Drought response of maize solar-induced chlorophyll fluorescence and its components and their coupling relationship with photosynthetic productivity

Solar-induced chlorophyll fluorescence (SIF) serves as a vital indicator of plant photosynthetic efficiency and carbon uptake, and is widely recognised as an effective tool for monitoring gross primary productivity (GPP). However, the coupling mechanism between SIF and GPP under drought stress remains unclear. This study utilised satellite-derived GPP, SIF, and Standardised precipitation evapotranspiration index (SPEI) data from June to September 2010–2020 to investigate the spatiotemporal coupling between SIF and GPP across varying drought intensities in spring maize crops in Northeast China. It further examined the relationships between SIF components—radiation, canopy structure, and physiological factors—and their respective contributions to the coupling dynamics with GPP. Results indicate: 1) Under non-drought conditions, the correlation between SIF and GPP was stronger during the entire growing season (June to September) (R² = 0.80) than in individual months, and stronger in June and September (R² = 0.59, 0.61) than in July and August (R² = 0.31, 0.38); During July and August, as well as throughout the entire growing season, canopy structural components contributed most significantly to the relationship between SIF and GPP, accounting for 48.6%, 39.1%, and 50.0% respectively. In June and September, physiological components contributed most significantly, accounting for 49.4% and 42.4% respectively. 2) Drought weakened the correlation between SIF and GPP in June and September, while strengthening it in July, August, and throughout the entire growing season. Over the entire growing season, the structural component of the canopy decreased by 0.4%, while the physiological and radiative components increased by 0.1% and 0.3% respectively; It increased the structural component by 3.9%, 0.9%, 4.2%, and 2.6% respectively in each month, while the contribution of the radiative component decreased by 5.3%, 3.2%, 0.7%, and 1.3% respectively. The physiological component increased by 1.4% and 2.3% in June and July respectively, but decreased by 3.5% and 1.3% in August and September respectively. 3) As drought intensified, the correlation between SIF and GPP exhibited a linear increase trend throughout the entire growing season and in June, with R² increasing from 0.80 to 0.83 and from 0.52 to 0.61 respectively. Non-linear trends were observed in the remaining months. Radiation components exhibited a linear decrease in August, while canopy structure components showed a linear decrease throughout the entire growth period and a linear increase in June, July, and August. Physiological components decreased linearly in August but increased linearly throughout the entire growth period and in September. Drought does not affect the primary contributing components of the SIF-GPP relationship, but alters the contribution rates of each component, determined by the correlation between the maximum contributing component and GPP. This study provides important reference for deepening the understanding of drought response mechanisms in the coupling relationship between SIF and GPP.