Study On Physiological Characteristics Of Hypnum Vaucheri In Biotic Crusts Of Huangfuchuan Watershed Primary Vegetation

Hypnum vaucheri Lesq., the dominant species of biotic crusts of Du songlin and sparse woody in hilly loess region of Erdos. Through studying physiological characteristics, such as water retention, photosynthetic pigments, photosynthetic characteristics, respiratory metabolism, reactive oxygen species metabolism and osmotic adjustment, especially the process of dehydration and rehydration, the article explored physiological mechanism adapting to arid and revealed the ecological adaptability of bryophytes living in semi-arid area. It can provide theoretical basis for the formulation and sustention of bryophyte crust. The results were as follows:1. Under the natural dehydration, water of Hypnum vaucheri Lesq. dissipated 21.11% in 15 minutes and reduced to 8.66% after 120minutes. Moisture content reduced to 2.51% after 165 minutes.2. Under natural water condition, the content of chlorophyll a and chlorophyll b were 0.82mg·g^-1(FW) and 0.42mg·g^-1(FW) respectively. The ratio of two was 1.93. The content of carotenoids and total chlorophyll were 0.20 mg·g^-1(FW) and 1.24 mg·g^-1(FW) respectively.3. Under natural water condition, maximum net photosynthetic rate was 76.45μmol CO₂·kg^-1(DW)·s^-1, light compensation point was 34.90μmol·m^-2·s^-1, light saturation point was 475μmol·m^-2·s^-1 and dark respiration rate was 11.40μmolCO₂·kg^-1(DW)·s^-1. Diurnal variation of photosynthetic rate showed bimodal curve with a midday photosynthetic break which caused by photoinhibition and had a certain relationship with high temperature and low concentration of CO₂. Normally, moisture content of Hypnum vaucheri Lesq. was 81.48% and net photosynthesis rate was 71.95μmolCO₂·kg^-1 . The moisture content reduce by 29.95%, 54.88%, 68.52%, 77.09%, 79.86% and net photosynthesis rate declined 26.55%, 44.11%, 55.20%, 58.33%, 61.23% after dehydration 15min, 30min, 45min, 60min and 90min respectively. The moisture content of Hypnum vaucheri Lesq. was 16.41% and net photosynthesis rate was 27.89μmolCO₂·kg^-1 before rehydration. But they increased by 1.12 times, 2.20 times, 3.11 times and 3.08%, 22.87%, 50.03% respectively after rehydration 15min, 30min and 45min than that before rehydration. It recovered basically to the level that before dehydration after rehydration 60 minutes (water content was 85.19%). The most suitable moisture content for net photosynthesis rate was about 80-85%.During the process of dehydration, Fo, Fv/Fm of Hypnum vaucheri lesq., actual yields of photosynthetic quantum and ETR appeared a short increase in the early period of dehydration, then decreased with the moisture content declined, this same with Fm. qP declined remarkably only after a certain dehydration. On the contrary, qN had certain retardation and started to increase rapidly only at middle or later stages of dehydration. The fluorescence parameters of Hypnum vaucheri Lesq recovered to the previous level gradually after dehydration. However, overhydration would cause inhibition phenomenon.4. Within 15 minutes dehydration, the moisture content of Hypnum vaucheri Lesq reduced from 81.64% to 59.08%, respiration rate decreased by 46.47% from 63.78 O₂μl·g^-1(DW)·h^-1 to 34.14 O₂μl·g^-1(DW)·h^-1 correspondingly. Respiratory rate in this period decreased with a sharp degree. With the moisture content reduces from 59.08% to 16.59%, respiratory rate of Hypnum vaucheri Lesq declined with a moderate trend. Compared with control group, respiratory rate decreased 53.41%, 59.97%, 73.18% and 73.23% after dehydrating 30min, 45min, 60min and 90min respectively, and increased rapidly after rehydration. Respiration rate before rehydration was 17.08 O₂μl·g^-1(DW)·h^-1, it increased by 17.45% , 46.82%, 60.86% after rehydration 15minutes, 30minutes, 45minutes respectively than origin. When moisture content reached 85% of all (rehydration 60min), respiration rate basically reached the original level, and remained relative stability.5. Dehydration caused by osmotic stress can lead superoxide anion of Hypnum vaucheri Lesq. generated rapidly, MAD content and membrane permeability increased. Activity of SOD, POD and CAT increased first, then decreased with the stress intensity increased, and had a good coordination. The change of the AsA and GSH content had a positive correlation with the change of superoxide anion (O₂-.) and showed a complementary trend with the changes in activities of three kinds of protection enzyme.6. Different intensity of osmotic stress caused the content of soluble protein first increasing and then decreasing, but content of its proline, free amino acids and soluble sugars showed a increasing trend with the strengthen of osmotic stress. The accumulation of Na⁺ was significantly greater than that of K ⁺, and the ratio of Na ⁺ and K ⁺ was 7.74. Na⁺ was the dominant osmotic regulator of cell fluid. The result showed that Hypnum vaucheri Lesq. had a strong osmotic adjustment skill.