Complexity and interactions of climatic variables affecting winter wheat photosynthesis in the North China Plain

Investigating the impact of climate change on crop photosynthesis is crucial for evaluating yield loss and ensuring food security. While previous studies have explored the effects of temperature, vapor pressure deficit (VPD), and soil moisture (SM) on crop photosynthesis in specific locations or ecosystems, large-scale analyses remain limited. This study provides a comprehensive evaluation of the sensitivities and contributions of temperature, VPD, plant available water (PAW), and other climatic factors to winter wheat photosynthesis in the North China Plain (NCP) from 2001 to 2019, utilizing remotely sensed solar-induced chlorophyll fluorescence (SIF) data. Our findings indicate a significant increase in SIF during both the vegetative growth period (VGP) and reproductive growth period (RGP), with trends in climatic factors influencing SIF over the past two decades. The sensitivity of SIF to temperature, VPD, and PAW was more pronounced during the VGP compared to the RGP, suggesting that climatic variability has a greater impact on photosynthesis prior to the heading stage of winter wheat. VPD emerged as a major negative contributor to SIF variation in both periods, followed by temperature during the VGP and PAW during the RGP. Notably, when VPD dropped below the thresholds of 0.83 kPa during the VGP and 1.11 kPa during the RGP, the sensitivity of photosynthetic capacity significantly decreased. Structural equation modeling (SEM) revealed that the negative indirect effects of temperature on SIF through VPD counterbalanced its positive direct effects, while the positive indirect effects of PAW via VPD enhanced its direct effects. Overall, the increase in VPD and the significant decrease in PAW had a substantial negative impact on winter wheat photosynthesis, particularly during the RGP in the NCP. These results offer a quantitative and comprehensive assessment of the influence of rising VPD on the photosynthetic capacity of winter wheat in the context of climate warming and diminishing SM, while also highlighting the significance of growth stage at the system scale for a more meaningful ecophysiological understanding.