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(American Journal of Botany. 2005;92:456-461.)
© 2005 Botanical Society of America, Inc.

Tropical Biology

Correlation but no causation between leaf nitrogen and maximum assimilation: the role of drought and reproduction in gas exchange in an understory tropical plant Miconia ciliata (Melastomataceae)1

Débora V. Aragão2, Lucas B. Fortini3, Stephen S. Mulkey4, Daniel J. Zarin3,5, Maristela M. Araujo2 and Cláudio J. R. de Carvalho6

2Universidade Federal Rural da Amazônia, Departamento de Ciências Florestais, Av. Presidente Tancredo Neves, n°2501—Bairro Terra Firme—Caixa Postal 917. CEP 66077-530—Belém—Pará—Brazil; 3School of Forest Resources and Conservation, Institute of Food and Agricultural Sciences, University of Florida, P.O. Box 11070, Gainesville, Florida 32611-0760 USA; 4Department of Botany, College of Liberal Arts and Sciences, University of Florida, P.O. Box 118526, Gainesville, Florida 34002-8526 USA; 6Laboratório de Ecofisiologia e Propagação de Plantas, Empresa Brasileira de Pesquisa Agropecuária—Amazônia Oriental, Trav. Dr. Enéas Pinheiro S/N—Bairro Marco—Caixa Postal 48. CEP 66092-100—Belém—Pará—Brazil

Alternative hypotheses were tested to explain a previously reported anomaly in the response of leaf photosynthetic capacity at light saturation (Amax) in Miconia ciliata to dry-season irrigation. The anomaly is characterized by an abrupt increase in leaf Amax for nonirrigated plants at the onset of the rainy season to values that significantly exceeded corresponding measurements for plants that were irrigated during the previous dry season. Hypothesis 1 posits that a pulse in leaf nitrogen increases CO2 assimilation in nonirrigated plants at the onset of the wet season and is dampened for irrigated plants; this hypothesis was rejected because, although a wet-season nitrogen pulse did occur, it was identical for both irrigated and nonirrigated plants and was preceded by the increase in assimilation by nonirrigated plants. Hypothesis 2 posits that a reproduction-related, compensatory photosynthetic response occurs in nonirrigated plants following the onset of the wet season and is dampened in irrigated plants; consistent with hypothesis 2, high maximum assimilation rates for control plants in the wet season were significantly correlated with fruiting and flowering, whereas irrigation caused flowering and fruiting in the dry season, spreading M. ciliata reproductive activity in irrigated plants across the entire year.

Key Words: Amazon • compensatory photosynthesis • drought seasonality • nitrogen content • reproductive phenology • secondary forest






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