| Plants growing on the Loess Plateau often suffer from water stress due to the low precipitation,changeable annual precipitation and large evaporation.Robinia Pseudoacacia L.is one of the main plantation tree species on the Loess Plateau,the adjustment of its photosynthetic and physiological characteristics under water stress is conductive to regulating the water transmission process of the Soil-Plant-Atmosphere Continuum(SPAC)and ensuring to maintain a high level of water,reducing plants mortalities.This research was to assess the adaptability of the Robinia pseudoacacia to water stress and explore the reasonable growth soil environment of the plants,which was carried out in the greenhouse on the changes of hydraulic characteristics of the Soil-Robinia pseudoacacia System under different soil water contents(SWC).In this study,8 water treatments(30±2.5%,40±2.5%,50±2.5%,60±2.5%,70±2.5%,80±2.5%,90±2.5%and 100±2.5%of field capacity,FC)were conducted using indoor pot experiments to study both the leaf and whole plant scales of Robinia pseudoacacia plants under water stress during the growing season(July to October).Changes in photosynthetic and ecological characteristics,water potential,hydraulic resistances and vulnerability curves,etc.,were chosen to analyze the adaptation of the physiological characteristics of Robinia pseudoacacia plants under water stress,and then to explore the hydraulic characteristics of the Soil-Robinia pseudoacacia System under water stress and the balanced hydraulic measurements of Robinia pseudoacacia.The main conclusions were as follows:(1)There was a water threshold(?c)for physiological and ecological indexes of Robinia pseudoacacia plants with the change of SWC.When the SWC was less than ?c,indexes increased with the increasing SWC;when the SWC was greater than ?c,indexes of the plants kept relative stable.The physiological and ecological indexes of Robinia pseudoacacia plants showed a trend of first increasing and then maintaining stable with the increase of SWC.The ?C for the normalized single leaf area(LA)of Robinia pseudoacacia was in the 85?90%of FC,while the ?c of the whole plant leaf area(PLA)was in 90-100%of FC.The maximum water availability of height increment(?H)and ground diameter increment(?D)were in the 60?80%and 50?70%FC,respectively.The ?c of the root indexes was 60?80%of FC;and ?c of stomatal conductance(Gs)and net photosynthetic rate(Pn)were 75?80%and 65?80%FC,respectively.Leaf transpiration rate(Trl)and whole plant transpiration rate(Trp)in the 75?85%and 70?85%of FC were in the peak of their increasing trends,and Trl was larger relatively;The indexes of plants raised from July to August or September,but reduced in October during the growing season were the results of the adaptation for plants to the environment;(2)The root water potential(?r)and leaf water potential(?l)increased with the increasing SWC firstly,and then remained stable;the hydraulic resistances of the Soil-Robinia pseudoacacia System decreased with the increase of SWC and then tended to be stable.?r and ?l increased rapidly with the increase of SWC during they were in 30?50%of FC,and then changed little.And both of them showed a decreasing trend from July to October,which was benefical to enhancing the efficiency of water absorption by roots and water transmission by plants;Hydraulic resistances(R)decreased with the increment of SWC.Soil resistance(Rs),total root resistance(Rr)and plant transmission resistance(Rp),all decreased rapidly with the increase of SWC in the range of 30?50%FC.but changes of them were little when SWC was above 50%;And leaf-air resistance(Rla)decreased fastest in the range of 30?50%FC,but the rate of the decrement becomed slower in the range of 50?70%,and there was less changes when SWC was above 70%.The trends of the total resistances for SPAC(Rt)were similar to Rla,as Rla accounted for more than 96.0%of Rt values and played a predominant role in the adjustment of Rt.The values of resistances decreased from July to September but increased in October,and the reduced water potential increased the water potential differences,which were conducive to water transmission;(3)Both the predawn leaf water potential(?pd)and the midday leaf water potential(?md)of Robinia pseudoacacia first increased and then stabilized with the increasing SWC,while the specific conductivity(Ks)of Robinia pseudoacacia first increased and then decreased with the increase of SWC.And the P50(water potential of the branches with 50%loss of the hydraulic conductivity)on the xylem vulnerability curves(VCs)of Robinia pseudoacacia increased with the increase of SWC.?pd and ?md increased with increasing SWC in the range of 30?50%of FC,and relatively stable changes were observed in the range of 50?100%FC.The values of ?pd were much greater than that of the ?md.The Ks of Robinia pseudoacacia was the highest in the range of 60?90%FC,but was relatively small in other SWC.The distribution of VCs of Robinia pseudoacacia followed a certain law:the P50 values corresponding to the VCs with the increasing SWC,and the drought resistance and the anti-embolism ability were weaker with the high water potential.The P50 values of Robinia pseudoacacia under each treatment in this study(except the treatment for 30%of FC)decreased in the following order:100%>90%>80%>70%>60%>40%>50%;(4)Robinia pseudoacacia cultivated with the range of 30?40%of FC was strongly inhibited by water stress,but it could maintain sustainable growth above 50%FC.Specially,the fallen leaves,the small Gs values,the suppressed photosynthic activities and inhibited growth occurred in the 30?40%FC.Based on the distribution and variation trend of Trl,?l and R of Robinia pseudoacacia during water stress process,it is speculated that Robinia pseudoacacia cultivated under SWC greater than 50%FC can ensure relatively high Trl and low R values to adapt to the drought or water shortage conditions on the Loess Plateau,which can favor the healthy and sustainable growth of Robinia pseudoacacia. |
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