Plant and Soil 202: 201–209, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 201 Can plants track changes in nutrient availability via changes in biomass partitioning? K.D.M. McConnaughay 1 and J.S. Coleman 2,3 1 Department of Biology, Bradley University, Peoria, IL 61625, USA ∗ and 2 Department of Biology, Syracuse University, Syracuse, NY 13244, USA. 3 Present address: Biological Sciences Center, Desert Research Institute, Reno, NV 89506, USA Received 15 September 1997. Accepted in revised form 5 May 1998 Key words: allocation, allometry, biomass partitioning, old-field annuals, ontogeny, plant growth Abstract Can old-field annuals exposed to temporally varying nutrient regimes adjust biomass partitioning programs in order to maximize growth? We grew three species of old-field annuals, Abutilon theophrasti, Chenopodium album, and Polygonum pensylvanicum, at low or high nutrient levels, and switched a subset of plants to the alternate nutrient regime after one or two weeks of growth. If plants were able to partition biomass in an optimal fashion, it would be predicted that changes in growth would be accompanied by changes in biomass partitioning programs. We found that when nutrient availability changes (e.g., from low to high) early in ontogeny, growth and partitioning to leaf area development are adjusted to be indistinguishable from those of plants grown at constant nutrient availabilities (e.g., always high). Root shoot partitioning, however, was developmentally fixed in two of the three species such that nutrient environment had no effect on root/shoot partitioning. Thus, although fluctuations in nutrient availability altered plant growth, the observed changes in growth occurred without concomitant adjustments to the root/ shoot partitioning program. These results imply that adjustments in allocation of biomass resources, at least between roots and shoots, are not necessary to effect alterations in plant growth in variable environments. Introduction Many plant growth models assume that changes in the partitioning of biomass among leaves, storage or- gans, support structures and roots drive changes in plant growth in response to fluctuating resource lev- els (e.g. Hilbert, 1990; Luo et al., 1994; Sharpe and Rykiel, 1991). For example, optimal partitioning mod- els predict that plants respond to resource variation by partitioning biomass in order to optimize resource capture, and thus growth, in a given environment (e.g., Bloom et al., 1985; Dewar, 1993; Hilbert, 1990; Hirose, 1987; Levin et al., 1989; Robinson, 1986; Szaniawski, 1987). Implicit in these models is that: (1) biomass partitioning programs determine resource capture and subsequently plant growth, and (2) ad- justments in biomass partitioning can happen continu- ∗ FAX No: (309) 677 3558. E-mail: kdm@bradley.edu ously throughout the vegetative life of a plant. While there is a lot of evidence that plants do respond to spe- cific resource environments, particularly with respect to nutrient availability, with concomitant changes in partitioning and growth (e.g., Mooney and Winner, 1991; Reynolds and D’Antonio, 1996), it is not clear whether changes in partitioning drive changes in plant growth, or whether plasticity in plant growth, cou- pled with the fact that partitioning of biomass among organs changes normally as a function of increas- ing plant size, drives apparent plasticity in biomass partitioning programs (Coleman and McConnaughay, 1995; Coleman et al., 1993, 1994). Our previous work suggested that both of these as- sumptions may not always apply (Gedroc et al., 1996). Increased nutrient availability at three weeks of growth resulted in substantially increased plant growth for two annual plant species, whereas decreased nutrient availability at three weeks resulted in substantially de-