Effects Of Na₂CO₃ Stress On Growth And Photosynthetic Characteristics Of Vicia Faba L.

Photosynthesis is always one hotspot in scientific researches. But few studies are carried out on the plant photosynthesis under alkaline salt stress and there are still many basic problems unresolved. Even fewer studies focus on the photosynthesis of halophytes. In this study we try to clarify the effect of alkaline salt stress on photosynthesis of Vicia faba L. and to further explore the detrimental mechanism of alkaline salt stress. Four groups of young seedlings of Vicia faba L. were treated with 0, 5, 10, 15 and 25 mmol/L Na₂CO₃ for 6 days, respectively. The photosynthetic rate, chlorophyll fluorescence parameter and other physiological indices were measured. The main results are shown as follows:1. Effect of Na₂CO₃ on growth of Vicia faba L.Under the treatment with 15 mmol/L Na₂CO₃, both fresh weight and dry weight of Vicia faba L. were higher than those of the control(P<0.05).But fresh weight of Vicia faba L. decreased significantly(P<0.05)under the treatment with 25 mmol/L Na₂CO₃. It indicates that low concentration of Na₂CO₃ (5mmol/L) promotes the growth of Vicia faba L. and high concentration of Na₂CO₃ inhibits its growth.2. Effect of Na₂CO₃ on photosynthetic gas exchange parameters of Vicia faba L.Pn, Gs and Ci declined in a range of 5-15mmol/L Na₂CO₃, while Ls increased, which proves that the stomata is the main limited factor. Under the high concentration of 25mmol/L Na₂CO₃, though Pn, Gs and Ls drop significantly, Ci rises. It proves that the stomatal limitation of photosynthesis decreases, but the non-stomatal limitation factor increases.3. Effect of Na₂CO₃ on chlorophyll fluorescence parameters of Vicia faba L.Fv/Fm decreased gradually with the increase of salt concentration and significant difference (P<0.05)could be observed between plants treated with 25mmol/L Na₂CO₃ and the control. It indicates that photochemical efficiency of PSâ…¡has been inhibited by high concentration of Na₂CO₃.φPSâ…¡, which reflects the actual quantum yield of electron transport in PSâ…¡,was higher in plants treated with 10mmol/L Na₂CO₃ than that in the control (P<0.05), then decreased gradually with the increase of salt concentration. The value at 25mmol/L Na₂CO₃ treatment was the lowest and significant difference (P<0.05) found compared with the control. It indicates that the actual quantum yield and the electron transport rate of PSâ…¡have been promoted under lower concentration of Na₂CO₃ but suppressed under higher concentration.4. Effect of Na₂CO₃ on Chlorophyll content of Vicia faba L.Content of chlorophyll a,b, and a+b slightly decreased under the treatment with 5 mmol/L Na₂CO₃ and then increased with the increase of Na₂CO₃ 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 15mmol/L and then decreased at 25 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.5. Effect of Na₂CO₃ on soluble sugar content of Vicia faba L.Soluble sugar content of the leaves increased with the increase of salt concentration and significant difference (P<0.05) was observed between the plants treated with 25mmol/L Na₂CO₃ and control. It indicates that feedback restriction caused by soluble sugar accumulation is probably one of the reasons why photosynthesis of Vicia faba L. decreased.6. Effect of Na₂CO₃ on plasma membrane of Vicia faba L.MDA content is used as an indicator of oxidative damage degree in environmental stresses. At low concentration of Na₂CO₃ [5 mmol/L], leaf MDA did not show distinct increase. Under the condition of high Na₂CO₃ concentration [15, 25 mmol/L Na₂CO₃], leaf MDA showed distinguished increase(P<0.05).While the relative electric conductivity of the plasma membrane showed the same trend with that of MDA content. Therefore, plasma membrane was not significantly affected by low concentration of Na₂CO₃. However, high concentration of the Na₂CO₃ caused distinct damage to the cell membrane of Vicia faba L. leading to accumulation of MDA.In conclusion, low concentration of Na₂CO₃ [5 mmol/L] promotes the growth of Vicia faba L. However, under high concentration of Na₂CO₃, both growth and photosynthesis are inhibited obviously. On the one hand, 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 PSâ…¡reaction center activity and the feedback restriction due to soluble sugar accumulation. On the other hand, the reduction of the RuBPCase activity maybe also lead to photosynthetic rate reduction under condition of Na₂CO₃. In addition, Fv/Fm only decreased by 9.3% compared with the control. It suggests that Na₂CO₃ stress did not obviously influence the PSâ…¡activity.