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Adaptative Variation Mechanism of Plants Photosynthetic Carbon Assimilation under Water Gradients in Arid Areas of Northwest China

Author: GongChunMei
Tutor: WangGenXuan
School: Lanzhou University
Course: Ecology
Keywords: Arid areas of northwest China C4 photosynthetic pathway Hedysarum scoparium Phragmites communis Interveinal distance Stomatal conductance Photosynthetic enzymatic activities Leaf mass per area The biomass ratio of shoot/leaf or green stem/leaf
CLC: Q945.11
Type: PhD thesis
Year: 2007
Downloads: 604
Quote: 3
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Environmental regulation plays significant roles in adaptative variation of photosynthetic carbon assimilation pathway. The transition/shift of photosynthetic pathway from C3 to C4 is involved in the evolution and adaptation of plants with water stress. Superior water use efficiency (WUE) of C4 photosynthesis relaxes hydraulic demands on the conducting pathway in the xylem, allowing for alteration of xylem structure and patterns of biomass allocation to exploit the environmental conditions more effectively. Thus, we propose hypothesises that C3 plant is induced to exhibit C4 photosynthetic characteristics by drought stress, which develop more C4 photosynthesis in rachis than in leaflet, and C3 plant shift photosynthetic mode from C3 to C4 pathway and vary anatomy, enhance WUE to adapt arid environment with increasing water stress based on these theories.From the viewpoint of plant photosynthetic physiology, using measurements and laboratory experiments, adaptative variation mechanism of photosynthetic carbon assimilation in Hedysarum scoparium and Phragmites communis was examined under water stress gradients in arid areas of northwest China. Meanwhile, some physiological indexes such as leaf mass per area, nitrogen mass per area and shoot/leaf or green stem/leaf ratio were made use of to appraise the extent of C4 photosynthesis at community level. The key results obtained are as follows:(1) C4 pathway in C3 species of H. scoparium presented a more and more thriving trend, PEPCase and NAD-ME activities in plant leaflet and rachis increase with water stress. These two assimilating organs especially rachis with C4 photosynthetic characteristics in C3 desert species were important ensurance for plant growth, survival and evolutional transition in the arid regions of northwestern China. Moreover, the sensitivity of gs to VPD was involved in C4 photosynthetic pathway of both C3 and C4 species.(2) With ecotype shifts of Phragmites communis from swamp reed via salt meadow reed to dune reed, the number of vascular bundle per unit leaf area and number of veins per unit transactional leaf area increased gradually, while interveinal distances decreased in salt meadow reed and dune reed. The volume of vessel bundle per unit leaf area increased significantly though the number of vessel bundle per unit leaf area showed no increase in dune reed. There was a significant positive correlation between increasing numbers of veins or volume of vascular bundle per unit leaf area and the extent of performed C4 photosynthetic pathway. It is essential at anatomical level that the number or volume of vascular bundle per unit leaf area increased for C3 plant when C4 photosynthesis evolved in plant life.(3) The main purpose that photosynthetic pathway of Phragmites communis shifted from C3 to C4 was to adapt to environment, enhancing water use efficiency (WUE). There were some correlations between the extent of performed C4 photosynthetic pathway and WUE and soil water potential surrounding root system. Obvious correlation was also found between the extent of performed C4 photosynthetic pathway and stomatal conduction. In addition, the sensitivity of gs to Ci increased with soil water potential stress. There were some significant correlations between the extent of performed C4 photosynthetic pathway and leaf specific porosity (LSP), leaf mass per area (LMA), nitrogen mass per area (Narea) and chlorophyll per area (Chlarea). With increase in performed C4 photosynthetic pathway, the index of log M-N increasd with the improvement of use efficiency of water which is limited resource for plant growth.(4) RuBPCase activity didn’t decline in dune reed even though they lived at extremely drought condition. During the integration of C3 and C4 cycles for dune reed, RuBPCase activity was maintained but PEPCase activity was enhanced, resulting in entire performance of C4 photosynthesis in dune reed when leaf PEPCase activity was equal to RuBPCase activity.(5) Shoot/leaf or green stem/leaf ratio at Lanzhou, Baiyin, Jingtai and Linze increased gradually with the decline of annual precipitation. So, the extent of performed C4 photosynthetic pathway was the highest at Linze trial site, followed by Jingtai and Baiyin, and the lowest appeared at Lanzhou trial site. This result was consistent with that the number of C4 species of dominent species was the most at Linze trial site and the number of C3 species of dominent species was the most at Lanzhou trial site. The shift of photosynthetic assimilation pathway from C3 to C4 would occur easier in the considerable severer drought stress if plant faced a prolonged water deficit stress.Our results demonstrated that H. scoparium has some C4 biochemical pathways of CO2 fixation, not only in plant leaflet but also in plant rachis. It is universal for the desert C3 species to extend their photosynthetic pathway in arid region. Different ecotypes of Phragmites communis showed that they lay at different stages of the evolution from C3 to C4 photosynthesis. Larger volume of vascular bundle sheath cell and active response of PEPCase activity to net carbon assimilation rate in swamp reed indicated that this reed was at the prophase of evolutional C4 photosynthesis. Different from swamp reed, salt meadow reed and dune reed showed different characters which indicated their evolutionary stages. The decrease in interveinal distance of salt meadow reed and increase in the number and volume of vascular bundle sheath cell of dune reed, the increases in inclusion of vessel bundle sheath cell and PEPCase activity of salt meadow reed and dune reed, and the increases in WUE and gs sensitivity to Ci in salt meadow reed and dune reed evidenced that salt meadow reed and dune reed were both at the final stage of evolutionary C4 photosynthesis. Individual plant has developed a coherent and optimized relationship between structure, function and adaptation. In addition, the ratio of PEPCase/RuBPCase activities reflected the extent of performed C4 photosynthesis (if PEPCase activity is beyond RuBPCase activity, it represents the plant performs C4 photosynthesis; vice versa). So, dune reed may perform an entire C4 photosynthesis, whereas evolutionary stage of C4 photosynthesis in salt meadow reed should be between swamp reed and dune reed. Shoot/leaf or green stem/leaf ratio increased from Lanzhou to Linze, implying that the extent of performed C4 photosynthesis increased gradually with drought stress. Thus, adaptative transition mechanism of photosynthetic carbon assimilation of plants was analyse entirely from individual via population to community under 3 different water gradients, which displayed the universality and superiority of C4 photosynthesis in arid areas.

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CLC: > Biological Sciences > Botany > Plant Physiology > Plant nutrition, metabolic and respiratory > Photosynthesis, carbon assimilation
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