ASSESSMENT OF PHOTOSYNTHETIC RADIATION-USE EFFICIENCY WITH SPECTRAL REFLECTANCE

Reflectance changes at 531 nm, associated with the zeaxanthin-antheraxanthin-violaxanthin interconversion and the related thylakoid energization, are widespread among plant species. We evaluated an index based on 531 nm reflectance ('PRI', Photochemical Reflectance Index, calculated as (R(531)-R(570))/(R(531)+R(570))) as an indicator of efficiency of photosynthetic radiation use in seven species representing both C-3 and CAM photosynthetic pathways. Leaves exposed to a dark-light-dark transition in a steady-state laboratory gas exchange system exhibited nearly parallel changes in PRI and PS II quantum yield (Delta F/F-m'). Similar PRI and Delta F/F-m' responses were seen in leaves exposed to diurnally changing sunlight levels outdoors. PRI was linearly related to Delta F/F-m', and both Delta F/F-m' and PRI were exponentially related to instantaneous efficiency of photosynthetic radiation-use in different species over a range of different field conditions. These results extend previous studies by indicating a functional relationship between PRI, Delta F/F-m', and photosynthetic radiation-use efficiency. The narrow-band PRI index offers a simple, portable means of assessing PS II radiation-use efficiency, analogous to Delta F/F-m', and with the potential for remote applications at scales larger than the leaf.