The Physiological And Growth Responses Of Robinia Pseudoacacia And Platycladus Orientalis To Rhythmic Drought Stress Under Lead Stress

In order to study the laws of the influence that drought stress of different genres, timeand intensity under lead stress has on the growth conditions of the seedlings of Robiniapseudoacacia and Platycladus orientalis at each growth stages, and discuss the adaptation andself-regulation mechanisms of the two trees’seedlings which response to different genres andintensity, and rhythmic drought stress. This research, through artificial water control indrought shed, adopts the ternary quadratic general rotary unitized design to study thechanges of Robinia pseudoacacia and Platycladus orientalis in water physiology,photosynthesis, hydraulic structure and biomass under the stresses of leadand rhythmic drought, viewing to enhance the efficiency of water application, achieveproduction potential of the tree species, and provide technical support andtheoretical foundations for theintroduction and cultivation of exotic forest trees and vegetationrestoration and reconstruction in arid and semi-arid lead-zinc mining area. The major researchconclusions are as follow:1. Under lead stress, when the soil relative moisture content is higher than87.84%, watersaturation deficit of Robinia pseudoacacia leaves increases with the decline in waterprovision, and when between52.16%and87.84%, it decreases with the aggravation ofdrought stress; while water saturation deficit of Platycladus orientalis leaves responsesdifferently, it increases with the aggravation of water stress. But when the soil relativemoisture content drops to40%, both the water saturation deficits increase to the largest,reaching17.04%and27.88%respectively. Transpiration rates of both tree spices increasefirstly before decreasing with the reduction in soil water supply, both reaching the largestwhen the soil relative moisture content is87.84%and the smallest when40%. Under the sametreatment, the transpiration rate of Robinia pseudoacacia is higher than that of Platycladusorientalis, which shows that the water consumption of Robinia pseudoacacia is bigger thanthat of Platycladus orientalis.2. Under lead stress, when the soil relative moisture content is lower than87.84%,the chlorophyll content and netphotosynthetic rate of Robinia pseudoacacia leaves, and thenetphotosynthetic rate of Platycladus orientalis decline with the aggravation of drought stress, and reduction extent increases, both reaching the smallest when the soil relative moisturecontent is40%; water use efficiency of both trees, however, increases gradually with theaggravation of drought stress; thechlorophyll content of Robinia pseudoacacia leaves changesmore complicatedly and without obvious regular laws. When the soil relative moisturecontent is higher than87.84%, with the increase in soil water provision, net photosyntheticrate of both trees, the chlorophyll content of Robinia pseudoacacialeaves, and the water useefficiency of Platycladus orientalis how a drop trend, while the water use efficiencyof Robinia pseudoacacia increases slightly, but the difference does not reach significantlevel(P>0.05),while the correlation to transpiration rate reaches the very significantlevel(P<0.01).3. Under lead stress, rhythmic drought stress has a significant impact on the hydraulicstructure of the branches of both trees. And water supply rate at the terminal growth stage hasthe largest impact on PLC of Robinia pseudoacacia branches, followed by the fast growthstage, and pre-growing stage the least. To achieve the smallest PLC of Robiniapseudoacacia branches, the relative water content was81.26%in pre-growing stage,85.29%in fast growth stage and82.70%in terminal growth stage. For Platycladus orientalis, watersupply rate at the fast growth stage has the biggest impact, followed by the terminal growthstage, and early growth stage the smallest. The frequency analysis indicated that to achieve<21.74%PLC of Platycladus orientalis branches, the relative water content range was64.72%~75.28%in pre-growing stage,76.42%~83.76%in fast growth stage and65.38%~75.26%in terminal growth stage. The interaction effect of soil water supply rates atpre-growing stage and fast growth stage on PLC of Robinia pseudoacacia branchesis significant, and that at fast growth stage and growth terminal stage on PLC of Platycladusorientalis is also significant.4. Under lead stress, the soil water supply rates at the three growth stages contribute tothe biomass of Robinia pseudoacacia as follows: the pre-growing stage>the fast growthstage>the terminal growth stage. To achieve the largest biomass of Robinia pseudoacacia,the relative water content was75.42%in pre-growing stage,85.17%in fast growth stage and71.82%in terminal growth stage. To Platycladus orientalis, the contributions are as follow:the fast growth stage>the terminal growth stage>the pre-growing stage. To achieve the largestbiomass of Platycladus orientalis, the relative water content was80.78%in pre-growing stage,79.62%in fast growth stage and95.21%in terminal growth stage. The pre-growing stage ofRobinia pseudoacacia is more sensitive to water supply, and the growth and development atpre-growing stage and with better water conditions, is the key to the biomass increase, while the fast growth stage is an important growth period of the year, the amount of water supplywill affect the ultimate biomass of Platycladus orientalis.