def psri(b3, b4, b6):
    """
    Plant Senescence Reflectance Index (Merzlyak et al., 1999).

    .. math:: PSRI = (b4 - b3)/b6

    :param b3: Green.
    :type b3: numpy.ndarray or float
    :param b4: Red.
    :type b4: numpy.ndarray or float
    :param b6: Red-edge 2.
    :type b6: numpy.ndarray or float

    :returns PSRI: Index value

    .. Tip::

       Merzlyak, M.N.; Gitelson, A.A.; Chivkunova, O.B.; Rakitin, V.Y. 1999. \
       Non-destructive optical detection of pigment changes during leaf \
       senescence and fruit ripening. Physiologia Plantarum 106, 135-141. \
       doi:10.1034/j.1399-3054.1999.106119.x.
    """

    PSRI = (b4 - b3)/b6
    return PSRI