Don't limit your background searches to light propagation in vegetables. Similar models are used in neutron transport, gas dynamics, and scattering in random media.
A nice article on the diffusion approximation to the transport
equation by A. Hielscher et al.
On Reserve at Marriott Library:
"Tissue Optics: light scattering methods and instruments for
medical diagnosis" by Valery Tuchin. QH642 .T83 2000
All the basics, but aimed at human tissue.
"Photon-Vegetation Interactions: applications in optical
remote sensing and plant ecology" R.B. Myneni and J. Ross, eds.,
QK757 .P46 1991
Mostly about radiation transport in leaf canopies. Don't
get hung up on those details. Contains some useful information
about scattering specific to plants.
"Photon Propagation in Tissues IV",
David A. Benaron et al, eds., QH642 .P76 1998
"A Monte Carlo Primer" by
S.A. Dupree and S.K. Fraley
QC175.25 R3 D87 2002
Information on Monte Carlo simulation, done mostly with
neutron transport in mind.
On Reserve in the Math Library:
"The Boltzmann Equation and its Applications" by C. Cercignani.
QA1 .A647 v.67
Much more information about Boltzmann transport
theory than you will need.