title: Seasonal changes in physiological performance in wild Clarkia xantiana populations: Implications for the evolution of a compressed life cycle and self-fertilization.
abstract: OBJECTIVE
  One explanation for the evolution of selfing, the drought escape hypothesis, proposes that self-fertilization may evolve under conditions of intensifying seasonal drought as part of a suite of traits that enable plants to accelerate the completion of their life cycle, thereby escaping late-season drought. Here, we test two fundamental assumptions of this hypothesis in Clarkia xantiana: (1) that a seasonal decline in precipitation causes an increase in drought stress and (2) that this results in changes in physiological performance, reflecting these deteriorating conditions.
  
  
  METHODS
  We examined seasonal and interannual variation in abiotic environmental conditions (estimated by ambient temperature, relative humidity, predawn leaf water potentials, and carbon isotope ratios) and physiological traits (photosynthesis, conductance, transpiration, instantaneous water-use efficiency, ascorbate peroxidase and glutathione reductase activities, quantum yield of photosystem II, PSII potential efficiency) in field populations of C. xantiana in 2009 and 2010.
  
  
  RESULTS
  In both years, plants experienced intensifying drought across the growing season. Gas exchange rates decreased over the growing season and were lower in 2009 (a relatively dry year) than in 2010, suggesting that the temporal changes from early to late spring were directly linked to the deteriorating environmental conditions.
  
  
  CONCLUSIONS
  Seasonal declines in transpiration rate may have increased survival by protecting plants from desiccation. Concomitant declines in photosynthetic rate likely reduced the availability of resources for seed production late in the season. Thus, the physiological patterns observed are consistent with the conditions required for the drought escape hypothesis.
authors.affiliation: Warren Wilson College, P.O. Box 9000, Asheville, North Carolina 28815 USA.
authors.forename: Leah S
authors.initials: LS
authors.lastname: Dudley
authors.affiliation: Warren Wilson College, P.O. Box 9000, Asheville, North Carolina 28815 USA.
authors.forename: Alisa A
authors.initials: AA
authors.lastname: Hove
authors.affiliation: University of St. Thomas, 2115 Summit Ave., St. Paul, Minnesota, USA 5105.
authors.forename: Simon K
authors.initials: SK
authors.lastname: Emms
authors.affiliation: University of St. Thomas, 2115 Summit Ave., St. Paul, Minnesota, USA 5105.
authors.forename: Amy S
authors.initials: AS
authors.lastname: Verhoeven
authors.affiliation: University of California, Santa Barbara, Santa Barbara, California 93106 USA.
authors.forename: Susan J
authors.initials: SJ
authors.lastname: Mazer
mesh_terms: D000704:Analysis of Variance; D000975:Antioxidants / Q000378:metabolism; D005075:Biological Evolution; D002140:California; D002734:Chlorophyll / Q000378:metabolism; D031663:Clarkia / Q000033:anatomy & histology / Q000254:growth & development* / Q000502:physiology*; D004777:Environment; D005453:Fluorescence; D005740:Gases / Q000378:metabolism; D016018:Least-Squares Analysis; D025341:Principal Component Analysis; D011891:Rain; D012621:Seasons; D057945:Self-Fertilization; D013696:Temperature; D014867:Water
chemical_list: D000975:Antioxidants; D005740:Gases; D014867:Water; D002734:Chlorophyll
keywords: California, Clarkia; Onagraceae; drought; gas exchange physiology; mating system evolution; outcrossing; selfing