Study On The Photoprotective Mechanisms In Limonium Bicolor Under Salt Stress

This study tries to explore the photoprotective mechanisms of Limonium bicolor under salt stress, through evaluating the effects of photosynthetic characteristics as well as thermal dissipation, metabolism of active oxygen and photochemical characteristics of PSⅡthat were impacted by different concentrations of NaCl. The main results are as follows:1. Effect of NaCl on the growth of Limonium bicolorWith the increase of salt treatment, the fresh weight and dry weight of the seedlings, the total length, surface area, total volume, top tips, average diameter, fresh weight and dry weght of roots of Limonium bicolor increased up to 100 mmol/L NaCl and then decreased. The reductive capability of TTC increased significantly under 100 mmol/L NaCl, and then gradually decreased with the increase of NaCl, which may be an adaptive reaction to low concentration of NaCl that could promote the growth of Limonium bicolor to some extent. Under the treatment with 400 mmol/L NaCl, the roots activities were lowered remarkably compared to the control, in which the growth metabolism greatly reduced.2. Effect of NaCl on the photosynthetic characters of Limonium bicolorUnder the treatments with 200, 400 mmol/L NaCl, Pn, Gs and Ci decreased remarkably compared with the control, while Ls increased sharply. It is obvious that at higher concentration of NaCl the decrease of photosynthetic rate closely correlate with stomatal limitation factors. However, oxygen evolution rate measured by Chlorolab-2 showed slight difference between plants under NaCl treatments and the control. Furthermore, these indicated that stomatal limitation factors are the important reason in the decrease of photosynthetic rate under salt stress.φPSⅡincreased with the increase of NaCl up to 100 mmol/L, and then decreased obviously at 400 mmol/L. It showed that the photoinhibition of photosynthesis in Limonium bicolor increased gradually with the increase of NaCl concentration.3. Effect of NaCl on the non-radiative energy dissipation (NPQ) and its three components of Limonium bicolorNPQ increased constantly with the increase of NaCl concentration and there was significant difference between plants under the treatment with 200, 400 mmol/L NaCl and the control. It indicated that thermal dissipation of Limonium bicolor obviously increased, which can protect the reaction centers of photosystemⅡfrom damage of photoinhibition.qE , namely△pH-dependent quenching, is the major composition of NPQ and comprises about 65% in Limonium bicolor. In this experiment, qE and NPQ exhibited the similar trend, which increased with the increase of salt treatment, and significantly increased under the treatments with 200, 400 mmol/L NaCl compared with the control. It indicated that which dependent on the energization of the thylakoid membrane might be one of efficient means to dissipate the heat.qT, state transition, is one of the composition and less than 10% in NPQ, but slightly reduced with the increase of NaCl concentration. This showsed that salt treatments inhibited the protective mechanisms of qT in Limonium bicolor.qI called photoinhibition is one of the composition and exhibited about 25% of NPQ, which gradually improved with the increase of salt concentration, specially had obvious decrease under the treatment with 400 mmol/L NaCl. It was possibly due to the increasing photoinhibition at higher concentration of NaCl. qI and DPS had the similar trend, which indicated that qI of Limonium bicolor probably related to zeaxanthin in the xanthophyll cycle.4. Effect of NaCl on the xanthophyll cycle of Limonium bicolorThe de-epoxidation content of xanthophyll cycle (DPS) increased gradually with increase of salt treatment, which had the same trend with NPQ and qE, especially increased remarkably under higher concentration. It is obvious that xanthophyll cycle under salt stress has the efficient role of protecting. One of the reasons may be that the increase of DPS showed the increase of zeaxanthin (Z) and antheraxanthin (A) which can dissipate excited excess energy. The second reason possibly relates to△pH affected by salt stress, the increase of which resulting in the dissipation dependent on xanthophyll cycle to increase. The relative content of Car affected by salt stress might be the third reason.5. Effect of NaCl on the reactive oxygen species of Limonium bicolorThe activities of SOD and CAT treated with 200, 400 mmol/L NaCl improved obviously compared with the control, while the relative contents of Car and Anth increased gradually, and the Car content increased distinctly under different salt concentrations. The relative content of Anth showed its significant increase under 400 mmol/L NaCl. The data suggested that the reactive oxygen species in Limonium bicolor played a great role in photoprotection under salt stress. The high antioxidant ability can enhance the salinity tolerance, meanwhile the content of non-enzymic system in ROS scavenging improves to adapt to NaCl stress.6. Effect of NaCl on the photochemical characters of PSⅡin Limonium bicolor The ratio of variable fluorescence FK to the amplitude FJ -Fo (WK) treated with 400 mmol/L NaCl decreased significantly compared with the control, which suggested the donor side of PSⅡin Limonium bicolor could not be damaged by NaCl stress. Under the treatment with 400 mmol/L NaCl, normalized complementary area corresponding to the J-P phase (Sm) reduced, turn over number of QA in the time span from J-step to P-step (N) sharply decreased, relative variable fluorescence at J-step (VJ) slightly decreased, and relative variable fluorescence at I-step (VI) obviously increased started from the third day compared with control. These indicated that the capacity of electron transport in acceptor side of PSⅡtreated with 400 mmol/L NaCl declined compared with control.Compared to the control, the density of active reaction centers per excited cross-section (RC/CSo) of Limonium bicolor increased sharply under the treatment with 400 mmol/L NaCl. In addition, absorption per active reaction centers (ABS/RC) decreased, trapping (TRo/RC) and electron transport per active reaction centers (ETo/RC) both decreased sharply, and heat dissipation per active reaction centers (DIo/RC) slightly decreased,which showed that DIo/RC might be correspondingly promoted. These suggested that under the treatment with 400 mmol/L NaCl the absorption per active reaction center reduced, while the density of active reaction centers per excited cross-section increased, and thus the total harvested light energy didn't reduce in order to retain photosynthesis.Performance index (PIABS) and driving force (D.F.) on absorption basis both increased under the treatment of 400 mmol/L NaCl, from which we can deduce that salt shock by 400 mmol/L NaCl scarcely damaged the primary photochemical reactions of PSⅡin Limonium bicolor.