Crassulacean Acid Metabolism and Photochemical Efficiency of Photosystem 11 in the Adaxial and Abaxial Parts of the Succulent Leaves of Kalanchoe daigremontiana Grown at Four Photon Flux Densities 1 Klaus Winter

Kalanchod daigremontiana, a species possessing crassulacean acid metabolism, was grown at four photon flux densities (1300, 400, 60, and 25 micromole photons per square meter per second). In leaves which had developed at 1300 and 400 micromole photons per square meter per second, CO2 was mainly incorporated through the lower, shaded leaf surfaces, and the chlorenchyma adjacent to the lower surfaces showed a higher degree of nocturnal acid synthesis than the chlorenchyma adjacent to the upper surfaces. In leaves acclimated to 60 and 25 micromole photons per square meter per second, the gradient in CAM activity was reversed, i.e. more CO2 was taken up through the upper than through the lower surfaces and noctumal acidification was higher in the tissue next to the upper surfaces. Total net carbon gain and total noctumal acid synthesis were highest in leaves which had developed at 400 micromole photons per square meter per second. Chlorophyll content was markedly reduced in leaves which had developed at 1300 micromole photons per square meter per second, especially in the exposed adaxial parts. There was also a sustained reduction in photosystem 11 photochemical efficiency as indicated by measurements of the ratio of variable over maximum chlorophyll a fluorescence. These findings suggest that, at high growth photon flux densities, the reduced activity of the exposed portions of these succulent leaves is caused by (a) the adverse effects of excess light, (b) together with a genotypic component which favors CO2 uptake and acid synthesis in the abaxial (lower) leaf parts even when light is not or only marginally excessive. This latter component is predominant at medium photon flux densities, e.g. at 400 micromole photons per square meter per second. It becomes overridden, however, under conditions of deep shade when strongly reduced light levels in the abaxial parts of the leaf chlorenchyma severely limit photosynthesis. Extensive studies on paradermal sections from mesomorphic, dorsiventral leaves of C3 species have revealed that chloroplasts from the adaxial part ofthe mesophyll may show distinct sun-type characteristics, which change to shade-type characteristics with increasing distance from the adaxial surface (9, 12-16). Gradients in light distribution (5) are particularly steep within the chlorenchyma of the succulent photosynthetic organs of plants exhibiting CAM (17). These plants 'Supported by the Deutsche Forschungsgemeinschaft. occur in both shaded and exposed natural habitats (21), and some species such as Kalanchooe daigremontiana have a large acclimation potential with respect to growth PFD2 (1). In a recent study (18), a marked gradation in the degree of CAM was observed within the chlorenchyma of the amphistomatic leaves of K. daigremontiana. In these experiments, the leaves had developed at about 25% offull sunlight incident on the upper surfaces. Only diffuse light had reached the lower surfaces during growth. Much more CO2 was incorporated via the lower, shaded surfaces, and nocturnal acidification was more pronounced in the chlorenchyma adjacent to the lower than in that adjacent to the upper surfaces. In the study presented here, the extent to which the differential expression ofCAM within the adaxial and abaxial parts of these leaves is determined environmentally was investigated. It is shown (a) that the reduced activity of the exposed upper portion of leaves from plants grown at high light was paralleled by photoinhibitory effects caused by excessive light and (b) that the gradient in the expression ofCAM within the chlorenchyma of these plants was reversed when they were grown under conditions of low light. MATERIALS AND METHODS Plant Material Juvenile plants of Kalanchoe daigremontiana Hamet et Perr. , having two pairs of leaves, were transferred from the glasshouse to a growth cabinet where they were grown in soil for 5 months prior to any experiments. Plants were kept well watered and irrigated with nutrient solution containing 12 mM NO3once every 3rd week. Air temperatures were 23°C during the light period (8:00 AM-8:00 PM) and 16°C during the dark period. The RH was not controlled and ranged between approximately 40% (light period) and 80% (dark period). Light was provided by metal halide lamps (HRI-T 1000 W/D; Radium Elektrizitiits-Ges., Wipperfurth, F.R.G.). Infrared radiation was reduced using 3 mm thick 1 3 Tempax heat reflecting glass (Schott Glaswerke, Mainz, F.R.G.). The spectral distribution of light incident on the plants is shown in Figure 1. Plants were exposed to four different growth 2Abbreviations: PFD, photon flux density (400-700 nm); A, rate of net CO2 assimilation; Fo, instantaneous fluorescence emission; FM, maximum fluorescence emission; Fv, variable fluorescence emission; g, leaf conductance to vapor transfer. 948 www.plantphysiol. org on July 22, 2017 Published by Downloaded from Copyright © 1989 American Society of Plant Biologists. All rights reserved. UPPER AND LOWER LEAF SURFACE IN KALANCHOE PFDs by varying the distance between lamps and plants and by insertion of brass screens to which a coating of carbon had been applied with a candle. Plants were grown at 1300, 400, 60, and 25 ,mol photons m-2 s-'. These PFDs refer to a horizontal orientation of the light sensor at the level of the experimental leaves (= fourth to the sixth leaf pair from bottom of a total of 10 to 12 leaf pairs per plant). Leaf position in K. daigremontiana is decussate, and, to prevent shading, leaves inserted directly above those selected for the experiments were detached soon after initiation. In the plants grown at the two high levels of PFD, the two halves ofthe lamina ofeach leaf, which were separated by the midrib, were folded upward. Thereby the PFD incident on the upper surfaces was reduced and that incident on the lower surfaces increased (Table I). By contrast, in the plants grown at 60 and 25 ,mol photons m'2 s-', the edges of the leaves were curled under. Unless stated otherwise, the growth PFDs referred to in the following text are the levels measured horizontally at the height of the experimental leaves. Gas Exchange Leaves were clamped between a double-sided, gas-exchange chamber made of glass (exposed leaf area of 13.5 cm2) and