LEAF OPTICAL PROPERTIES OF RAINFOREST SUN AND EXTREME SHADE PLANTS

The optical properties of the leaves of twelve tropical sun species and thirteen tropical extreme shade species were examined with an integrating sphere attached to a spectroradiometer. Measurements of diffuse reflectance and transmittance allowed calculations of absorptance, 3501,100 nm. Although some shade species absorbed higher percentages of quantum flux densities for photosynthesis (400-700 nm, PPFD) than the mean for the sun species, the sun and shade species as groups were not significantly different from each other: 90.2, S.D. 3.6% for shade species and 88.6, S.D. 2.4% for the sun species. The groups of species did not differ in total absorptance of energy 350-1,100 nm. Furthermore, the sun and shade species were identical in their shift of absorptance at wavelengths between 650 and 750 nm. The anthocyanic coloration of the leaf undersurfaces of two species polymorphic for this characteristic (Trionela hirsuta and Ischnosciphon pruinosus) is correlated with increased absorptance at the upper end of the action spectrum of photosynthesis. Although sun and shade species have similar optical properties, the energy investment (as documented by dry wt per unit area of leaf surface) is much less for the shade species. LEAF OPTICAL PROPERTIES are frequently neglected in studies on photosynthesis or plant productivity, yet it is only possible to determine the efficiency of these processes when the amount of radiation usable in photosynthesis (photosynthetic photon flux density = PPFD) absorbed by the leaf is known. Studies on leaf optical properties go back over fifty years (Willstatter and Stoll, 1918; Shull, 1929; Seybold, 1933; Billings and Morris, 1951; Loomis, 1965). Much of the impetus in studying leaf optics has been its relevance to remote sensing (Gausman, Escobar and Rodriguez, 1981; Smith, 1981), with a continuing interest in the effect of leaf surface features, as pubescence, on the absorptance of radiation. Several studies (Ehleringer, Bj6rkman and Mooney, 1976; Eller and Willi, 1981; Ehleringer, 1981) have demonstrated that surface features can significantly reduce absorptance of radiant energy, reduce leaf temperatures, and thus reduce water loss from transpiration. Other than the work of Billings and Morris (1951) along an elevation gradient in the Sierra Nevada and that of Ehleringer (1981) in the Mohave and Sonoran deserts, there have been no comparisons of leaf optics from plants from different ecosystemsand none of tropical forest plants-despite the relevance of such properties for photosyntheI Received for publication 15 May 1985; revision accepted 31 January 1986. This research was supported by a grant from the Whitehall Foundation. Ms. Sara Tarsis and Mr. Anthony Man Son-Hing provided technical assistance. sis, and energy and water relations (Mooney et al., 1980). In this study we compared the optical properties of leaves from extreme shade and sun-adapted plants of tropical forests to determine if shade-adapted plants absorb more energy, and if differences in absorptance occur at specific wavelengths. Although all leaves have similar optical properties (Gates et al., 1965; Loomis, 1965; Wooley, 1971; Gausman and Allen, 1973; Ehleringer, 1981; Fig. 1), it is important to consider the precise optical properties of leaves in relationship to the light environments in which the plants live. Because of the selective filtering oflight through foliage, plants ofextreme shade encounter a drastically different light environment than those to full sunlight (Chazdon and Fetcher, 1984a, b; Lee, 1986a). Full sunlight near solar zenith on a clear day has a PPFD in excess of 2,200 Iu mol s-5 m-2 and a total irradiance of over 1,000 w m-2 (Fig. 2). Because wavelengths of radiant energy in the red and far-red wavelengths affect the equilibria of phytochrome, which may have profound effects on plant development (Smith, 1981, 1982), it is useful to describe the spectral distribution of this radiation in terms of the quantum ratio at 660 compared to 730 nm (the red: far-red ratio or R:FR in the sense of Smith, 1982). For sunlight the R:FR is approximately 1.25 (Tasker and Smith, 1977; Holmes, 1981; Lee, 1986a; Fig. 2). In contrast, the light climates of rainforest understory are deficient in total radiant energy, especially in PPFD, and have a much lower R:FR (Lee, 1986a; Fig. 2).