title: Spatial patterns of photosynthesis in thin- and thick-leaved epiphytic orchids: unravelling C3-CAM plasticity in an organ-compartmented way.
abstract: OBJECTIVE
  A positive correlation between tissue thickness and crassulacean acid metabolism (CAM) expression has been frequently suggested. Therefore, this study addressed the question of whether water availability modulates photosynthetic plasticity in different organs of two epiphytic orchids with distinct leaf thickness.
  
  
  METHODS
  Tissue morphology and photosynthetic mode (C3 and/or CAM) were examined in leaves, pseudobulbs and roots of a thick-leaved (Cattleya walkeriana) and a thin-leaved (Oncidium 'Aloha') epiphytic orchid. Morphological features were studied comparing the drought-induced physiological responses observed in each organ after 30 d of either drought or well-watered treatments.
  
  
  RESULTS
  Cattleya walkeriana, which is considered a constitutive CAM orchid, displayed a clear drought-induced up-regulation of CAM in its thick leaves but not in its non-leaf organs (pseudobulbs and roots). The set of morphological traits of Cattleya leaves suggested the drought-inducible CAM up-regulation as a possible mechanism of increasing water-use efficiency and carbon economy. Conversely, although belonging to an orchid genus classically considered as performing C3 photosynthesis, Oncidium 'Aloha' under drought seemed to express facultative CAM in its roots and pseudobulbs but not in its leaves, indicating that such photosynthetic responses might compensate for the lack of capacity to perform CAM in its thin leaves. Morphological features of Oncidium leaves also indicated lower efficiency in preventing water and CO2 losses, while aerenchyma ducts connecting pseudobulbs and leaves suggested a compartmentalized mechanism of nighttime carboxylation via phosphoenolpyruvate carboxylase (PEPC) (pseudobulbs) and daytime carboxylation via Rubisco (leaves) in drought-exposed Oncidium plants.
  
  
  CONCLUSIONS
  Water availability modulated CAM expression in an organ-compartmented manner in both orchids studied. As distinct regions of the same orchid could perform different photosynthetic pathways and variable degrees of CAM expression depending on the water availability, more attention should be addressed to this in future studies concerning the abundance of CAM plants.
authors.affiliation: Departamento de Botânica, Instituto de Biociências, Universidade de São Paulo, 05508-900, São Paulo, SP, Brazil.
authors.forename: Maria Aurineide
authors.initials: MA
authors.lastname: Rodrigues
authors.forename: Alejandra
authors.initials: A
authors.lastname: Matiz
authors.forename: Aline Bertinatto
authors.initials: AB
authors.lastname: Cruz
authors.forename: Aline Tiemi
authors.initials: AT
authors.lastname: Matsumura
authors.forename: Cassia Ayumi
authors.initials: CA
authors.lastname: Takahashi
authors.forename: Leonardo
authors.initials: L
authors.lastname: Hamachi
authors.forename: Lucas Macedo
authors.initials: LM
authors.lastname: Félix
authors.forename: Paula Natália
authors.initials: PN
authors.lastname: Pereira
authors.forename: Sabrina Ribeiro
authors.initials: SR
authors.lastname: Latansio-Aidar
authors.forename: Marcos Pereira Marinho
authors.initials: MP
authors.lastname: Aidar
authors.forename: Diego
authors.initials: D
authors.lastname: Demarco
authors.forename: Luciano
authors.initials: L
authors.lastname: Freschi
authors.forename: Helenice
authors.initials: H
authors.lastname: Mercier
authors.forename: Gilberto Barbante
authors.initials: GB
authors.lastname: Kerbauy
mesh_terms: D002244:Carbon / Q000378:metabolism; D055864:Droughts; D008291:Malate Dehydrogenase / Q000378:metabolism; D029595:Orchidaceae / Q000033:anatomy & histology* / Q000378:metabolism* / Q000502:physiology; D010730:Phosphoenolpyruvate Carboxylase / Q000378:metabolism; D010788:Photosynthesis / Q000502:physiology*; D018515:Plant Leaves / Q000033:anatomy & histology* / Q000378:metabolism; D018517:Plant Roots / Q000033:anatomy & histology / Q000378:metabolism / Q000502:physiology; D014867:Water / Q000378:metabolism
chemical_list: D014867:Water; D002244:Carbon; D008291:Malate Dehydrogenase; D010730:Phosphoenolpyruvate Carboxylase
keywords: Cattleya walkeriana; Oncidium ‘Aloha’; crassulacean acid metabolism; drought; epiphytic orchid; leaf succulence; non-leaf photosynthesis; photosynthetic plasticity