Document: WIL-3-29-6

Physiological and morphological limitations to growth: A study of three Asters.

PFITSCH, W.A.*, P.BUDIHARDJO and J.A.EVANS

Hamilton College, Clinton, NY USA ¹

Abstract:
Our objective was to determine whether three Aster species that differ in distribution from open-field to closed-forest habitats have different inherent relative growth rates (RGR), and if so whether the causes are physiological (photosynthetic or nutrient uptake rates) or morphological (biomass allocation to leaves and roots). We estimated adult plant RGR by repeated leaf and stem measurements on plants transplanted in pots in two shrubby old field sites. Seedling RGR and biomass allocation were estimated by harvesting individuals grown outside under shade cloth or in a growth chamber. We found that when grown under these moderately high light levels (10 to 15 mol photons m^-2 d^-1) both adults and seedlings of the shade tolerant A. divericatus had lower RGR than two old field species, A. prenanthoides and A. novae-angliae. There were no differences among species, however, in maximum photosynthetic rates of leaves from adult plants (A_max averaged (15 to 18 mol m^-2 s^-1). Adult stems of A. divericatusproduced smaller leaves and grew more slowly as the growing season progressed, while the field species had less seasonal decline in growth rate as their leaves increased in size higher on the stem. Morphological constraint, therefore, appears responsible for species differences in RGR of adult plants. By contrast, seedling A_max of the forest species (7 to 10 mol m^-2 s^-1) was significantly lower than the field species (15 mol m^-2 s^-1). The higher seedling RGRs of the field species were associated with greater allocation to roots and less allocation to leaf mass and area. Morphological and physiological differences in nutrient uptake, therefore, appear responsible for species differences in A_max and RGR of seedlings. It is unwise to assume that causes of differences in RGR remain constant as plants develop.

Keywords: shade tolerance, relative growth rate, biomass allocation, nutrient uptake, photosynthesis

This abstract is being presented at: 10:30 AM in session:
Poster Session #1: Light Relations.