Photoacoustic Analysis Indicates That Chloroplast Movement Does Not Alter Liquid-Phase CO2 Diffusion in Leaves of Alocasia brisbanensis

Abstract

Abstract Light-mediated chloroplast movements are common in plants. When leaves of Alocasia brisbanensis (F.M. Bailey) Domin are exposed to dim light, mesophyll chloroplasts spread along the periclinal walls normal to the light, maximizing absorbance. Under high light, the chloroplasts move to anticlinal walls. It has been proposed that movement to the high-light position shortens the diffusion path for CO2 from the intercellular air spaces to the chloroplasts, thus reducing CO2 limitation of photosynthesis. To test this hypothesis, we used pulsed photoacoustics to measure oxygen diffusion times as a proxy for CO2 diffusion in leaf cells. We found no evidence that chloroplast movement to the high-light position enhanced gas diffusion. Times for oxygen diffusion were not shorter in leaves pretreated with white light, which induced chloroplast movement to the high-light position, compared with leaves pretreated with 500 to 700 nm light, which did not induce movement. From the oxygen diffusion time and the diffusion distance from chloroplasts to the intercellular gas space, we calculated an oxygen permeability of 2.25 × 10–  6 cm2 s–  1 for leaf cells at 20°C. When leaf temperature was varied from 5°C to 40°C, the permeability for oxygen increased between 5°C and 20°C but changed little between 20°C and 40°C, indicating changes in viscosity or other physical parameters of leaf cells above 20°C. Resistance for CO2 estimated from oxygen permeability was in good agreement with published values, validating photoacoustics as another way of assessing internal resistances to CO2 diffusion.