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We studied the effects of n-propyl gallate, which is a plastid terminal oxidase inhibitor involved in chlororespiration, on photosynthesis and physiological parameters in Dunaliella salina grown under different salinities and under low or high irradiance using chlorophyll a fluorescence transient measurements and pigment analysis. The inhibitor up to 1 mM had an additive significant effect on the photosynthetic efficiency in the cell suspensions grown under low salinity and irradiance. However, in the presence of high n-propyl gallate concentration (4 mM), there was a negative effect on all physiological aspects. In contrast, this high concentration of the inhibitor could enhance efficiency of electron transport and growth parameters under high irradiance. On the other hand, with salinity increase, the unfavorable effects of high inhibitor concentration on the efficiency of photosystem II were less evident than of low salinity. Interestingly, n-propyl gallate high concentration had a positive effect on fluorescence and on physiological parameters when high salinitiesgrown cells were exposed to high irradiance. The results suggest that there is a rational correlation between increase of salinity and algae ability to bypass n-propyl gallate inhibited plastid terminal oxidase function and also direct influence of its lethal concentration on photosystem II compartment. The ability is especially substantial when the increase of salinity is accompanying high irradiance. Furthermore, these data show that algal responses to inhibitor concentrations are different under various environmental conditions.

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The objective of present work was to study the role of n-propyl gallate (PG), a synthetic antioxidant, in antioxidative responses and salinity tolerance in Dunaliella salina. Algal cultures containing three level of salinity (1, 2 and 3 M NaCl) were treated with two level of 0 and 1 mM of PG for 48 h. 3 M NaCl-grown cells exhibited a minor increase in cell density in comparison to other salt treatments. PG treatment had no effect on cell growth under different salinities. However, the chlorophyll and b-carotene contents significantly increased in PG-incubated cells. Protein concentration clearly reduced in PG-incubated cells grown at 1 and 2 M NaCl compared with those of PG-free, whereas, no significant change influenced by PG was obtained at 3 M NaCl. Ascorbate peroxidase assay showed a minor increase at 3 M NaCl compared with 1 and 2 M NaCl-grown cells. Catalase activity decreased concurrently with salt concentrations, while superoxide dismutase activity pronouncedly increased in response to 2 M NaCl accompanied by a statistically equal increase at 1 and 3 M NaCl. However, the activity of all three enzymes significantly decreased in all PG-incubated algae compared with PG-free ones. Along with increase in total and reduced ascorbate in response to salinity and PG treatments, oxidized ascorbate content was significantly decreased.Hydrogen peroxide and malonyldialdehyde accumulation increased concomitantly with salinity. However, a large decrease in these metabolites occurred in response to PG added into the algal cultures. The results suggest that antioxidant enzymes are not pivotal in improving salinity tolerance and only have short-term adjustment effects to protect against salt stress in D. salina. These data also provide the first direct evidence that PG pretreatment ameliorates salinity stress by electron donation to free radical oxidants, and by inducing b-carotene, chlorophyll, and ascorbic acid biosynthesis, rather than activation of antioxidant enzymes.

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