title: Physiological responses of the CAM epiphyte Tillandsia usneoides L. (Bromeliaceae) to variations in light and water supply.
abstract: In an effort to understand the mechanisms that sustain rootless atmospheric plants, the modulation of Crassulacean acid metabolism (CAM) in response to variations in irradiance and water supply was investigated in the epiphyte Tillandsia usneoides. Plants were acclimated to three light regimes, i.e. high, intermediate and low, with integrated photon flux densities (PFD) of 14.40, 8.64 and 4.32 mol m-2 d-1 equivalent to an instantaneous PFD of 200, 100, and 50 mumol m-2 s-1, respectively. Daily watering was then withdrawn from half of the plants at each PFD for 7 d prior to sampling. In response to the three PFD treatments, chlorophyll content increased in plants acclimated to lower irradiances. Light response curves using non-invasive measurements of chlorophyll fluorescence demonstrated that photosystem II efficiency (phi PSII) was maintained in high PFD acclimated plants, as they exhibited a larger capacity for non-photochemical dissipation (NPQ) of excess light energy than low PFD acclimated plants. Net CO2 uptake increased in response to higher PFD, reflecting enhanced carboxylation capacity in terms of phosphoenolpyruvate carboxylase (PEPc) and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities. After water was withdrawn, nocturnal net CO2 uptake and accumulated levels of acidity declined in all PFD treatments, concomitant with increased respiratory recycling of malate. Examining the strategies employed by epiphytes such as T. usneodies to tolerate extreme light and water regimes has demonstrated the importance of physiological mechanisms that allow flexible carboxylation capacity and continued carbon cycling to maintain photosynthetic integrity.
authors.affiliation: School of Biology, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK. richard.haslam@bbsrc.ac.uk
authors.forename: Richard
authors.initials: R
authors.lastname: Haslam
authors.forename: Anne
authors.initials: A
authors.lastname: Borland
authors.forename: Kate
authors.initials: K
authors.lastname: Maxwell
authors.forename: Howard
authors.initials: H
authors.lastname: Griffiths
mesh_terms: D000064:Acclimatization / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D002245:Carbon Dioxide / Q000378:metabolism; D002734:Chlorophyll / Q000378:metabolism; D008027:Light; D008293:Malates / Q000378:metabolism; D010730:Phosphoenolpyruvate Carboxylase / Q000378:metabolism; D010788:Photosynthesis / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D045322:Photosynthetic Reaction Center Complex Proteins / Q000145:classification / Q000187:drug effects / Q000528:radiation effects; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism; D031232:Tillandsia / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D014867:Water / Q000494:pharmacology*
chemical_list: D008293:Malates; D045322:Photosynthetic Reaction Center Complex Proteins; D014867:Water; D002734:Chlorophyll; D002245:Carbon Dioxide; C030298:malic acid; D010730:Phosphoenolpyruvate Carboxylase; D012273:Ribulose-Bisphosphate Carboxylase