Effects Of Seasonal Drought On Photosynthetic Characteristics And Competition Mechanism Of Amaranthus Retroflexus L.and Glycine Max(Linn)Merr.

The change of precipitation pattern refers to the change of rainfall,rainfall intensity a nd frequency.The change of precipitation pattern has become one of the global phenomena.Most farmland in northeast China is rain-fed farming,which is obviously affected by natural precipitation,interannual and annual precipitation fluctuates greatly,d rought occurs frequently.However,there is no research on the effects of different seasonal drought on exotic weeds in farmland and the comparison of photosynthetic characteristics between C 4 exotic weeds and C3 crops under different seasonal drought cond itions.These studies will help us better understand the invasion mechanism of exotic weeds C4.The objective of this study was to investigate response mechanism of C4 exotic weeds and C3 crops to different seasonal drought and the competition mechanism between them.A C4 weed A.retroflexus L.and a C3 crop G.max(Linn)Merr.were used as study materials.A experiment was carried out to simulate the dry year four precipitation patterns(under the same amount of rain,IP is the pattern of initial peak prec ipitation in the growing period,MP is the pattern of medium peak precipitation in the growing period,TP is the pattern of terminal peak precipitation in the growing period,ITP is the pattern of initial and terminal peak precipitation in the growing period).The leaf water content(LW),specific leaf area(SLA),gas exchange parameters,chlorophyll content and composition of the two plants were compared under seasonal drought and two competition modes.The results showed that seasonal drought had a significant effect on LW and SLA of A.retroflexus L.and G.max(Linn)Merr.The LW and SLA of the two plants were higher than those of four seasonal drought treatments under the CK.During the same growth period,the SLA of G.max(Linn)Merr.was 1.86-3.11 times of that of Amaranthus retroflexus L.,which may be due to the different strategies of competing for light.G.max(Linn)Merr.tends to form larger SLA,whereas Amaranthus retroflexus L.got more light energy through higher plant height.Under the same precipitation pattern,A.retroflexus L.had higher Pn,Gs,E,WUE and PNUE than G.max(Linn)Merr.,especially at the beginning of growing season the A.retroflexus L.Pn was 2-5 times higher than that of G.max,thus the photosynthetic capacity of A.retroflexus L.was in a dominant position in interspecific competition.A.retroflexus L.had no significant WUE fluctuation under different seasonal drought treatments,while the WUE of G.max(Linn)Merr.decreased with the decrease of rainfall,indicating that A.retroflexus L.was more adaptable todrought environment.The ITP precipitation pattern inhibited the photosynthesis of the two plants.Such as in the middle of growth,the Pn of A.retroflexus L.in the ITP was only 54% to 58% of that in the CK,and the Pn of G.max in the ITP was only 37-39% of that in the CK.At the beginning of growth,chl(a+b)content of the two plants in four seasonal drought patterns were lower than or not much different from the CK.In the middle growth stage,the chl(a+b)content of the two plants in sole planting was higher than that of other precipitation pattern in the ITP,which may be because the low rainfall resulted in higher chlorophyll concentration in the leaves of the two plants.Seasonal drought had a significant effect on the total biomass of the two plants,the total biomass of both plants decreased under the seasonal drought treatment.At the beginning of growth,the total biomass of both species was the minimum under the TP,At the later growth stage,the total biomass of A.retroflexus L.was the smallest in the ITP,and that of sole G.max(Linn)Merr.was the smallest in the ITP,and the minimum of mixed G.max(Linn)Merr.appeared in the TP or MP.Under the same rainfall pattern,the total biomass of A.retroflexus L.was significantly higher than that of G.max(Linn)Merr.Seasonal drought and growth stage had significant effects on the relative biomass of the two plants.The relative biomass of A.retroflexus L.increased gradually with the growth stage,while that of G.max(Linn)Merr.reached the maximum at the early growth stage and then decreased significantly.The relative biomass of A.retroflexus L.was the largest in ITP from July to August.G.max(Linn)Merr.had the largest relative biomass under IP of July.In August,the relative biomass was largest in ITP.Interspecific competition had a significant effect on LW of the two plants.The LW of A.retroflexus L.in sole planting was lower than that in mixed planting,while the LW of Glycine max(Linn)Merr.in sole planting was higher than that in mixed planting.The effect of interspecific competition on the SLA of A.retroflexus L.in mixed planting was significant,but the effect on G.max(Linn)Merr.was not significant,and the SLA of A.retroflexus L.in mixed planting was higher than that in sole planting.Interspecific competition had significant effect on gas exchange parameters and photosynthetic pigment of A.retroflexus L.and G.max(Linn)Merr.The Pn,E,PNUE of A.retroflexus L.in mixed planting were higher than those in sole planting,but Pn,E,PNUE of G.max(Linn)Merr.in mixed planting were lower than those in sole planting.The Chla content of A.retroflexus L.in sole planting was lower than that in mixed planting,and chla,chlb,chl(a+b)content of G.max(Linn)Merr.in sole planting were higher than those in mixed planting.Interspecific competition had a significant effect on the total biomass of the two plants.At the middle and the end of the growing season,the total biomass of mixed A.retroflexus L.was significantly higher than that of sole A.retroflexus L.,while the total biomass of mixed G.max(Linn)Merr.was significantly lower than that of sole G.max(Linn)Merr.The relative biomass of A.retroflexus L.was significantly greater than 1 and that of G.max(Linn)Merr.was significantly less than 1,indicating that interspecific competition promoted the growth of A.retroflexus L.but inhibited the growth of G.max(Linn)Merr.In conclusion,under Seasonal drought conditions,C4 weed A.retroflexus L.had stronger adaptability to drought in the different seasonal drought conditions,drought may intensify theinvasion of A.retroflexus L.in soybean fields of northeast China.