Kontrasterende invloed van chroniese osoonblootstelling op fotosintetiese elektronoordrag en CO2-assimilering in gevoelige (S156) en bestande (R123) Phaseolus vulgaris L. genotipes

Abstract

Troposferiese osoon (O3) word beskou as een van die belangrikste lugbesoedelstowwe aangesien dit wêreldwyd meer skade aan landbougewasse aanrig as al die ander besoedelstowwe saam. Weens die oksiderende aard daarvan verooraak O3 blaarskade en ’ n afname in fotosintese. Bestandheid van gewasse teen O3 verskil aansienlik tussen spesies en genotipes. Met hierdie studie is nuwe inligting oor die fisiologiese en biochemiese grondslag van die skadelike effek van O3 verkry deur chroniese blootstelling van twee bosboongenotipes met bekende gevoeligheid vir O3 , naamlik S156 (gevoelig) en R123 (bestand), aan onderskeidelik houtskool-gefiltreerde lug en 80 dpm O3 . Die studie is uitgevoer in oop-dek-groeikamers (OTCs) vir die volle groeiperiode tot wasdom. Die status van die fotosintetiese apparaat van die proefplante is bepaal deur analise van chlorofil a-fluoressensiekinetika en CO2 -afhanklikheidskrommes (A:Ci ). Fisiologiese effekte is by S156 waargeneem lank voor die verkyning van nekrotiese vlekke op die drieledige blare. Fotosintese is aansienlik gerem in S156, hoofsaaklik weens ontkoppeling van die suurstofvrystellingskompleks (OEC), remming van fotosintetiese elektronoordrag, gevolglike afname in die reduksie van eind-elektronontvangers (ferredoksien, NADP+) en die gepaardgaande afname in die karboksilering en die regenerering van ribulose-1,5- bisfosfaat. Saad- en peulopbrengs het nou ooreengestem met die fotosintetiese gedrag van die proefplante. Ofskoon blare van albei genotipes sigbaar aangetas was, was dit S156 wat ernstige nekrotiese vlekke vertoon het. Hierdie data bied nuwe inligting en komplementeer bestaande kennis oor die prosesse onderliggend aan die fitotoksisiteit van O3 , nodig vir die ontwikkeling van bestande genotipesTropospheric ozone is currently regarded as one of the most important air pollutants, since it causes more damage to vegetation world-wide than all the other pollutants combined (Ashmore and Bell 1991). Due to its oxidative nature ozone causes leaf damage and a decrease in photosynthesis. Ozone tolerance varies widely between species and genotypes. The aim of this study was to identify and quantify the physiological and biochemical constraints imposed by chronic ozone exposure of two bush bean (Phaseolus vulgaris L.) genotypes with known difference in sensitivity, namely S156 (sensitive) and R123 (resistant), to charcoal-filtered air and 80 nmol.mol-1 O3. The study was conducted in open-top growth chambers (OTCs) over the entire growth period by measuring chlorophyll a fluorescence (JIP-test) and photosynthetic gas exchange of the test plants weekly. The status of the photosynthetic apparatus was assessed by analysis of chlorophyll a fluorescence kinetics (JIP test) and CO2 response curves (A:Ci). O3-induced physiological effects were detected in S156 long before appearance of necrotic spots on the trifoliate leaves. Photosynthesis was substantially inhibited in S156, mainly due to disengagement of the oxygen evolving complex (OEC), inhibition of intersystem electron transport and the reduction of end-electron acceptors of PSI (ferredoxin, NADP+), causing the concomitant decrease in the carboxylation and regeneration of ribulose-1,5-bisphosphate. Seed and pod yield closely reflected the photosynthetic response of the test plants. Although leaves of both the genotypes were affected visually, it was S156 that displayed severe necrotic ozone injury on the trifoliate leaves. Our data contribute to and complement the existing knowledge on the processes underlying the phytotoxicity of O3 needed for development of tolerant genotypes