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 leaflevel photosynthesis in sun leaves of both C3 and C4 plants can be 10-15% higher under direct light compared to equivalent absorbed irradiances of diffuse light. High-lightgrown leaves showed significant photosynthetic enhancement in direct light, while shade-adapted leaves showed no preference for direct or diffuse light at any irradiance. Highlight-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.

We used the Mixfa+ and mixfa-lines of

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 CO 2 from the intercellular air spaces to the chloroplasts, thus reducing CO 2 limitation of photosynthesis. To test this hypothesis, we used pulsed photoacoustics to measure oxygen diffusion times as a proxy for CO 2 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 ϫ 10 Ϫ6 cm 2 s Ϫ1 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 CO 2 estimated from oxygen permeability was in good agreement with published values, validating photoacoustics as another way of assessing internal resistances to CO 2 diffusion.Light-mediated chloroplast movements in the leaves of some plants are so striking that they create patterns visible to the naked eye. They have attracted the attention of plant physiologists for more than a century. Chloroplast movements of all types have been the subject of numerous reviews (Britz, 1979; Haupt and Scheuerlein, 1990; Wada et al., 1993; Yatsuhashi, 1996; Haupt, 1999; Wada and Kagawa, 2001; Kagawa and Wada, 2002). In leaves, chloroplasts spread along the periclinal walls of mesophyll cells (the face position) in low light, whereas in high light, they move toward the anticlinal walls (the profile position), effectively forming cylinders of chloroplasts in palisade tissue. In darkness, the chloroplasts are generally in an intermediate position, although this varies among species (Inoue and Shibata, 1974) and depends on the growth environment (Trojan and Gabrys, 1996). Because of the optical sieve effect (Britz and Briggs, 1987), these marked chloroplast movements alter light absorption (Zurzycki, 1961), which, in many leaves, gives rise to the visible color changes that have attracted attention for so long.Chloroplast movements are widespread in algae, mosses, ferns, and seed plants. Among seed plants, they are common in both monocots and dicots, and they occur in plants with widely differing leaf anatomy, from submerged aquatic plants (Zurzycki and Lelatko, 1969) to sclerophyllous evergreens (Del Hierro et al., 2000). Chl...

Corn seedlings were grown in white light in the absence and presence of the chlorosis-inducing herbicide San 9789. The resulting green and achlorophyllous seedlings were used to investigate phytochrome-mediated responses to end-of-day far red irradiation and reversal of these responses by subsequent red irradiation. Mesocotyl and coleoptile elongation increased in response to end-of-day far red irradiation, whereas the anthocyanin content of the coleoptiles was decreased. All three responses were reversible by red irradiation following the far red. Dose-response curves for far red induction and red reversal of these responses did not differ significantly for plants grown in the presence or absence of San 9789. Thus, San 9789