Performance of Unidirectional View Digital Photography to Retrieve Crop LAI Using Greencroptracker Software.

The optimal acquisition of LAI using non-destructive commercial instruments needs to be made under diffuse light conditions. This requirement limits the operational data acquisition and the extent of spatial sampling to evaluate in situ crop growth variability. However, this constraint disappears when a standard digital camera is used to quantify canopy structural variables. When the camera is equipped with a fisheye lens, it captures gap fraction over a range of zenith angles, permitting simultaneous determination of several canopy structural descriptors, such as LAI, mean leaf inclination angle and foliage clumping. However, digital photography taken at a restricted field of view provides better spatial resolution than the corresponding hemispherical photos and thus would improve the differentiation of green vegetation tissues from others and from soil. Moreover, it is well known that foliage projection, and thus LAI measurement, is insensitive to leaf angle distribution at the 57.5° view angle. In this study, the performance of unidirectional view photography using standard digital camera was evaluated against destructive measurements to quantify green LAI and green plant area index (GAI) at nadir and with a slant angle of 57.5° over corn, wheat and soybean canopies. A histogram-based threshold method was developed to differentiate gaps from plant tissues, in order to derive structural descriptors. The image analysis method was implemented in GreenCropTracker software (download for free at www.flintbox.com/public/project/5470/) to process unidirectional view digital colour photos taken of agricultural crops. The software can be used to process digital photos taken upward and downward looking, at a vertical direction or with a slant angle of 57.5°.The GAI derived for corn, soybean and wheat crops was strongly linearly correlated with the destructive GAI for both the nadir and the 57.5° photographic methods (R² > 0.83, RMSE < 0.63). Because the destructive LAI and GAI exhibited strong linear correlations for soybean and corn, LAI could be predicted from photographic methods. The LAI and GAI of the wheat canopy were poorly correlated because of a significant proportion of non-leaf tissues contributing to photosynthesis after the start of stem elongation. Clumping had a greater effect on the nadir photographic method than on the 57.5° method. Both photographic methods were able to provide satisfactory measurement of the total GAI, although the 57.5° photographic method underestimated it by 7% and the nadir photographic method overestimated it by 25%. This approach is complementary to the conventional instrumentation, when intensive in situ field campaigns are carried out, especially during the early crop growth stages.