Reflectance PRI Does Not Equal Transmittance PRI

Starting from leaf reflectance and transmittance measurements, we calculated two substantially different values of the Photochemical Reflectance Index (PRI) for each of 22 corn leaves having leaf chlorophyll concentrations ranging from high to low. Leaf reflectance and transmittance were measured simultaneously, precluding changes in leaf light use efficiency (LEU) between measurements. We propose the proportion of leaves reflecting v. transmitting direct beam sunlight toward a canopy reflectance sensor is a potential source of variability in estimates of canopy bidirectional PRI.

1. Introduction

In remote sensing, use of the Photochemical Reflectance Index (PRI) provides an evaluation window into the photosynthesis processes that occur inside the leaves of a plant canopy [1]–[14]. PRI is linked to photosynthetic light use efficiency (LUE), which is dry matter produced per unit of absorbed radiation [4]. Photosynthetic LUE varies with leaf environmental conditions, including incident light level and deviations from both optimal temperature and optimal amounts of available water and nutrients. Thus, PRI provides indication of the level of stress in a remotely sensed canopy due, for example, to environmental conditions that are nonoptimal for plant growth such as too much or too little water, nutrients or incident sunlight [1], [2], [9].