Irradiance and water deficit effects on gas exchange behavior of two C3 Amazonian weeds.

The gas exchange behavior or two perennial weed species, a vine Ipomoea asarifolia (Desr.) Roem. & Schultz and a shrub Stachytarpheta cayennensis (L.C. Rich) Vahl., both common in the Brazilian Amazonia, was compared for plants grown under well watered and water limited conditions. Under well-watered conditions, light saturated rates of photosynthesis (A) and stomatal conductance (g) in I. asarifolia were, respectively, 15 and 83% higher than in S. cayennensis. When water stress was imposed, A and g in I. asarifolia declined, respectively, by 29 and 57% in relation to S. cayennensis. Dark respiration and light compensation point were similar on well-watered plants. Under water limited conditions, S. cayennensis had 60% higher dark respiration and 35% higherlight compensation point than I. asarifolia. The observed responses are in accord with the known distribution of the species in the field and may provide important information for designing differential management strategies for controlling these weeds