Water stress altered photosynthesis‐vegetation index relationships for winter wheat

The photosynthetic rate (P n ) of winter wheat ( Triticum aestivum L.) is directly affected by water conditions, which can be effectively estimated by coupling vegetation indices (VIs). Establishing sound empirical models between VIs and P n of winter wheat under different water conditions therefore is a sound approach for predicting P n for broad applications. A rainout‐shelter field experiment was conducted to investigate the changes in P n and VIs (normalized difference vegetation index [NDVI], enhanced vegetation index [EVI], ratio vegetation index [RVI], photochemical reflectance index [PRI]) under five water stress conditions. We found that: (a) heavy and moderate drought stress inhibited P n , with their accumulated effects became more significant with the duration of treatments while the light drought stress and slight water logging produced minor effects on P n . Heavy and moderate drought stress also caused the peak time of P n preceded during the growth period; (b) NDVI showed low sensitivity to vegetation coverage while EVI explained more seasonal variation than other VIs when leaf area index (LAI) was >3; (c) the linear models between P n and EVI under heavy ( P n = 2.0449 EVI − 1.2906 ) and moderate ( P n = 1.7742 EVI − 1.7021 ) water stress were selected as the best model for P n , while a power function for P n ‐NDVI performed better under sufficient water supply (P n = 37.982NDVI 3.0101 ) and slight water logging (P n = 28.024NDVI 2.5646 ). Our results demonstrated that P n –VI relationships varied by water stress for a winter wheat over the growing season. These results could be used for the prediction of winter wheat P n under water stress at a large spatial scale.