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Sun-induced chlorophyll fluorescence (SIF) is a promising remote sensing signal for early stress detection due to its close link with photosynthesis. Canopy SIF signals are controlled by leaf physiology, canopy structure, radiation intensity and sun-observer geometry. Variations in SIF observations are affected by variations in these controlling factors besides water stress. Mitigating the interference of non-drought factors on the variations in canopy SIF to accurately evaluate drought degree is still challenging. In this study, we explore the response of apparent SIF yield (SIFy) to progressive drought in maize. With experimental evidence, we show that the difference between noon and morning SIFy was a better indicator of drought than mono-temporal SIFy measurements. We proposed the noon-to-morning ratio (NMR) to characterize diurnal dynamics and assess the severity of drought. The results show that midday measurements of SIFy were the most affected by water stress, and morning measurements were the least. The NMR of SIFy successfully revealed water stress by tracking the timing of the transition from light-limited to water-limited conditions of SIF within a day. Hence, the NMRs of SIFy were considerably more sensitive to drought than their mono-temporal values, and traditional vegetation indices, especially during the early phase of drought. This demonstrates that the use of multi-temporal or diurnal SIF measurements is more reliable than mono-temporal observations for stress detection.

Diurnal Pattern of Sun-Induced Chlorophyll Fluorescence as Reliable Indicators of Crop Water Stress