Photosynthetic responses to light in an understory herb, Aster acuminatus

Previous studies of the understory herb, Aster acuminatus, have shown that variation among natural patches in ramet growth, reproduction and population dynamics reflects environmentally induced responses to differences in patch light regimes. This set of experiments was undertaken to determine if photosynthetic responses to variation in light could provide a physiological explanation for the observed differences among patches. Ramets were raised in pots under different light regimes in either the field or growth chambers and then were taken to the lab for analyses of photosynthetic light responses. Ramets acclimated to the light conditions they experienced. Low-light plants had lower maximum photosynthetic rates, light saturation points and light compensation points compared to high-light plants. These differences were more pronounced for growth chamber-grown plants than for those from the field. JDifferences between ramets grown in deep shade and partial sun in the field were greatest early in the season and then became less as photosynthetic rates for all plants declined. Photosynthetic rates were strongly correlated with leaf nitrogen level, and there was no evidence that nitrogen was any more limiting for one treatment compared to others. These results demonstrate a physiological basis for patch differences in growth, reproduction and dynamics. PREVIOUS STUDIES OF the forest understory herb, Aster acuminatus Michx., have demonstrated that light availability on the forest floor is very important in regulating the occurrence of patches and the growth, reproduction and population dynamics of ramets (Pitelka, Stanton, and Peckenham, 1980; Ashmun and Pitelka, 1984; Ashmun, Brown, and Pitelka, 1985; Pitelka, Ashmun, and Brown, 1985). Within single forest stands, patches of A. acuminatus can be found distributed over a wide range of light levels as well as over other environmental gradients. A reciprocal transplant experiment (Ashmun and Pitelka, 1984) and other experimental analyses of growth and reproduction (Pitelka et al., 1985; L. Pitelka, unpubl.) have revealed that variation among local patches in ramet growth, reproduction and dynamics is environmentally induced and is mostly explained by differences in light regime. Normal, natural levels of variation in other environI Received for publication 22 July 1985; revision accepted 31 October 1985. We are grateful to Otto T. Solbrig of Harvard University for providing necessary equipment and space, as well as advice and encouragement. J. W. Ashmun, R. L. Brown and S. B. Hansen assisted at various stages of the work. The work was supported by National Science Foundation grant BSR-7923 148 to LFP. 2 Present address: Electric Power Research Institute, P.O. Box 10412, Palo Alto, CA 94303. 3Present address: Science and Medical Editorial Department, Oxford University Press, 200 Madison Ave., New York, NY 10016. mental parameters have negligible effects. Aster acuminatus appears to be unusual both in terms of the range of light levels over which it can grow and persist, and in the degree to which light level alone explains most patterns of distribution, growth, reproduction and dynamics. In this paper we report results from analyses of the photosynthetic responses of ramets of A. acuminatus to variation in light. Our goal was to determine whether the photosynthetic light response characteristics were consistent with the results of other studies on the species. Specifically, we asked: 1) Is photosynthesis highly sensitive to variation in light? 2) Do ramets acclimate to the light regime they experience in a manner that would be adaptive in the forest understory? 3) Do the photosynthetic characteristics help explain how plants are able to persist in environments receiving anywhere from 1 to 25% of full sun? and 4) How do the photosynthetic patterns vary seasonally with respect to the status of the deciduous canopy trees? Studies of photosynthetic responses to low light have been conducted by a number of investigators (e.g., Bjorkman and Holmgren, 1963; Boardman, 1977; Chabot and Chabot, 1977; Gross and Chabot, 1979; Teramura and Strain, 1979; Young and Smith, 1980; Bazzaz and Carlson, 1982; Curtis, 1984). From these studies and others we understand a great deal about how plants respond to variation in light and how they can adapt to low light. However,