title: Responses of chlorophyll fluorescence parameters of the facultative halophyte and C3-CAM intermediate species Mesembryanthemum crystallinum to salinity and high irradiance stress.
abstract: Mesembryanthemum crystallinum L. (Aizoaceae) is a facultative annual halophyte and a C(3)-photosynthesis/crassulacean acid metabolism intermediate species currently used as a model plant in stress physiology. Both salinity and high light irradiance stress are known to induce CAM in this species. The present study was performed to provide a diagnosis of alterations at the photosystem II level during salinity and irradiance stress. Plants were subjected for up to 13 days to either 0.4M NaCl salinity or high irradiance of 1000 micromol m(-2)s(-1), as well as to both stress factors combined (LLSA=low light plus salt; HLCO=high light of 1000 micromol m(-2)s(-1), no salt; HLSA=high light plus salt). A control of LLCO=low light of 200 micromol m(-2)s(-1), no salt was used. Parameters of chlorophyll a fluorescence of photosystem II (PSII) were measured with a pulse amplitude modulated fluorometer. HLCO and LLSA conditions induced a weak degree of CAM with day/night changes of malate levels (Deltamalate) of approximately 12mM in the course of the experiment, while HLSA induced stronger CAM of Deltamalate approximately 20 mM. Effective quantum yield of PSII, DeltaF/F'(m), was only slightly affected by LLSA, somewhat reduced during the course of the experiment by HLCO and clearly reduced by HLSA. Potential quantum efficiency of PSII, F(v)/F(m), at predawn times was not affected by any of the conditions, always remaining at 0.8, showing that there was no acute photoinhibition. During the course of the days HL alone (HLCO) also did not elicit photoinhibition; salt alone (LLSA) caused acute photoinhibition which was amplified by the combination of the two stresses (HLSA). Non-photochemical, NPQ, quenching remained low (<0.5) under LLCO, LLSA and HLCO and increased during the course of the experiment under HLSA to 1-2. Maximum apparent photosynthetic electron transport rates, ETR(max), declined during the daily courses and were reduced by LLSA and to a similar extent by HLSA. It is concluded that M. crystallinum expresses effective stress tolerance mechanisms but photosynthetic capacity is reduced by the synergistic effects of salinity and light irradiance stress combined.
authors.affiliation: Department of Chemistry and Biochemistry, Institute of Biosciences, São Paulo State University (UNESP), 18618-000 Botucatu, SP, Brazil. broetto@ibb.unesp.br
authors.forename: Fernando
authors.initials: F
authors.lastname: Broetto
authors.forename: Heitor
authors.initials: H
authors.lastname: Monteiro Duarte
authors.forename: Ulrich
authors.initials: U
authors.lastname: Lüttge
mesh_terms: D002734:Chlorophyll / Q000378:metabolism* / Q000528:radiation effects; D005453:Fluorescence; D008027:Light; D008293:Malates / Q000378:metabolism; D035723:Mesembryanthemum / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects; D010788:Photosynthesis; D045332:Photosystem II Protein Complex / Q000378:metabolism; D012965:Sodium Chloride / Q000494:pharmacology*
chemical_list: D008293:Malates; D045332:Photosystem II Protein Complex; D002734:Chlorophyll; D012965:Sodium Chloride; C030298:malic acid