Vertical profile of chlorophyll in maize canopy: Technique, quantification, and implications for remote sensing

The production of dry matter by pasture and crop species has been demonstrated to be ultimately limited by the amount of chlorophyll (Chl) due to the strong relationship of this pigment content with the photosynthetic processes. The vertical profile of Chl content in the canopy is closely related to the amount of photosynthetic light absorbed by each leaf layer and thus affects the total canopy photosynthesis. Therefore, chlorophyll content has the potential to be used as an objective indicator of crop "vigor" since it is a proxy of the actual photosynthetic capacity of a canopy. However, there is still a lack of accurate, rapid, and practical methodologies available to quantify total Chl content in the canopy. The objectives of this study were (a) to develop a technique to quantify chlorophyll in maize canopy based on leaf Chl content (b) to characterize the vertical profile of Chl content in maize canopy, and (c) to understand the implication of the vertical profile of Chl in maize canopy for the remote estimation of canopy chlorophyll content. Three experimental sites were used for this study on fields under irrigated and rainfed maize hybrids in eastern Nebraska. Chl in the canopy was measured non-destructively proximately and remotely weekly during the growing season using a hyperspectral sensor in the range 400 nm - 800 nm. A rapid and non-destructive technique based on Chl retrieval from leaf reflectance at two wavelengths was calibrated and validated to quantify Chl in maize leaves. Canopy Chl content was accurately estimated from Chl content in a single leaf: the collar (i.e. the most recently fully expanded leaf) or the ear leaf during the vegetative or reproductive periods, respectively. In reproductive and senescence stages, Chl content was more sensitive than LAI and green LAI to changes in the "vigor" of the crop; for the same LAI, Chl content might varies more than two-folds. A spectral vegetation index, computed from remotely measured reflectance, was closely related to total canopy Chl content. It was shown that this index was affected by the reflectance of the upper seven leaves of the canopy.