Adaptative Variation Mechanism Of Plants Photosynthetic Carbon Assimilation Under Water Gradients In Arid Areas Of Northwest China

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 g_s 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 g_s to C_i 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 (N_area) and chlorophyll per area (Chl_area). 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 g_s sensitivity to C_i 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.