title: Natural foliar variegation without costs? The case of Begonia.
abstract: OBJECTIVE
  Foliar variegation is recognized as arising from two major mechanisms: leaf structure and pigment-related variegation. Begonia has species with a variety of natural foliar variegation patterns, providing diverse examples of this phenomenon. The aims of this work are to elucidate the mechanisms underlying different foliar variegation patterns in Begonia and to determine their physiological consequences.
  
  
  METHODS
  Six species and one cultivar of Begonia were investigated. Light and electron microscopy revealed the leaf structure and ultrastructure of chloroplasts in green and light areas of variegated leaves. Maximum quantum yields of photosystem II were measured by chlorophyll fluorescence. Comparison with a cultivar of Ficus revealed key features distinguishing variegation mechanisms.
  
  
  RESULTS
  Intercellular space above the chlorenchyma is the mechanism of variegation in these Begonia. This intercellular space can be located (a) below the adaxial epidermis or (b) below the adaxial water storage tissue (the first report for any taxa), creating light areas on a leaf. In addition, chlorenchyma cell shape and chloroplast distribution within chlorenchyma cells differ between light and green areas. Chloroplasts from both areas showed dense stacking of grana and stroma thylakoid membranes. The maximum quantum yield did not differ significantly between these areas, suggesting minimal loss of function with variegation. However, the absence of chloroplasts in light areas of leaves in the Ficus cultivar led to an extremely low quantum yield.
  
  
  CONCLUSIONS
  Variegation in these Begonia is structural, where light areas are created by internal reflection between air spaces and cells in a leaf. Two forms of air space structural variegation occur, distinguished by the location of the air spaces. Both forms may have a common origin in development where dermal tissue becomes loosely connected to mesophyll. Photosynthetic functioning is retained in light areas, and these areas do not include primary veins, potentially limiting the costs of variegation.
authors.affiliation: Department of Life Sciences, National Chung Hsing University, 250, Kuo Kuang Rd, Taichung 402, Taiwan. crsheue@gmail.com
authors.forename: Chiou-Rong
authors.initials: CR
authors.lastname: Sheue
authors.forename: Shang-Horng
authors.initials: SH
authors.lastname: Pao
authors.forename: Lee-Feng
authors.initials: LF
authors.lastname: Chien
authors.forename: Peter
authors.initials: P
authors.lastname: Chesson
authors.forename: Ching-I
authors.initials: CI
authors.lastname: Peng
mesh_terms: D031112:Begoniaceae / Q000033:anatomy & histology* / Q000235:genetics / Q000378:metabolism*; D002736:Chloroplasts / Q000648:ultrastructure*; D005110:Extracellular Space; D030681:Ficus / Q000033:anatomy & histology / Q000378:metabolism; D014644:Genetic Variation; D005838:Genotype; D010788:Photosynthesis; D010858:Pigmentation / Q000502:physiology*; D019441:Plant Epidermis / Q000166:cytology / Q000378:metabolism; D018515:Plant Leaves / Q000166:cytology* / Q000378:metabolism*; D013045:Species Specificity