Measuring incident light on grape clusters using photosensitive paper and image analysis

Digital imaging and analysis was used to quantify and characterize the light exposure patterns of photosensitive paper tubes placed in representative cluster positions in two grape (Vitis vinifera L.) canopies: a minimally pruned and a vertically trained canopy. Blue pixel values of the captured images had a negative correlation with the log of irradiance from an integrating quantum sensor (r2 = 0.9308). The spectral response of the photosensitive paper was not measured. Histograms of incident light distribution on individual paper tubes were developed using imaging software. Histograms were able to quantify the distribution of incident light on individual tubes and were clearly related to the tube's exposure in the canopy. Average population curves of pixel light distribution of 20 tubes in each canopy were able to differentiate the typical cluster light environment in the two canopies. Tubes in the minimally pruned canopy had a larger proportion of their surface exposed to irradiances > 50 micromoles.s-1 m-2 and 65% higher average irradiance than the vertical canopy. Image analysis of photosensitive paper appears to be a workable method to record the distribution of incident light in plant canopies and may have utility in a range of ecological studies