Effect Of NaCl On The Photosynthesis Of Kosteletzkya Virginica (L.) Presl

More and more studies on the plant resistant physiology focus on thephotosynthesis,stomatal conductance and substomatal CO2 concentration in theseyears. But less studies are carried out on the plants of CO2 saturation point, CO2compensation point, light saturation point, light compensation point, carboxylationefficiency, apparent quantum yield, photorespiration rate and colony photosynthesis,but all these indexes are very important to the light utilization efficiency and bio-yieldof plants. Thus, this experiment tries to study the effects of NaCl on thephotosynthesis of the Kosteletzkya virginica (L.) Presl through analyzing the changesof CO2 compensation point,rate of photorespiration,glycolate oxidase (GO) activityand the changes of chlorophyll fluorescence parameters that were impacted bydifferent NaCl concentrations. The main results are as follows:1. Phasic response of photosynthesis of Kosteletzkya virginica under salt stress.The Pn,Gs,Ci of Kosteletzkya virginica decreased simultaneously at the earlydays of salt-treatment (the 0-1st day after arriving at the terminal concentration ofNaCl), these changes indicate that stomatal limitation factor is the dominant factor ininfluencing the photosynthesis of Kosteletzkya virginica under the early days of saltstress. This is the alarm phase of plant photosynthesis response to the salt stress.Along with the extension of salt-treatment, Pn,Gs,Ci of Kosteletzkya virginicaincreased significantly, which is the restoration phase of the plant. After that, Pn,Gsdecreased obviously, while the Ci of Kosteletzkya virginica decreased first and thenincreased significantly. This is the exhaustion phase of the plant. Thus, non-stomatalfactor became the main factor that influences the photosynthesis at the late stages ofsalt stress.2. Effect of NaCl treatment on the growth of Kosteletzkya virginica seedlings.The growth indexes-fresh weight,dry weight and leaf area of Kosteletzkyavirginica decreased with the increase of NaCl concentrations. However, the growthidexes of Kosteletzkya virginica treated with 100 mmol/L NaCl decreased slightlycompared with control, and then decreased significantly with the increase of NaClconcentration (above 200mmol/L NaCl). These indicated that Kosteletzkya virginicacan bear with low concentrations of NaCl.Kosteletzkya Virginica seedlings accumulated Na+ in saline environment, Na+contents of Kosteletzkya virginica increased with the concentration of NaCl (0-300mmol/L), the changing trend of osmotic potential of the cell sap of leaves wasidentical to that of Na+ contents. Therefore, Kosteletzkya virginica modulate theosmolarity by absorbing abundant Na+.3. Effects of NaCl treatment on the photosynthetic characters of Kosteletzkyavirginica seedlings.Pn and Gs of Kosteletzkya virginica decreased with the increase of NaClconcentration, while Ci didn't show the same trend with the Pn and Gs. Ci decreasedwith the increase of NaCl concentration up to 100 mmol/L, and then increasedsignificantly at higher concentration, reaching the maximum value at the 300 mmol/LNaCl treatment. These all indicated that the decrease of the Pn caused the decrease inthe utilization of the CO2, a large amount of CO2 were accumulated in theintercellular space. Besides, the concentration of Ci has a positive correlation to theNaCl concentration.Chlorophyll content increased significantly with the increase of NaClconcentration up to 200 mmol/L and then decreased slightly at 300 mmol/L, and allthe values under salt stress were higher than that of control. It might be due to therestraint of the growth under salt stress that led to the relatively increasing ofchlorophyll content;while Chla/Chlb has the same changing trend with the content ofchlorophyll under salt stress, but the maximum value presented to the NaCl treatmentof 100 mmol/L;Fv/Fm slightly decreased with the increase of NaCl concentration upto 200 mmol/L, and then decreased significantly at higher concentration;the variationof Fo is not obvious;φPSⅡ,φPSⅡR all increased first and then decreased with theincrease of NaCl concentration. However, the maximum of φPSⅡ reached at 200mmol/L NaCl, while the peak of φPSⅡR reached at 100 mmol/L. In addition, qPincreased first and then decreased, while the NPQ was just the opposite, the maximumand the minimum values all presented at 100 mmol/L. These results made clear thatsalt stress decreased the energy conversion efficiency of antenna pigment, influencedthe transfer of the electron, but exerted no damage to the photosynthetic reactioncenter. Furthermore, salt stress also significantly increased the CO2 compensationpoint. All of these brought about the decrease of photosynthesis in Kosteletzkyavirginica, making the non-stomatal factors became the main factor that influenced thephotosynthesis at the later stage of salt stress.4. Oxidative damage to Kosteletzkya virginica seedlings.Llipid peroxidation (MDA) was used as an indicator of NaCl stress to test theoxidative damage. At low NaCl concentration (100 mmol/L), leaf MDA did not showdistinct increase. Under the condition of high NaCl concentration (200-300 mmol/LNaCl), leaf MDA showed distinguished increase. While the relative electricconductivity of the plasma membrane showed the same trend with that of MDA.Therefore, plasma membrane was not significantly affected by low NaClconcentration, and then, with the increase of the NaCl concentration, salt stresspressed distinct damage to the cell membrane of Kosteletzkya virginica by increasingMDA.5. Effect of salt stress on the photorespiration of Kosteletzkya virginicaThe GO activity of plants exposed to NaCl was quite lower than that of control.Accordingly, the rate of photorespiration of plants also showed distinguished decreaseunder salt stress;However, spraying the seedling leaves with 1.0 mmol/L Na2Saccelerated the activity of the GO, and then promoted the photorespiration of theplants under stress;the increase of the rate of photorespiration also caused theincrease of the contents of the chlorophyll and lowered MDA, decreased relativeelectric conductivity of cell membrane. On the other hand, Na2S was proved to haveno damage on the photosynthetic reaction center. All of these indicated thatphotorespiration imposed protection on the plants under salt stress. Overall, thedecrease of the photorespiration under salt stress can't play the protecting role in thephotosynthetic system any more.In conclusion, at the early days of salt stress, stomatal limitation factor was thedominant factor in influencing the photosynthesis of Kosteletzkya virginica. While atthe later stage of salt stress, salinity lowered the GO activity and the rate ofphotorespiration. The weakening of photorespiration lowered the protecting role tophotosynthetic system. Likewise, salinity decreased the energy conversion efficiencyof antenna pigment, influenced the transfer of the electron, but had no damage on thephotosynthetic reaction center;In addition, salinity significantly rose the CO2compensation point and lowered the usage of CO2. All of these resulted in thedecrease of net photosynthetic rate and plant growth, at that non-stomatal limitationbecame the restriction factor.