Monitoring Of Water Status With Plant-Air Temperature Difference In Wheat

Non-destructive monitoring of crop water status is an important research area in information agriculture, and is essential for optimization of crop irrigation and precision farming management. In this study, a series of experiments with different wheat varieties and water levels were carried out with pool culture in different years. The relationships of canopy-air temperature difference and leaf-air temperature difference with water status in winter wheat were established, and estimating models were developed for indicating water status of wheat plant. The results help to provide a technique and theoretical basis for quick and non-destructive estimation of wheat water status.Firstly, the changes of leaf water content, leaf equivalent water thickness, leaf-air temperature difference of each functional leaf on main stem of wheat plant under different water levels were investigated. And then the models were developed for predicting the leaf water content and leaf equivalent water thickness based on leaf-air temperature difference of the upper three leaves in wheat. The results showed that at anthesis and filling stages the leaf water content and leaf equivalent water thickness increased gradually with increasing soil water content, but leaf-air temperature difference decreased. When the soil water was under deficit condition, leaf-air temperature difference of the flag was the smallest, and the leaf water content and leaf equivalent water thickness of the third leaf descended firstly. When the soil water content was adequate, the leaf-air temperature difference of the second leaf and the third leaf was lower than in the flag leaf. The monitoring models of leaf water content and leaf equivalent water thickness based on leaf-air temperature difference gave high accuracy in the models of y=-0.159x2-2.053x+74.632, y=-0.012x+0.128, with coefficient of determination(R2) as0.56and0.77, and standard errors(SE)as4.22and0.017, respectively. Testing of the equations with independent experiment data gave R2values of 0.47and0.61, and RRMSE (relative root mean square error)of7.59%and12.70%, with best performance for the leaf equivalent water thickness. Overall, the model can accurately estimate the wheat water status at the single leaf level, which is helpful for the non-destructive and fast monitoring of the water status in wheat.Further, investigations were conducted on the changes of net photo synthetic rate(Pn), stomatal conductance(Cond), intercellular CO2concentration(Ci), transpiration rate(Tr)and water use efficiency(WUE)under different water levels. The quantitative relationships between the leaf-air temperature difference and the net photosynthetic rate, stomatal conductance, intercellular CO2concentration, transpiration rate, water use efficiency were clarified. The results showed that the net photosynthetic rate, stomatal conductance and transpiration rate reduced with the decreased soil water content. The intercellular CO2concentration reduced with the decreased soil water content from booting to midlle grain-filling stages, and higher under W4and W1treatments at the late grain-filling stage. The water use efficiency went up with the decreased soil water content at the booting and heading stages, enhanced under W2, W1treatment at the stages of anthesis, initial filling and middle filling, and was higher under W2,W3treatment than that under W1,W4treatment at late grain-filling stage. The relationships between leaf-air temperature difference and net photosynthetic rate, transpiration rate, stomatal conductance, water use efficiency, intercellular CO2concentration were significant (P<0.05). In particular, the stomatal conductance, transpiration rate were well related with leaf-air temperature difference, and the regression models for the whole growth period were y=-0.208x+0.076, y=-3.338x+1.339, respectively, with R2of0.56and0.91, and the SE of0.126and0.704. The relationships between leaf-air temperature difference and net photosynthetic rate, intercellular CO2concentration and water use efficiency are relatively weak. When independent data were fit to the derived equations, the estimating models on stomatal conductance and transpiration rate gave perfect testing performance, withR2between the measured and estimated values as0.79and0.82, respectively, and RRMSE as36.88%and22.82%.Finally, studies were made on the changes of canopy-air temperature difference(at90°,60°angles against the horizon), canopy equivalent water thickness, plant water content, canopy leaves water content at different water levels. The relationships of canopy-air temperature difference at different angles to canopy equivalent water thickness(CEWT), canopy leaves water content, and plant water content were quantified. The ability of monitoring water status based on the canopy-air temperature difference at two angles was compared. The results showed that with the decreased soil water content the canopy equivalent water thickness, canopy-leaf water content and plant water content decreased gradually and the canopy-air temperature difference went up except at the jointing stage. The plant water status of winter wheat can be indicated by the canopy-air temperature difference at booting to grain filling stages, especially at the late growth stage. The plant water status can be indicated better when the canopy temperature was measured with the thermal infrared sensors from a vertical view than from an oblique view of60°against the horizon at the heading stage, with R2above0.62. During the post-heading stage of winter wheat the canopy-air temperature can be more usable for judging the plant water status with the sensors at a view of60°against the horizont than from a vertical view, and equation(R2)was greater than0.76. Overall, the canopy-air temperature difference measured with the sensor at a view of60°can indicate plant water status of winter wheat better than at90°angle against the horizon.