Linking light capture with CO2 fixation: new methods enable integration from the chloroplast to the leaf.

Abstract

Photosynthetic properties of a leaf are determined by the profiles of light absorption and photosynthetic capacity through the leaf. The aim was to measure these two profiles and incorporate them into a model. We used a new method to directly measure light absorption through intact leaves, which previously has not been possible. A transversely cut leaf was mounted vertically on an inverted microscope, which captured an image of chlorophyll fluorescence from the cut surface. Light could be applied to either epidermis or to the transverse surface to excite fluorescence, which represented either light absorption or chlorophyll content, respectively. Light absorption peaked between 50 and 100 m m into the mesophyll and then declined rapidly for blue light or more slowly for green light. Profiles of chlorophyll measured from the fluorescence image using epi-illumination were qualitatively similar to those from paradermal sectioning. By analysing the profile of light absorption with respect to chlorophyll, we found that light absorption could be approximated by the Beer-Bouguer law. The apparent extinction coefficient varied depending on the wavelength of light and mesophyll anatomy. To complement the optical approach, profiles of 14CO2 fixation were measured by paradermally sectioning labelled leaves. The profile of photosynthetic capacity was assumed to equal the pattern of 14CO2 fixation through the leaf when light was applied to the transverse surface of the leaf. Photosynthetic capacity closely matched green light absorption. There was good agreement between the model and observed 14CO2 fixation profiles measured under a range of conditions.