A cytophotometric technique for measuring photosynthetic potential of leaves: preliminary research and development

A novel combined photographic and cytophotometric technique provides information on the physiological state of leaves and a permanent record of each leaf measured in field studies. The method has the ability to determine the distribution of photosynthetic potential within various portions of the leaf and to scale up to the canopy level as well as down to the cellular level. Photographic transparencies are taken in the field, then brought back to the lab to be analyzed cytophotometrically at the investigator's convenience. The image encoded on the film yields an absorption curve that is similar to that of intact leaves with peaks in the blue (450 nm) and red (660 nm) for paper birch. Internal standards of fluorescein and first surface mirrors combined with standardized magnification and illumination (e.g., ring flash) are used to insure precision and accuracy. Two wavelength and plug cytophotometric equations have been modified for use in this technique. Some problems with the two-wavelength method remain, but the usefulness of the plug method for cytophotometry has been expanded through the use of portable leaf area and chlorophyll meters and portable photosynthesis laboratories. Total photosynthetic potential (TPP) is shown to equal leaf area multiplied by the mean optical density of the leaf. With the use of internal standards TPP can be expressed in fluorescein units or adjusted by optical density of the image of the first surface mirror.