| Karst landscape is distributed very widely in the world, with the area of 22 million km2. The karst region in the southwest of China is very concentrated, which is one of the three largest karst areas all over the world. The geological environment in karst is extremely fragile. On one hand, the capacity of making soil is very low, resulting in very thin and limited soil, that is, a “congenital defect”. Coupled with the adverse conditions of severe surface karst fissures, tattered land form,accumulated depressions, soil caves and others, the soils distribute discontinuity and vary in their thicknesses dramatically. On the other hand, the special “karst drought” phenomenon is easy to appear even under the warm and wet climate conditions due to the poor ability of soil water retention capacity, severe permeation of surface water, deep buried groundwater and high exposure of rocks.In addition, with the global climate changing, extreme drought events has been becoming more frequent, and global precipitation will lead to “dry places are drier, and wet places are wetter”. Reducing rainfall in the southwest of China will further aggravate the influence of “karst drought” and soil layer loss in karst regions. In order to investigate the adaptations of plants to “karst drought” and the change of soil layer thickness, perennial Lolium perenne L. was selected as the research object in this experiment. Three soil depths(designed by using different sizes of the container including 15 cm normal soil depth as CK, 60 cm deep soil as D and 3cm shallow soil as S,pair-wise combinations to CK-CK, CK-S, CK-D, D-D, S-D and S-S) and three water treatments(including W0, W1 and W2, representing well-watering, moderate drought and severe drought,respectively) were established to study the response of root growth, biomass accumulation and allocation, photosynthetic physiology and nutrients absorption to different water contents and soil thicknesses, to explore whether the reduction of water availability modulate plants adaptation to soil thickness.The main results are as follow:1.Under well-watering treatment, compared with the combination of two normal soil depths(CK-CK), root growth of L.perenne in the combinations of other containers(CK-S,CK-D,D-D, S-D and S-S) were inhibited. Root length, root surface area, root volume and root diameter were allreduced. With the moisture reduction in moderate drought and severe drought conditions, root elongation and absorption area increased in the combinations with deep containers(CK-D and D-D),however, the roots growth including root length and root absorption surface area was inhibited under shallow soil treatment(S-S). Further, the negative effect of shallow soil and the positive effect of deep soil increased with the decline of water availability.Under well-watering treatment, compared with the combination of two normal soil depths(CK-CK), the root biomass and total biomass of L.perenne were lower in the combinations of other containers.Aboveground biomass decreased in the combinations with one shallow container(CK-S,S-D and S-S), but shoot to root ratios were not significantly affected. With the water decreased, the total biomass and root biomass were significantly stimulated in the combination with shallow and deep container(S-D). Aboveground biomass increased significantly in the combination with normal and deep soil depth(CK-D). It indicated that the increase of soil thickness could promote biomass accumulation. when water availability declined. On one hand, in the deeper soil,roots could grow rapidly to deep soil to get more water; on the other hand, in the combination of shallow and deep container, the horizontal extension could decrease in the shallow soil and allocate more resources to the root in the deep soil so that accelerate the growth of the plant.2.Under well-watering treatment, compared with the combination of two normal soil depths(CK-CK), net photosynthetic rate of L.perenne was almost not affected in the combinations of other containers(CK-S, CK-D, D-D, S-D and S-S). In the combinations with one shallow container(S-D,S-S), the stomatal conductance and intercellular CO2 concentration were lower significantly, but transpiration rate was not significantly affected. With the water decreased, stomatal conductance of L.perenne increased in the combinations with one deep container(CK-D, D-D and S-D) due to reducing stomatal resistance, as a result, net photosynthetic rate and transpiration rate increased. In the severe drought conditions, the photosynthesis only increased significantly in the combinations with two deep container(D-D). It indicated that the increase of soil thickness could promote stomatal conductance, and then net photosynthetic rate and transpiration rate. The reduce of soil thickness could decreased net photosynthetic rate, transpiration rate of L.perenne. Instantaneous water use efficiency and intrinsic water use efficiency were lower in the moderate drought conditions of the combinations of other containers compared with the combination of two normal soil depths(CK-CK), but showed an increasing trend in the deep soil treatment. In the severe drought conditions,photosynthetic rate, stomatal conductance and transpiration rate increased, which led to more water loss so that water use efficiency decreased in the combinations with one deep container.3. Under well-watering treatment, root C and N of L.perenne were lower significantly in all the other combinations compared with the combination of two normal soil depths(CK-CK), but root Preduced in the combinations with one deep container(CK-D and S-D) while shoot C in the combinations with one deep container(CK-D, D-D and S-D), shoot P in the deep soil treatment(D-D),and shoot N in the other combinations(D-D, CK-S) all increased significantly. In the moderate drought and severe drought conditions, root C fixation in the deep soil treatment(D-D) increased,while shoot C increased significantly in severe drought conditions of shallow soil treatment(S-S).With the water decreased, root P increased in the combinations of other containers(CK-S, CK-D,D-D, S-D and S-S), and shoot P decreased in the combinations with one shallow container(CK-S and S-S). Root N of L.perenne increased in the combinations with one deep container, and shoot N increased significantly in the deep soil treatment(D-D). Root C/N decreased in deep soil treatment,shoot C/N had no significant in the deep soil. In the deep soil conditions,the plant could enhance plant photosynthesis, so that leaf C absorption increased and supply more biomass to the underground so that could absorb more nutrients. In addition, with the water decreased, root N/P of L.perenne in the combinations of other containers were inhibited. Shoot N/P was decreased in the combinations with one deep container compared with the combination of two normal soil depths(CK-CK), but increased in shallow soil treatment.In general, soil depths could influence growth, photosynthetic physiology and nutrients absorption of L.perenne. With the water changing, root growth, photosynthesis and nutrients absorption were influenced. When the water is enough, root growth, biomass accumulation and nutrition element content of L.perenne in the combinations of other containers were inhibited; in the moderate drought, root length, root surface area and root volume increased while stomatal conductance and net photosynthetic rate and transpiration rate also increased. Plant nutrients absorption could enhance in the combinations with one deep container, especially in the severe drought. Plant photosynthesis, N and P absorption could be promoted significantly in the deep soil,but in shallow soil they were inhibited. Obviously, the negative effect of shallow soil and the positive effect of deep soil increased with the decline of water availability. |
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