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In arid areas drought conditions and warmer temperatures will alter the competitive balance between crops and some weed species. The objective of this study was to study water competition and its effect on canopy relationship of a C4 weed (pigweed) and a C3 weed (bindweed) towards a C3 crop (pepper) in a Mediterranean area. The experiment was carried out in 2008 in Matera, Southern Italy. Pigweed and bindweed were studied within a naturally occurring weed population in a bell pepper field where a rainfed treatment (V0) was compared to a full irrigated one (V100), the latter corresponding to the restoration of 100% of the maximum crop evapotranspiration, (ETc). Soil water content was measured periodically; leaf water potential, net assimilation rate (A), stomatal conductance (gs), transpiration rate (T), Ci (intercellular CO2 concentration) and A/Ci curves were also determined on pigweed, bindweed and pepper leaves. All gas exchange parameters differed between irrigated and rainfed treatments and between the three species.Water use efficiency was higher in pigweed than in pepper and bindweed. Between the considered weeds, pigweed competed for water with pepper significantly since, unlike bindweed, pigweed began to reduce stomatal conductance only when its leaf water potential achieved very negative values, lower than -2.00 MPa. Unlike C4 crops already saturated for CO2, pigweed photosynthesis is not completely saturated for CO2. Consequently, since atmospheric CO2 is increasing, when pigweed is grown in mixed stands where competition occurs, it can further limit other slow-growing species, both crops and weeds.
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