The Study Of Influencing Mechanism Of Drought Stress On Cucumber Plants Growth And Spectra Estimation Model

Drought stress is an important factor affecting crop growth.In order to study influencing mechanism of drought stress on cucumber leaf photosynthesis and plants growth,and used spectral parameters to estimate total chlorophyll contents of cucumber leaves,an artificial controlled experiment was conducted to investigate the effect of drought stress on cucumber plants.Four different soil moisture levels were designed over the whole stage,which were normal irrigation(CK,soil relative humidity was 70%-80%),light drought stress(T1,soil relative humidity was 60%-70%),moderate drought stress(T2,soil relative humidity was 50%-60%)and severe drought stress(T3,soil relative humidity was 35%-45%),respectively.The light response curves,growth indexes,chlorophyll contents and canopy spectra of cucumber plants were measured at seedling stage,tendril stage,flowering stage,initial ripening stage and mature stage,respectively.And selected a growth with physiological parameter-total chlorophyll content,which had the most significant correlation with soil moisture content,then established an estimation model of of total chlorophyll content based on spectral parameters.The main results are as follows:(1)Took the seedling stage and the mature stage as examples,light response process of cucumber plants leaves was fitted by rectangular hyperbola,non-rectangular hyperbola,exponential and Ye Z P model,the results showed that Ye Z P model fitted best.Ye Z P model was applied to simulate light response process of cucumber plants leaves over the whole growth stage,drought stress decreased the net photosynthetic rate,light saturation point and maximum net photosynthetic rate,and raised the light compensation point of leaves.The initial quantum efficiency of leaves all increased at first and dropped afterwards with the aggravation of drought stress.With the increase of stress,stomatal conductance and transpiration rate decreased gradually.Intercellular CO2 concentration(Ci),stomatal limitation value(Ls)of cucumber leaves under light and moderate stress was lower,higher than CK,respectively.From seedling stage to initial ripening stage,Ci and Ls of leaves under severe drought stress had same trends as light and moderate drought stress,while at mature stage,severe drought stress elevated Ci and declined Ls of cucumber leaves.The higher the stress was,the lower chlorophyll contents were.(2)Under drought stress,intemode lengths of stems,leaf area indexes of cucumber plants decreased by 14.05-47.89%,41.66%-80.88%,respectively.Drought stress reduced stems,leaves and fruits dry matter accumulations significantly,and delayed cucumber-set time.Drought stress reduced cucumber plants root lengths,root tips,root surface areas and root average diameters.Drought stress induced the occurrence of capillary and thick roots,and inhibited the occurrence of fine and mid roots in cucumber plants.Except for the increased dry matter accumulation in root under T1 to adapt to drought stress at tendril,flowering stage,cucumber plants root dry matter accumulations of other treatments over the whole stage were significantly lower than that of CK.Root/shoot ratios of cucumber plants increased under drought stress.(3)Drought stress reduced reflectance and red edge reflectance change rate of cucumber plants canopy spectra in near-infrared band.Under drought stress,the red edge of cucumber plants canopy spectra shifted from long to short wavelength(characterized as blue shift),and the red edge area and red edge amplitude of cucumber plants canopy spectra decreased compared to CK.The correlation between soil moisture content and total chlorophyll content was the strongest,the total chlorophyll content can be used as a symbol to reflect the degree of drought stress,thus total chlorophyll content estimation models based on spectral parameters were established,according to the parameters correlation(|r|>0.8)and model determination coefficient(R2>0.8),four better models were selected for testing,the results of models test indicated that the estimation of chl(a+b)content achieved the highest precision by the multiple stepwise regression model based on first derivative spectra,which got the lowest precision model based on DR779/DR542 ratio index.