Studies On The Photosynthesis Of Nostoc Flagelliforme (Cyanophyceae) Subjected To Desiccation And Rehydration And The Metabolism Of Reactive Oxygen Species

Nostoc flagelliforme (Berk. & Curtis) Bornet & Flah. is an edible terrestrial blue-green alga with great economic value. It is distributed worldwide and in China mainly in the arid or semiarid areas of northern and west-northern parts. During the past twenty years, a number of studies have been carried out in China on the ecology, physiology, morphology, reproduction, culture and resources of N. jlagelliforme. Unfortunately its resources have been over-exploited due to the increasing market demands. Over-exploitation of this species has seriously damaged extensive ranges of vegetation in western and west-northern China. As a consequence, the State Council of the People's Republic of China announced in June 2000 the prohibition of further collection and trading in N. flagelliforme, so as to halt the progress of desertification in northern China. Its protection degree has also been elevated to the first-rank.The metabolism of reactive oxygen species in N. flagelliforme and effects of exogenous oxidants on its photosynthetic recovery were investigated to obtain insight into oxidative stress in desiccation and its possible damaging impact on photosynthetic apparatus. No ascorbate acid was detected with ascorbate oxidase in N. flagelliforme. Superoxide dismutase (SOD) remained active even after 3 years of drying storage and its activity was 78% of that in fully recovered samples. The SOD activity decreased during desiccation or in drying storage. Intracellular active oxygen production was studied by incubating samples in BGu medium for 2 h and measuring the oxidation of 2', 7'-dichlorohydrofluorescein diacetate. The production rate of DCF was 38.11 nmol DCF g (d.wt)-1h-1 in dried field samples and was significantly higher than in fully recovered samples or air-dried samples. The balance between intracellular active oxygen production and the defense system might break down in air-dried and dried field samples. Treatment with exogenous oxidants slowed the photosynthetic recovery especially with singlet oxygen. Oxidative stress might play an important role in desiccation-induced damages to the photosynthetic apparatus.The photosynthetic response of Nostoc flagelliforme to desiccation and rehydration was investigated in this paper. The initial, variable and maximal fluorescence increased from the beginning (fully hydrated), reached the maximuminat a water loss of about 35%, and then decreased with further water loss. The Fv/Fm value and the area over fluorescence induction curve reached the maximum at a water loss of about 70%. Pigment contents of chlorophyll a, phycocyanin and allophycocyanin were constant during the photosynthetic recovery. A fast initial phase of recovery was completed within half an hour and further change of 77K fluorescence emission spectra was mainly related to the fluorescence level at 695 nm. The decrease in fluorescence intensities of phycobiliproteins was not detected from 0.5 to 24 h. The initial fluorescence was high at the beginning of rehydration and decreased sharply in the recovery. Its decrease was mainly related to the activation or repair of PS II rather than changes in the coupling and assembly of phycobilisomes. The changes of variable fluorescence and Fy/Fm value correlated well with the recovery of PS II activity during rehydration. N. flagelliforme required light and the de novo protein synthesis to fully recover its photosynthetic activity. In darkness, FV/FOT recovered 3.4% in dried field samples but 18.9% in samples dry for two days. When light was provided in the presence of chloramphenicol, it could recover 10.1% or 39.5% respectively in dried field samples or in samples dry for two days.