Effects Of Water Stress On The Characteristic Of Light Interception, Temperature And Humidity Of Canopies Of Winter Wheat And Maize

Solar radiation is the only energy source in farmland ecosystem.Photosynthetically active radiation(PAR)with a wavelength of 400-700 nm,which accounts for approximately 50%of total solar radiation,can be absorbed by green plants and used for photosynthesis.Crops absorb and convert light energy into dry matter through photosynthesis.Both aboveground biomass and final yield are dependent on the ability of canopy to intercept PAR and the radiation use efficiency(RUE).Under non-stressed conditions,the dry matter production of crop is linearly related to the PAR intercepted by crop canopy.The slope of this linear relationship is the RUE of the crop.The fraction of canopy PAR interception dynamically changes with leaf area and stem growth during the whole growing season of crops.People generally use the Lambert-Beer law to calculate the fraction of PAR interception.In additon,extinction coefficient can quantitatively describe the crop canopy morphology and leaf distribution.The empirical parameter of RUE reflects crops'utilization efficiency of PAR and can be affected by factors such as crop species,planting structure,irrigation regime,fertilization rate,and climatic conditions.At the same time,RUE also changes at different crop growth stages.This relationship between crop canopy PAR interception and RUE has been applied to calculate crop dry matter accumulation and yield formation in crop models.Besides,the solar radiation received by crops can further affect the distributions of temperature,relative humidity,and soil moisture in the farmland.Temperature in crop canopy can be regulated through leaf transpiration and soil evaporation.At the same time,under different conditions of soil water stress,the vertical distribution of relative humidity of winter wheat and corn will change accordingly.In order to investigate the effects of soil water stress on the light interception,temperature,and relative humidity in the canopy of winter wheat and maize at different growth stages,field experiments of winter wheat(2015-2016 and 2016-2017)and maize(2015 and 2016)were conducted under a rainout shelter in two growing seasons in Yangling,Shaanxi province.The experiment included the treatments of vagetative stage drought(early stress for wheat,ES;jointing stress for maize,JS),reproductive stage drought(later stress for wheat,LS;filling stress for maize,FS),and non-irrigation(WS)during the whole growing season.An additional treatment of full irrigation was set up as a contrast(CK).The irrigation amount was 80 and 110mm per event,respectively.The PAR flux,temperature,and relative humidity in the canopy were measured around the clock with the weather stations installed at the center of each plot selected.The main conclusions of this study were drawn as follows.(1)The relative soil content of winter wheat under water stress was significantly lower than that of other treatments with no water stress during the same period.The maximum leaf area index(LAI)for ES,LS,and WS treatments was 31%,15%,and 58%lower than the CK treatment,respectively.The maximum fraction of canopy PAR interception for CK,ES,LS,and WS treatments were 0.90,0.88,0.79,and 0.42,respectively,and WS treatment was significantly lower than the other three treatments.Water stress had the greatest negative effect on the extinction coefficient of WS treatment.The average above-ground biomass for the two years of CK,ES,LS and WS treatment was 1532,1443,1403 and 1077 g/m~2,respectively.The crop growth rate(CGR)of winter wheat showed a similar pattern to above-ground biomass accumulation,and there was a good correlation between the two(R~2=0.93).For RUE of winter wheat,ES and LS treatments were 17%and 2%lower than CK treatment,but WS treatment was 66%higher than CK treatment.The RUE reduction of winter wheat caused by water stress in the vegetative stage was greater than that in the reproductive stage under water stress.The RUE of winter wheat under the WS treatment showed very different responding mechanisms to water stress,which needs further study.This study confirmed that the extinction coefficient and RUE should be treated as functions of growth period or thermal time rather than a single constant.Thus,the estimation accuracy of dry matter in crop models could be improved and the uncertainty of simulation results could be reduced.(2)Among the treatments of water stresses at different growth stages,the effect of water stress on the LAI of maize at the jointing stage was higher than that at the filling stage.The impact of water stress on LAI was the most serious in the WS treatment.The water stress at different growth stages of maize had a significant impact on its fraction of canopy radiation interception and RUE.Among them,drought at the jointing stage helped improve the fraction of radiation interception of maize canopy,and the peak value is 18%higher than that of CK treatment.However,water stress at the grain filling stage significantly reduced the fraction of canopy radiation interception of maize,and the peak value was 39%lower than that of CK treatment.Water stress at single growth stage had little effect on the above-ground biomass of maize.The RUE of the maize obtained in this study ranged from 1.6 to 4.74 g/MJ.Among them,water stress at the jointing stage significantly reduced the RUE of maize,but water stress at the grain filling stage had little effect on it.The impact of water stress on the grain yield of maize was negative.The negative impact of water stress at jointing stage on grain yield was higher than that of water stress at the grain filling stage.Thus,the maize harvest index under water stress at jointing stage was lower than that under water stress at grain filling stage..(3)Differences in canopy air temperatures can be a good indicator for the water status of winter wheat and maize.This study established the relationship between canopy air temperature difference and the soil moisture for winter wheat and maize,respectively.Water stress affected the development of crop leaves and then affected the distribution of relative humidity within canopy.Especially when water stress in vegetative stage and in the whole growing season,poor leaf development resulted in lower relative humidity at the bottom of the canopy,which was not good to the crop transpiration in the bottom.In addition,The vertical distribution of relative humidity within the canopies of maize and winter wheat increased as the growth stage advanced.