title: Photosynthetic and growth responses of reciprocal hybrids to variation in water and nitrogen availability.
abstract: OBJECTIVE
  Fitness of plant hybrids often depends upon the environment, but physiological mechanisms underlying the differential responses to habitat are poorly understood. We examined physiological responses of Ipomopsis species and hybrids, including reciprocal F(1)s and F(2)s, to variation in soil moisture and nitrogen. •
  
  
  METHODS
  To examine responses to moisture, we subjected plants to a dry-down experiment. Nitrogen was manipulated in three independent experiments, one in the field and two in common environments. •
  
  
  RESULTS
  Plants with I. tenuituba cytoplasmic background had lower optimal soil moisture for photosynthesis, appearing better adapted to dry conditions, than plants with I. aggregata cytoplasm. This result supported a prediction from prior studies. The species and hybrids did not differ greatly in physiological responses to nitrogen. An increase in soil nitrogen increased leaf nitrogen, carbon assimilation, integrated water-use efficiency, and growth, but the increases in growth were not mediated primarily by an increase in photosynthesis. In neither the field, nor in common-garden studies, did physiological responses to soil nitrogen differ detectably across plant types, although only I. aggregata and hybrids increased seed production in the field. •
  
  
  CONCLUSIONS
  These results demonstrate differences in photosynthetic responses between reciprocal hybrids and suggest that water use is more important than nitrogen in explaining the relative photosynthetic performance of these hybrids compared to their parents.
authors.affiliation: Department of Ecology & Evolutionary Biology, University of California, Irvine, California 92697 USA.
authors.forename: Diane R
authors.initials: DR
authors.lastname: Campbell
authors.forename: Carrie A
authors.initials: CA
authors.lastname: Wu
authors.forename: Steven E
authors.initials: SE
authors.lastname: Travers