CHLOROPHYLL-A FLUORESCENCE, ENZYME AND ANTIOXIDANT ANALYSES PROVIDE EVIDENCE FOR THE OPERATION OF ALTERNATIVE ELECTRON SINKS DURING LEAF SENESCENCE IN A STAY-GREEN MUTANT OF FESTUCA-PRATENSIS

Mutation of the sid gene in Festuca pratensis prevents chlorophyll deg radation, The senescing leaves retain their chlorophyll complement and stay green, Nevertheless, CO2 assimilation and ribulose-bisphosphate carboxylase/oxygenase content decline in both mutant and wildtype plan ts, Photosynthesis and chlorophyll a fluorescence measurements were pe rformed in air and at low oxygen to prevent photorespiration, The maxi mum extractable activity of ribulose 1,5 bisphosphate carboxylase was higher in the senescent mutant leaves than in those of the wild-type c ontrol but was much lower than that observed in the mature leaves of e ither genotype, The activation state of this enzyme was similar in mut ant and wild-type lines at equivalent stages of development, Analysis of chlorophyll a fluorescence quenching with varying irradiance showed similar characteristics for mature leaves of the two genotypes, Genot ypic variations in photosystem II (PSII) efficiency were observed only in the senescent leaves, Photochemical quenching and the quantum effi ciency of PSII were gl eater in the senescent mutant leaves than in th e wild type at a given irradiance, The calculated electron flux throug h PSII was substantially higher in the mutant with a greater proportio n of electrons directed to photorespiration, Maximum catalytic activit ies of ascorbate peroxidase decreased in senescent compared to mature leaves of both genotypes, while glutathione reductase and monodehydroa scorbate reductase were unchanged in both cases, Superoxide dismutase activity was approximately doubled and dehydroascorbate reductase acti vity was three times higher in senescent leaves compared with the matu re leaves of both genotypes, In no case was there a difference in enzy me activities between mutant and wild type at equivalent growth stages , The pool of reduced ascorbate was similar in the mature leaves of th e two genotypes, whereas it was significantly higher in the senescent leaves of the mutant compared with the wild type, Conversely, the hydr ogen peroxide content was significantly higher in the mature leaves of the wild type than in those of the mutant, but in senescent leaves si milar values were obtained, In leaves subjected to chilling stress the reduced ascorbate pool was higher in both mature and senescent leaves of the mutant than in their wild-type counterparts, Similarly, the hy drogen peroxide pool was significantly loner in both mature and senesc ent leaves of the mutant than in the wild type, We conclude that, in s pite of deceased CO2 assimilation, the mutant is capable of high rates of electron flow, The high ascorbate/hydrogen peroxide ratio observed in the mutant, particularly at low temperatures, suggests that the se nescent leaves are not subject to enhanced oxidative stress.