Effect Of NaCl On Photosynthesis Of Limonium Bicolor

Photosynthesis is always one hotspot in scientific researches. But few studies are carried out on the plant photosynthesis in salt stress and there are still many essential problems unresolved. Even fewer studies focus on the photosynthesis of halophytes. In this study we try to clarify the effect of salt stress on Limonium bicolor photosynthesis and to further explore the detrimental mechanism of salinity on it. Four groups of young seedlings of Limonium bicolor were treated with 0, 100, 200, 400 mmol/L NaCl respectively, and the photosynthetic rate,Chlorophyll fluorescence parameter and other physiological indices were measured two weeks later. The main results are shown as follows:1. Effect of salt stress on growth of Limonium bicolorUnder the treatment with 100 mmol/L NaCl, both fresh weight and dry weight of Limonium bicolor were higher than those of the control (P<0.05). But fresh weight of Limonium bicolor decreased significantly (P<0.05) under the treatments with 200, 400 mmol/L NaCl. It indicates that low concentration of NaCl (100 mmol/L) promotes the growth of Limonium bicolor and high concentration of NaCl inhibits its growth.2. Effect of salt stress on photosynthetic gas exchange parameters of Limonium bicolorUnder the treatment with 100 mmol/L NaCl, Pn of Limonium bicolor slightly increased compared with the control (P>0.05), but decreased significantly under the treatments with 200, 400 mmol/L NaCl (P<0.05, P<0.01). With the increase of salt concentration, Gs and Ci decreased gradually and reached the lowest at the 400 mmol/L NaCl treatment, but changes of Ls presented an opposite trend. It is obvious that at higher concentration of NaCl (200, 400 mmol/L NaCl) the decrease of Pn closely correlate with stomatal limitation factors.3. Effect of salt stress on CO2 compensation point, light compensation point, light saturation densities of Limonium bicolorCO2 compensation point and light compensation point increased gradually with the increase of NaCl concentration. It indicates that abilities of using light and CO2 decreased under the salt treatment. But decrease of light saturation densities with the increase of NaCl concentration indicates that photoinhibition is more likely to occur in strong light. It negatively affects the accumulation of photoproducts and thus inhibits the growth of Limonium bicolor.4. Effect of salt stress on apparent photosynthetic quantum yield (AQY) and carboxylation efficiency (CE) of Limonium bicolorAQY and CE of Limonium bicolor decreased gradually with the increase of salt concentration and there were significant difference (P<0.05, P<0.01) between plants under the treatments with 200, 400 mmol/L NaCl and the control, but plants under the treatment with 100 mmol/L NaCl were slightly different (P>0.05) from the control. The decrease of AQY indicates that the photoinhibition has happened and the decrease of CE shows that activity of RuBPCase has been inhibited.5. Effect of salt stress on Chlorophyll fluorescence parameters of Limonium bicolorSlight decrease of Fo with the increase of NaCl concentration indicates that the reaction center of PSⅡhas not been destroyed under salt stress. Fv/Fm decreased gradually with the increase of salt concentration and significant difference (P<0.05, P<0.01) could be observed between plants under the treatments with 200, 400 mmol/L NaCl and the control. It indicates that photochemical efficiency of PSⅡhas been inhibited by higher salt concentration.φPSⅡ, which reflected the actual quantum yield of electron transport in PSⅡ,was higher in plants under the treatment with 100 mmol/L NaCl than that in the control(P<0.05), then decreased gradually with the increase of salt concentration. The value at 400 mmol/L NaCl treatment was the lowest and significant difference (P<0.01) appeared compared with the control. It indicates that the actual quantum yield and the electron transport rate of PSⅡhave been promoted in lower concentration of NaCl but inhibited in higher concentration. With the increase of salt concentration, qP significantly decreased under the treatments with 200, 400 mmol/L NaCl compared with the control (P<0.05). This indicates that the ratio of opening PSⅡreaction center have reduced and activity of PSⅡhas decreased. NPQ increased with the increase of salt concentration and the highest value appeared in plants under the treatment with 400 mmol/L NaCl. This indicates that the ability of dissipating excessive light energy of PSⅡhas increased at the higher concentration of NaCl.6. Effect of salt stress on Chlorophyll content of Limonium bicolor Content of chlorophyll a, b, a+b slightly decreased under the treatment with 100 mmol/L NaCl and then increased with the increase of NaCl concentration. It is possible that salinity has promoted the formation of chlorophyll, and/or the inhibited growth of leaf area under salt stress has caused relative increase of chlorophyll content. Chlorophyll a/b content increased significantly with the increase of salt concentration up to 200 mmol/L and then decreased at 400 mmol/L, but all the values under salt stress were higher than those of the control. This indicates that thylakoids still had higher stack extent possibly under the salt treatment.7. Effect of salt stress on soluble sugar content of Limonium bicolorSoluble sugar content of lamina increased with the increase of salt concentration and significant difference (P<0.05) could be observed between the treatment with 400 mmol/L NaCl and the control. It indicates that feedback restriction caused by soluble sugar accumulation is probably one of the reasons why photosynthesis of Limonium bicolor has decreased.8. Effect of salt stress on plasma membrane of Limonium bicolorMDA content is used as an indicator of oxidative damage degree in salt stress. At low NaCl concentration (100 mmol/L), leaf MDA did not show distinct increase. Under the condition of high NaCl concentration (200, 400 mmol/L NaCl), leaf MDA showed distinguished increase (P<0.01). While the relative electric conductivity of the plasma membrane showed the same trend with that of MDA. Therefore, plasma membrane was not significantly affected by low NaCl concentration, and then, with the increase of the NaCl concentration, salt stress caused distinct damage to the cell membrane of Limonium bicolor by increasing MDA.9. Effect of salt stress on photorespiration of Limonium bicolorPhotorespiration rate and GO activity exhibited the similar trend, which decreased gradually with the increase of salt concentration in which the values of 400 mmol/L NaCl treatment were the lowest (P<0.05). The higher photorespiration can promote dissipating excessive light energy and further protect plants from light oxidation. So decrease of photorespiration in salt stress is also one of reasons that the growth of Limonium bicolor is inhibited.10. Effect of salt stress on daily changes of photosynthesis and fluorescence of Limonium bicolorThe daily change of photosynthesis in Limonium bicolor was a double peaks curve and there was the distinct phenomenon of"noon break"under the condition of strong light. But a single peak curve appeared at the treatment with 400 mmol/L NaCl. No significant difference was observed between plants under the treatment with 100 mmol/L NaCl and the control, but the photosynthetic rate decreased significantly at the treatments with 200, 400 mmol/L NaCl. Variations of Gs and Ci indicate that photosynthesis decreases at noon mainly because of non-stomatal factors. Changes of Fv/Fm show that photoinhibition of PSⅡreaction centre is possibly one of reasons why Pn decreases.In conclusion, low concentration of NaCl (100 mmol/L) promotes the growth of Limonium bicolor. But in higher concentration of NaCl its growth and photosynthesis are inhibited obviously. The reduced photosynthetic rate results from stomatal limitation factors and non-stomatal limitation factors, such as the block of electron transport, the down-regulation of PSⅡreaction center activity, the feedback restriction due to soluble sugar accumulation, as well as the reduction of the RuBPCase activity. In addition, Fv/Fm only decreased by 1.4% compared with the control. It shows that salt stress did not obviously influence the PSⅡactivity. But carboxylation efficiency decreased by 43.0% compared with the control. These results indicate that dark reaction of Limonium bicolor may have suffered the more severe inhibition than light reaction under the salt stress. In addition, salt stress decreases GO activity and inhibits photorespiration, thus lowers the protection of photorespiration to photosystem. The increase of plasma membrane permeability influences the balance of ion and small molecular metabolites in cells, which further influences the photosynthesis.