The role of chloroplast movement in C4 photosynthesis: A theoretical analysis using a 3-D reaction-diffusion model for maize

Chloroplast movement within mesophyll (M) cells in C4 plants is
hypothesized to enhance the CO2 concentrating mechanism (CCM), but this is
difficult to verify experimentally. A three-dimensional (3-D) leaf model
can help analyze how chloroplast movement influences the operation of CCM.
The first volumetric reaction-diffusion model of C4 photosynthesis that
incorporates: detailed 3-D leaf anatomy, light propagation, ATP and NADPH
production and CO2, O2 and bicarbonate concentration driven by diffusional
and assimilation/emission processes, was developed and implemented for
maize leaves to simulate various chloroplast movement scenarios within M
cells: the movement of all M chloroplasts towards bundle-sheath (BS) cells
(aggregative movement) and movement of only those of interveinal M cells
towards BS cells (avoidance movement). Light absorbed by bundle-sheath
(BS) chloroplasts relative to M chloroplasts increased in both cases.
Avoidance movement decreased light absorption by M chloroplasts
considerably. Consequently, total ATP and NADPH production and net
photosynthesis rate increased for aggregative movement and decreased for
avoidance movement case compared to the default case of no chloroplast
movement at high light intensities. Leakiness increased in both
chloroplast movement scenarios due to the imbalance in energy production
and demand in M and BS cells. These results suggest the need to design
strategies for coordinated increases in electron transport and Rubisco
activities for an efficient CCM at very high light intensities.