Measuring leaf light absorptance is central to many areas of plant biology including photosynthesis and energy balance. Absorptance is calculated from measured values of transmittance and reflectance, and most such measurements have used direct beam light. However, photosynthesis and other processes can differ under direct and diffuse light. Optical properties under diffuse light may be different, but there have been technical difficulties involved in measuring total reflectance of diffuse light. We developed instrumentation to measure this reflectance using a chopped measuring beam delivered alternately to sample and reference integrating spheres, and lock-in detection. We also built instrumentation for measuring transmittance of diffuse light. We developed standards to calibrate our instruments and correct for substitution error, a known systematic error with integrating sphere-based measurements. Helianthus annuus leaves measured under diffuse light reflected 5-10% more and transmitted a few percent less 400-700 nm light than under direct light. Overall absorptance was only a few percent higher under direct light, but leaves may utilize absorbed direct and diffuse light differently. For example, of the light entering the leaf, significantly more direct light than diffuse light is transmitted through the leaf, suggesting differences in localization of absorption within the leaf.

Global-change scenarios suggest a trend of increasing diffuse light due to expected increases in cloud cover. Canopy-level measurements of plant-community photosynthesis under diffuse light show increased productivity attributed to more uniform distribution of light within the forest canopy, yet the effect of the directional quality of light at the leaf level is unknown. Here we show that leaf-level photosynthesis in sun leaves of both C(3) and C(4) plants can be 10-15% higher under direct light compared to equivalent absorbed irradiances of diffuse light. High-light-grown leaves showed significant photosynthetic enhancement in direct light, while shade-adapted leaves showed no preference for direct or diffuse light at any irradiance. High-light-grown leaves with multiple palisade layers may be adapted to better utilize direct than diffuse light, while shade leaf structure does not appear to discriminate light based on its directionality. Based upon our measurements, it appears that leaf-level and canopy-level photosynthetic processes react differently to the directionality of light, and previously observed increases in canopy-level photosynthesis occur even though leaf-level photosynthesis decreases under diffuse light.

Tree saplings, two groups of three species from each of two deciduous tree communities, were grown in competition at three CO2 concentrations and two light levels. After one growing season, biomass was measured to assess the effect of CO2 on community structure, and nitrogen and phosphorus concentrations were measured for leaves, stems, and roots of all trees. Gas-exchange measurements were made on the same species grown under the same CO2 concentrations.

Tree saplings, two groups of three species from each of two deciduous tree communities, were grown in competition at three CO concentrations and two light levels. After one growing season, biomass was measured to assess the effect of CO on community structure, and nitrogen and phosphorus concentrations were measured for leaves, stems, and roots of all trees. Gas-exchange measurements were made on the same species grown under the same CO concentrations.Photosynthetic capacity (rate of photosynthesis at saturating CO and light) tended to decline as CO concentration increased, but differences were not statistically significant. Stomatal conductance declined significantly as CO increased. Nitrogen and phosphorus concentrations generally declined as CO increased, but there were some unexpected patterns in roots and stems. CO concentration did not significantly affect the overall growth of either community after one season, but the relative biomass of each species changed in a complex way, depending on CO light level, and community.

Journal Article A Book by any other Name Get access Biophysical Plant Physiology and Ecology. Nobel Park. W. H. Freeman, Salt Lake City, UT, 1983. 576 pp., illus. $36.95 (cloth). Holly L. Gorton, Holly L. Gorton Biological Sciences Group U-42, University of Connecticut, Storrs, CT 06268 Search for other works by this author on: Oxford Academic Google Scholar William E. Williams William E. Williams Department of Plant Biology, University of Illinois, 505 S. Goodwin Ave. Urbana, IL 61801 Search for other works by this author on: Oxford Academic Google Scholar BioScience, Volume 34, Issue 11, December 1984, Page 718, https://doi.org/10.2307/1309667 Published: 01 December 1984