Detection and measurement of vegetation photoprotection stress response using PAR reflectance

Optical reflectance from leaf surfaces has been known for decades to increase with decreasing leaf water content. Experimental results show this increase consistently in maize in the visible (photosynthetically active radiation-PAR) and middle infrared (MIR) spectral regions, and with weaker correlation in the near infrared (NIR) region. Changes in chlorophyll concentration have been shown to be too small to substantially contribute to increasing reflectance during the duration of these experiments. Therefore, the reflectance responses to water deficit are perplexing since reflectance in the PAR region is dominated by pigment absorption, while reflectance in the MIR region is dominated by water molecule absorption. We report on recent experiments with maize that indicate that the reflectance changes during water stress in the PAR and MIR regions are due to changing optical absorption, whilst those in the NIR region are due to changing optical scatter. In addition, reflectance in PAR and MIR appears to be influenced by down-welling irradiance. These findings are consistent with the idea that increasing PAR reflectance induced by water stress may result, primarily, from chloroplast avoidance movement, a plant photoprotective response.