Studies On Morphology And Function Of Root Of Typical Desert Plant And Its Drought-resistant Physiology Characteristics On Northwest China

The root system is the vegetative organ from which the plants absorb water and nutrition;moreover, it offers the partial material base for photosynthesis. The root system’smorphological characteristics, spatial distribution and architecture show a great difference indifferent habitats conditions. The study of morphology structure, nutrition characteristics andphysiological characteristics against drought of root system in different habitats conditionsplay a significant role in understanding the root morphological structure and functions inresponding mechanism to environmental factors and the strategy of physiological adaptationwithin the different environments.Based on the typical desert plants Reaumuria soongorica and Nitraria tangutorun in aridand semi-arid regions of northwestern China, according to the precipitation gradient, themethod which combined trenching with root-tracking method were used to mine thewhole-plant roots in this research, and its morphological characteristics and architectureparameters were measured, and its root distribution was drew. At the same time, fine rootsunder different habitats were collected and classificated according to the Pregitzer,sclassification method, and then the root analysis software was used to measure themorphology parameters, and the nutrient characteristics（the content of C、N）of the fine rootwas measured. Under different habitats, morphological characteristics and architectureparameters of two desert plants were discussed, and the response mechanism of them toenvironmental variations was revealed, fine root architecture and morphology and functionalcharacteristics were also clarified. Combined with the water gradient experiment of artificialcontrol conditions, the morphology and function of R. soongorica root were compared underdifferent soil moisture conditions. We further prove the response strategy of the morphologycharacteristics and functions of R. soongorica root to soil moisture variations.1. The taproot growth of the two types of desert plants was restrained with the decrease inprecipitation and increase in drought stress. However, there were some discrepancies amongdifferent species, among which the taproot growth of R.soongorica is evidently restrained.These two types of desert plants can adapt the environmental stress through the morphologicalchange in arid and semi-arid regions in Northwest, China. These two types of desert plants inloess hilly and gully region can adapt the environmental stress through the increase of lateralbranches, the increase of the number of lateral branches and the total root length. Nevertheless, these two types of desert plants in sandy area of Hexi Corridor can adapt the bioticenvironment mainly through prolong the average connecting length which demonstrate theobvious morphological plasticity of the two types of desert plants.2. The root topological indices of the two desert plants were small, and the root branchingpatterns were herringbone-like. The roots of the two desert plants had obvious fractalcharacteristics, with the fractal dimension of R.soongorica and N.tangutorum being (1.18±0.04) and (1.3±0.06), respectively. The root fractal dimension and fractal abundance weresignificantly positively correlated with the root average link length. The root average linklengths of the two plants were long，which enlarged the plants’ effective nutrition space，andthus, made the plants adapt to the dry and infertile soil environment. The sums of the rootcross-sectional areas before and after the root bifurcation of the two desert plants were equal,which verified the principle of Leonardo da Vinci. A total of17parameters of rootarchitecture were analyzed by the principal component analysis. The parameters of roottopological structure，numbers of root links，stepwise branching ratio，and root diameter couldwell present the root architecture characteristics of the two desert plants.3. Topological indices of R.soongorica root system are small at all the habitats, and rootbranching pattern tends to be dichotomous. The topological indices of R.soongorica rootsystem gradually increase in the Minqin windblown sand region and the Zhangye Gobi regionin Hexi Corridor, which indicates that drought makes root branching pattern tend to beherringbone-like. Fractal dimension values of R.soongorica root system all are small in theMinqin windblown sand region and the Zhangye Gobi region in Hexi Corridor and the valuesare1.1778and1.1169, respectively. While the fractal dimension values are relative large inJiuzhoutai semi-arid hilly and gully region of the Loess Plateau and the values is11.3104.Total branching ratios of the R.soongorica root system in arid region of Hexi Corridor aresmaller than that in the Jiuzhoutai semi-arid hilly and gully region of the Loess Plateau. Itshows that the root branching ability in the semi-arid region is stronger, and it decreases atsome degree with drought increase. The root link lengths of R.soongorica root system arelong at all the habitats, but there are significant differences between the different habitats. Theroot link length at the Minqin windblown sand region is the longest. It is concluded thatR.soongorica adapts to arid environment by decreasing root branching, decreasing rootoverlap and increasing the root link length, which makes its root branching pattern tend to beherringbone-like to reduce compete degree in root internal environment for nutrients and toenhance root absorption rate to nutrients, and ensure the effective nutrition space, thus it can absorb enough water and nutrients in resource-poor settings to ensure its normal physiologicalrequirements.4. The morphology structure of fine root of two desert plants had an obvious differenceunder the natural condition, but the first order lateral root had high proportion in all the rootorders. Regression analysis showed that there was an exponential relationship between all fineroots of every order and the order for two desert plants. With increasing root order, thediameter and length of fine roots of two desert plants increased, whereas the specific rootlength (SRL) and tissue density decreased. The diameter and root length of fine roots indicateda regular change with increasing of root order, and had a good character with exponentialfunctions, while the change of the specific root length and tissue density could be depicted asthe quadratic function. With increasing root order, the C content and C/N of two desert plantsincreased gradually, whereas the N concent decreased. Moreover, C content and N concenthad significant differences between the first order and order of2ndand3rd. This showed thatfirst order roots mainly undertook the function of absorbing nutrient and water due to theirhigh respiration, while higher order of fine roots played an important role in nutrient andwater transportation and carbohydrate storage owing to the high degree of lignifications.5. The morphological characteristics of R.soongorica seedling roots demonstrated greatvariations under different water gradient and root order. The growth of taproot was inhibitedwith the increase of stress of soil water, but the total length of lateral roots increasedobviously under higher water stress. The diameters and volumes of root also influenced by thedegree of water stress, and specific surface area and specific root length increased with theincrease of water stress, especially for the roots of order1and order2, this results indicatedgreater lateral root length, root surface area, root volumes, and smaller root diameters appearto be important indicators of morphological adaptation of root system in an extreme soilcondition. The morphological characteristics of R.soongorica seedling roots also weredifferent with the change of root orders, and the root length and specific root length decreasedwith increase of root orders, however, the root with higher order has greater root diameter andvolume. The total root surface area and specific surface area showed a more complexvariability under different root orders, and increased in lower order root, followed by adecrease in higher order root. These phenomena indicated the highly morphological variationof fine root system of R. soongorica, and the high development of lateral root with lowerorder was an important strategy in response to the water stress conditions.6. The nutrient characters of fine roots of R. soongorica seedling changed with the different degree of drought stress and root order. The total carbon content decreased with theincrease of soil water stress, but the total nitrogen content firstly decreased and then increasedwith the increase of soil water stress. This showed that respiration of fine roots ofR.soongorica seedling was restrained under different soil water stress condition. Under samesoil water stress condition, the total carbon content and C/N of fine roots of R.soongoricaseedling gradually increased with the increase of root order, but the total nitrogen contentgradually decreased with the increase of root order. This showed that the low roots havestronger respiration and metabolic activity. Under different soil water stress condition, thetotal fine roots and lateral root biomass of R.soongorica seedling were all increased, andlateral root made the greatest contribution to the increase of the total roots biomass, however,under soil water stress condition, the growth of aerial parts of R.soongorica seedling wasrestrained, especially, abscission of the leaves caused the decrease of aerial parts biomass. Sothe root shoot ratio was increased. With the increasing of soil water stress, water consumptionof R.soongorica seedling deceased, but water use efficiency was improved.7. Soil water stress has not done hurt to the root of R.soongorica seedling to some extent,and the root activity of R.soongorica seedling increased with the increase of soil water stress.In compared with the control, the root activities under middle and severe water stress wereincreased by12.89%and17.42%, respectively. This showed, under soil water stresscondition, that the root of R.soongorica seedling still had relatively high TTC deoxidizingability which provides water and nutrients for the growth of the aerial portion to ensure thenormal growth of plant in the hostile environment. SOD and POD activity of roots issomewhat reduced with the increase of soil water stress, but the differences were notsignificant, however, CAT activity and proline content increased with the increase of soilwater stress. This showed, under soil water stress condition, that the root of R.soongoricaseedling mainly decreased lipid peroxidation and clear intracellular excess H2O2by increasingCAT activity to make it maintain at a low level, in order to protect the construction of the cellmembrane under soil water stress, in the meantime, increased ability of osmotic regulation byincreasing proline content, the higher drought resistance ability was represented. R.soongoricaseedling still remained low MDA content under soil water stress condition, and this showedits membranes did not produce lipid peroxidation. So, under soil water stress condition,R.soongorica seedling mainly adapt to soil drought stress by maintaining high root activity,CAT activity and proline content and remaining low MDA content, the high droughtresistance ability was represented. This is main reason why R.soongorica can largely exist anddistribute on arid or semi-arid.