Remote sensing of solar-excited plant fluorescence as a measure of photosynthetic rate

Leaf level net photosynthetic rates (P_N) of laurel oak (Quercus hemispherica) juveniles grown under contrasting nutrient and CO₂ regimes were negatively correlated with red to far-red ratios, R/FR (690/760 nm), steady-state, solar-excited fluorescence ratios (r² = 0.66, n = 12) measured across 12 plant canopies. Laurel oak juveniles that had been subjected to nitrogen stress over a period of a year demonstrated higher R/FR than their counterparts that had been provided with sufficient nitrogen. Plants that had been grown at elevated CO₂ concentrations, EC [700 μmol(CO₂) mol^-1] also exhibited significantly higher R/FR when subjected to normal ambient carbon dioxide concentrations than their counterparts grown under ambient concentrations, AC [380 μmol(CO₂) mol^-1]. All fluorescence measurements were obtained by observing a multi-plant canopy using a unique solar-blind passive sensor. This sensor, which utilizes Fraunhofer-line discrimination techniques, detects radiation at the cores of the lines comprising the atmospheric oxygen A- and B-bands, centered at 762 and 688 nm, respectively. These results support the use of solar-excited steady-state plant fluorescence as a potential tool for remote measurement of canopy radiation use efficiency.