Continuous monitoring of rice responses to elevated temperature and CO2 in a T-FACE experiment using tower-based hyperspectral observation

The global atmospheric CO₂ concentration and the surface temperature keep rising since the industrial era. This ongoing change profoundly affects crop photosynthesis and yield formation. Therefore, accurately and timely monitoring the combined effects of elevated CO₂ (eCO₂) and warming on crop production is of great scientific and practical importance. In recent years, the development of continuous spectral observation technology, which captures a range of vegetation indices (VIs) and the solar-induced chlorophyll fluorescence (SIF) signal, has provided a new approach for vegetation dynamic monitoring. However, the responses and underlying mechanisms of SIF and various vegetation indices to the interactive effects of eCO₂ and warming remain unclear.

This study investigates the responses of two rice cultivars, Ningxianggeng (NXG) and Yongyou (YY), to elevated CO₂ (C⁺) and warming (T⁺), both individually and in combination, using a T‑FACE system in Nanjing, Jiangsu Province. The experiment included four treatments: ambient control (CT), elevated CO₂ (C⁺T), warming (CT⁺), and combined elevated CO₂ and warming (C⁺T⁺). Continuous canopy spectral observations, covering both full‑range (400–1000 nm) and hyperspectral (650–800 nm) measurements, were integrated with key physiological parameters such as net photosynthetic rate (Aₙ), biomass, and yield.

SIF proved to be a more sensitive and earlier indicator of photosynthetic dynamics than VIs. Under elevated CO₂ alone (C⁺T), SIF increased in both cultivars, reflecting a clear CO₂ fertilization effect. The interaction with warming (C⁺T⁺), however, revealed a diurnal dual effect: SIF was higher in C⁺T⁺ than in C⁺T during the morning, but slightly lower in the afternoon, indicating the complex effects of temperature on modulating photosynthetic activity. YY generally exhibited higher photosynthetic capacity than NXG across treatments, with a marked afternoon enhancement (up to 40%) under C⁺T⁺ during certain growth stages, though this advantage varied seasonally. In contrast, NXG showed a stronger positive response in photosynthetic efficiency under the combined C⁺T⁺ treatment. The Photochemical Reflectance Index (PRI) indicated that light‑use efficiency (LUE) declined at midday during periods of sustained high temperatures (mid‑July to early August), particularly for NXG under C⁺T, while YY under C⁺T⁺ maintained relatively higher LUE, suggesting a greater warming tolerance in YY.

The approach by integrating SIF with multiple VIs may provide a robust methodology for rapid, non‑invasive assessment of cultivar‑specific climate adaptability, offering valuable insights for precision agriculture management and climate‑resilient breeding strategies that deserve further investigation.