of an indirect contribution to fitness. In the non-competitive environment, there were fewer differences in selection differentials between the two plant genotypes, suggesting fewer indirect effects. Path analysis showed that variation in photosynthetic biotype had indirect effects on reproductive biomass, via growth traits, and that there were no direct effects. Photosynthetic rate appears to have fitness consequences primarily through multiple contributions to growth throughout development.

Original languageEnglish (US)
Pages (from-to)323-330
Number of pages8
Issue number3
StatePublished - 1998

Bibliographical note

Funding Information:
Acknowledgements We thank Michelle Coles and Allison Cheese-man for assistance in the field. Carol Augspurger, Jeff Conner, Don Ort, and Art Zangerl provided useful comments on many phases of this project. We also thank the anonymous reviewers whose insights improved the manuscript. This research was supported by a DOE/NSF/USDA Interdisciplinary Research Training Grant on Integrative Photosynthesis Research (DOE 92ER20095) to A.M.A.


  • Atrazine
  • Fitness
  • Path analysis
  • Photosynthesis
  • Resistance
  • Selection


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