| Neyraudia reynaudiana is a pioneer Plant for water and soil conservation in the southern collapsing hill area. They can recovery rapidly in the bare soil and then become dominance species. Neyraudia reynaudiana can survive and grow in nutrient deficiency and drought soil, which show it may possesses specific mechanisms to find the nutrient and moisture in poor soil. However, the adaption mechanism of Neyraudia reynaudiana to nutrients deficiency and drought condition is still not determined clearly.For these reasons, the morphological and physiological characteristics of Neyraudia reynaudiana were determined under nutrient and drought stress based on the method of Stress Botany. The differences between the morphological and physiological features were analyzed in different stress treatments. The strategy to find the nutrients and moisture in poor soil was discussed for Neyraudia reynaudiana, which would be benefit for understanding its adaption mechanism to nutrients and drought stress. The main results were as follows:(1) There was significant effect on the growth of Neyraudia reynaudiana among the different water stress treatments. In the early stage of drought stress, the moisture content in the soil had a greater effect on the number of tillers and leaves of Neyraudia reynaudiana. Compared with adequate water supply treatment, the numbers of tillers and leaf of Neyraudia reynaudiana were significantly more both in moderate and slight water stress treatment, however, the growths of their height were significantly reduced (p<0.05). The growth of leaf length was weaned in the severe drought stress treatment, however, the growth of its leaf width was speeded. AS the drought stress goes, the growth of its height, tiller ability, leaf number and maximum leaf width were inhibited, however, leaf elongation ability weakened significantly.(2) There was significantly difference in total root length, root surface area, root average diameter, root biomass of Neyraudia reynaudiana among different drought stress treatments, however, not in the root volume. As the drought stress increased, the volume and diameter of roots reduced gradually in different drought stress. Compared with the adequate water treatment, the total length, surface area, biomass of roots, and root-shoot ratio were high significantly under different drought stres. The total length, surface area, biomass of roots, aboveground biomass, root-shoot ratio were the highest under moderate drought stress. Neyraudia reynaudiana had strong drought-tolerant because of large root system with root length and root surface area. The root characters of elongation growth and enlargement root-soil interface of Neyraudia reynaudiana may be one of morphology response mechanisms to drought stress.(3) The effects of drought stress on photosynthetic characteristics of Neyraudia reynaudiana were significantly different.. Chlorophyll content of Neyraudia reynaudiana increased and chlorophyll a/b was decrease under moderate and slight drought stress. Chlorophyll fluorescence parameters F0, Fm, Fv increased and Fv/Fm, Fv/F0decreased under drought stress. The average intercellular CO2content increased as the drought stress increasing. They increase slowly in the morning and rapidly increase in the afternoon to maximum at16:00. The diurnal variation curves of net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) were increasing, and then decreasing, a single-peak at12:00. Pn, Gs, and Tr decreased as drought stress increasing. The WUE of Neyraudia reynaudiana increased under moderate and slight drought stress, and decreased slightly under heavy drought stress. Therefore, Neyraudia reynaudiana had strong drought-resistant capacity.(4) In the different drought stress treatments, there were significantly differences among the physiological process of Neyraudia reynaudiana. The effects of drought stress on protection enzymes of Neyraudia reynaudiana were significant. The activities of POD, SOD and CAT in leaves of Neyraudia reynaudiana under adequate water supply treatment were significantly higher than the slight, moderate and severe stress. From the leaf membrane system, the more serious the drought stress, the higher MDA content of Neyraudia reynaudiana.The MDA content showed the increasing trend under adequate water supply. The relative Conductivity of leaf gradually increased with increasing of drought stress. The effects of drought stress on the leaf soluble proteins and secrete acidic substances in roots of Neyraudia reynaudiana were showed as follows: their contents were decreased with increasing of drought stress.(5) Among the different low phosphorus stress treatments, there were significant effects on biomass distribution pattern and root morphology indicators of Neyraudia reynaudiana. The total root length, total root surface area and total root volume of Neyraudia reynaudiana were significantly higher than other treatments under moderate stress. The moderate stress was more benefit for growth of root length, root surface area and root volume, however, not conducive to root diameter growth. The effects on stalks, leaves, roots, and biomass accumulation of Neyraudia reynaudiana under the low phosphorus stress showed an upward trend with increase of phosphorus stress and then gradually decline.(6) Among the different nitrogen stress treatments, there was significantly different in total root length, root surface area, root average diameter, root biomass of Neyraudia reynaudiana, However, not in the average diameter. The root length, root surface area, root volume and root biomass decreased with increase of nitrogen stress. The Neyraudia reynaudiana grew slowly under the extreme shortage of nitrogen in the soil, however, root biomass distribution ratio was higher than the aerial parts which may be an ecological resistance mechanism of Neyraudia reynaudiana under nitrogen stress.(7) There was significantly difference in biomass allocation pattern and root morphology indicators of Neyraudia reynaudiana under different potassium stress treatments. The total root length, total root surface area and total root volume of Neyraudia reynaudiana without potassium supply treatment were significantly higher than other treatments. The root average diameter of Neyraudia reynaudiana showed a decreasing trend with the decrease of potassium. The whole plant biomass showed an increasing trend and the root-shoot ratio showed a declining trend with the increase of potassium.(8) There was significantly difference in the leaves physiology indicators of Neyraudia reynaudiana among different phosphorus stress treatments. To some extent, the leaf protection enzymes of Neyraudia reynaudiana were enhanced under low phosphorus stress. Compared with normal phosphorus supply treatment, the activities of POD, SOD and CAT in Neyraudia reynaudiana leaves under low phosphorus stress were significantly increased (p<0.05), however, with the intensification of phosphorus stress, each protection enzymes activities consistently decreased under severe phosphorus stress and not phosphorus supply treatments. From the leaf membrane system, the MDA content and relative conductivity gradually increased with the intensification of phosphorus stress, while the phosphorus stress reaches a certain level, both showed a decreasing trend. The leaf soluble proteins content and secrete acidic substances content in roots of Neyraudia reynaudiana were significantly higher than other treatments under moderate stress.(9) There was significantly difference in the leaves physiology indicators of Neyraudia reynaudiana among different nitrogen stress treatments. The effects of low nitrogen stress on the leaf protection enzymes of Neyraudia reynaudiana were significantly. The activities of POD, SOD and CAT in Neyraudia reynaudiana under normal nitrogen supply, slight, moderate stress were gradually increased, however, with the intensification of nitrogen stress, each protection enzymes activities decreased in turn among severe nitrogen stress and not nitrogen supply treatments. The MDA content and relative conductivity gradually increased with the intensification of nitrogen stress. When the nitrogen stress reaches a certain level, both showed a decreasing trend. The leaf soluble proteins content and secrete acidic substances content in roots of Neyraudia reynaudiana under severe and moderate stress treatments were significantly higher than other treatments (p<0.05).(10) There was significantly difference in the leaves physiology indicators of Neyraudia reynaudiana among different potassium stress treatments. To some extent, under low potassium stress the protection enzymes of Neyraudia reynaudiana were enhanced, the activities of POD, SOD and CAT inNeyraudia reynaudiana leaves under moderate potassium stress were higher, and there were the lowest under normal supply potassium treatment, which significantly lower than the low potassium stress treatment (p<0.05). The MDA content was the highest under moderate stress among different treatments from the leaf membrane system of Neyraudia reynaudiana. While the leaf relative conductivity showed a gradually increasing trend with the intensification of low potassium stress. Similarly, the MDA content and the leaf relative conductivity showed a gradually increasing and then decreasing trend with the intensification of potassium stress.Based on the above results, the response strategies to environmental stress for pioneer plant Neyraudia reynaudiana in the collapsing hill area. were as follows:when field moisture capacity less than40%and moderate or severe drought stress, the tillers number of Neyraudia reynaudiana increased, which improved the coverage of Neyraudia reynaudiana to reduce soil moisture loss by increasing the leaves number and leaf width. At the same time, the average intercellular CO2content, chlorophyll, MDA content and relative conductivity significantly increased, which improved water use efficiency to some extent.The MDA content and the relative conductivity showed a gradually increasing and then decreasing trend with the intensification of drought stress, while the root biomass showed a significantly increasing trend, root-shoot ratio increasing, which mainly through the amplification of the root surface area and root length extending outward to enhance the searching ability for moisture in soi; and meet the demand of its normal growth.It showed a similar response strategy for Neyraudia reynaudiana to phosphorus nitrogen and potassium stress. When phosphorus content was less than0.50mmol·L-1, the nitrogen content was less than3.75mmol·L-1, and the potassium content was less than3.0mmol·L-1in the soil, or under a moderate or severe nutrient stress, the activities of POD, SOD and CAT in Neyraudia reynaudiana leaves were significantly increased, the leaf soluble proteins content and secrete acidic substances content in roots increased. At the same time, stalks, leaves, roots, and biomass accumulation of Neyraudia reynaudiana also increased, the root-shoot ratio, total root length, total root surface area and total root volume of Neyraudia reynaudiana significantly increased, and then improved root searching ability of Neyraudia reynaudiana under the nutrient stress. However, the active response mechanism to nutrient stress only last for a short time. With nutrient stress for a long time, the enzyme activity and number of the leaves might decrease, and the growth of Neyraudia reynaudiana became slow. |
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