IRRADIANCE AND WATER-DEFICIT EFFECTS ON GAS-EXCHANGE BEHAVIOR OF 2 C-3 AMAZONIAN WEEDS

The gas exchange behavior of two perennial weed species, a vine Ipomoe a asarifolia (Desr.) Roem. and Schultz and a shrub Stachytarpheta caye nnensis (L.C. Rich) Vahl., both common in the Brazilian Amazonia, was compared for plants grown under well watered and water limited conditi ons. Under well-watered conditions, light saturated rates of photosynt hesis (A) and stomatal conductance (g) in I. asarifolia were, respecti vely, 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 compensati on point were similar on well-watered plants. Under water limited cond itions, S. cayennensis had 60% higher dark respiration and 35% higher light compensation point than I. asarifolia. The observed responses ar e in accord with the known distribution of the species in the field an d may provide important information for designing differential managem ent strategies for controlling these weeds.