Seasonal alteration of photosynthetic unit sizes in three herb layer components of a deciduous forest community.

The pigment content and photosynthetic unit size of three herbs on the floor of a mature yellow oak (Quercus meuhlenbergii) forest were measured during most of the growing season. Two species, dutchman's breeches (Dicentra cucullaria) and Virginia bluebell (Mertensia virginica) are spring ephemerals, while Hepatica acutiloba has a semi-evergreen habit. Although pigment composition changed in Dicentra, photosynthetic unit size only varied slightly. Mertensia was nearly invariant in pigment composition, but unit size was modified as the growing season progressed. The semi-evergreen herb, Hepatica, underwent a remarkable alteration in pigment content, photosynthetic unit size, and number following overstory leaf expansion. The results indicate that an alteration of photosynthetic unit size is possible in fully expanded leaves of two forest floor herbs. THE FUNCTIONAL relationship between the light-gathering chlorophyll and the reaction centers of photosynthetic electron transport has been expressed as the photosynthetic unit. The concept of a photosynthetic unit developed from the saturating flash experiments of Emerson and Arnold (1932). The unit size was initially interpreted as the number of chlorophylls functioning in concert for assimilation of a CO2 molecule or evolution of an 02 molecule. Increased resolution of the photosynthetic electron transfer mechanism allowed redefinition of the photosynthetic unit. The unit is now defined as the chlorophyll per P700, the photochemical center of photosystem I or chlorophyll per Q, the immediate acceptor of the system II photoreaction (Bjorkman, 1973). Photosynthetic unit size has been measured in various plant groups (Alberte, McClure, and Thornber, 1976), which include species native to diverse environments (Boardman, 1977) and plants grown under differing regimes of light intensity (Bjorkman, 1973). The values were determined for species which do not experience drastic shifts of quantum dose during the growing season. Alteration in pigment quality and quantity have been related to change in 1 Received for publication 8 May 1979; revision received 21 November 1979. The author wishes to thank the Glen Helen Association and and its director Ralph Ramey for allowing the field portion of the study in the Glen Helen Preserve. The laboratory work was performed at the Charles F. Kettering Research Laboratory in Yellow Springs, Ohio. The aid of Dr. Donald L. Keister, Dr. Berger C. Mayne and Elwood Shaw with the use of equipment and for valuable advice is gratefully acknowledged. 2 C.I.W.-D.P.B. Publication Number 687. light environment (Rabinowitch, 1945; Egle, 1960; Bjorkman and Holmgren, 1963). These shifts in pigment composition suggest a remodeling of photosynthetic unit size dependent upon the radiant environment of the plant leaf. The deciduous forest is typified by radical differences in insolation penetrating to the forest floor as a function of the phenological development of the overstory (Tasker and Smith, 1977). Herb layer species have developed different light utilization strategies in response to overstory cycles. Spring ephemerals take full advantage of the period of high radiant flux preceding overstory leaf expansion (Muller, 1978). Summer green and semi-evergreen species make some use of the high insolation period and remain active through the following period of low radiant flux (Mahall and Bormann, 1978). The herb components of the deciduous forest provide attractive features for the study of dynamic alteration of the pigment composition and photosynthetic unit size. Spring ephemerals which complete the active cycle before overstory leaf emergence could be rather invariant of unit size. Semi-evergreen and summer green herbs which have leaves fully established before canopy leaf expansion might alter either photosynthetic unit size or number after the onset of the summer season of low radiant flux. Pigment composition and photosynthetic unit size were measured in three deciduous forest herbs over the bulk of the growing season to detect alteration in the pigment matrices. MATERIALS AND METHODS-Field site description-The forested area chosen is within the Glen Helen Preserve of Antioch University