A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Photosynthesis light curves of sun and shade plants of transitional tropical forest(cerradão) in Mato GrossoClóvis Lasta Fritzen - UFMS - Campus de Corumbá; DEX - Departamento de CiênciasExatas Av. Rio Branco, N. 1270 Cx. Postal 252 Corumbá – MS CEP: 79304-020(clovislf@bol.com.br)Eduardo Jacusiel Miranda, José Holanda Campelo Jr., José de Souza Nogueira, NicolauPriante Filho -Universidade Federal de Mato Grosso - Depto. de Física - Grupo deFísica e Meio Ambiente - Av. Fernando Correa da Costa s/n, 78060-900 -Cuiabá -MTBrasil.George Louis Vourlitis (georgev@csusm.edu) Biological Sciences Program- CaliforniaState University- San Marcos, CA 92096-0001, USAThe photosynthesis light response curves for several sun and shade plants ofQuiina pteridophylla and D. exelcia were measured during portions of the wet and dryseasons in the transitional tropical forest of northern Mato Grosso. Although thephotosynthesis curves of sun and shade plants show the same qualitative trend, plantsgrowing in full sun had a higher rate of light-saturated photosynthesis (P max ) than theplants growing in shade. In measurements made during the peak of the dry season(July), Q. pteridohylla plants growing in full sun had a P max value of on average of 5µmol m -2 s -1 , while plants growing in shade had a P max value of around 4 µmol m -2 s -1 .During the wet season (December), Q. pteridohylla plants growing in full sun had aP max value of 9 µmol m -2 s -1 while shade plants had a P max of 7 µmol m -2 s -1 . Similarly,measurements during the wet season of D. exelcia individuals growing in full sun hadP max values of on average 13 µmol m -2 s -1 , while individuals growing in shade had P maxvalues of 10 µmol m -2 s -1 . During the dry season, however, P max values wereapproximately 40% lower for plants growing in sun and shade, indicating that seasonaldeclines in precipitation led to corresponding declines in P max . Plants growing in sunalso exhibited dark respiration rates that were approximately 1.5 times higher thanplants growing in shade. These data indicate that spatial variations in sun exposure toplants growing in the sub-canopy of transitional tropical forests have importantimplications for the maximum rates of leaf photosynthesis, and these differences appearto be consistent over seasonal variations in rainfall. Our data also indicate that theseasonal variation in rainfall also cause substantial variation in the rate of maximumphotosynthesis, as there is a significant increase in the photosynthesis for all the plantsas the water availability increases.