  Spectropolarimetry of intact plant leaves allows to probe the molecular
architecture of vegetation photosynthesis in a non-invasive and non-destructive
way and, as such, can offer a wealth of physiological information. In addition
to the molecular signals due to the photosynthetic machinery, the cell
structure and its arrangement within a leaf can create and modify polarization
signals. Using Mueller matrix polarimetry with rotating retarder modulation, we
have visualized spatial variations in polarization in transmission around the
chlorophyll a absorbance band from 650 nm to 710 nm. We show linear and
circular polarization measurements of maple leaves and cultivated maize leaves
and discuss the corresponding Mueller matrices and the Mueller matrix
decompositions, which show distinct features in diattenuation, polarizance,
retardance and depolarization. Importantly, while normal leaf tissue shows a
typical split signal with both a negative and a positive peak in the induced
fractional circular polarization and circular dichroism, the signals close to
the veins only display a negative band. The results are similar to the negative
band as reported earlier for single macrodomains. We discuss the possible role
of the chloroplast orientation around the veins as a cause of this phenomenon.
Systematic artefacts are ruled out as three independent measurements by
different instruments gave similar results. These results provide better
insight into circular polarization measurements on whole leaves and options for
vegetation remote sensing using circular polarization.
