Ozone Impacts on Competition between Tomato and Yellow Nutsedge: Above‐ and Below‐Ground Effects

Tomato ( Lycopersicon esculentum L.) production in the San Joaquin Valley (SJV) of California is challenged by air pollution and weeds. Differential ozone (O 3 ) tolerance of tomato cultivars and weed species may alter crop–weed competition. A study was conducted in open top chambers (OTCs) at the Kearney Research and Extension Center, Parlier, CA, to assess O 3 impacts on competition between tomato and a C 4 weed, yellow nutsedge ( Cyperus esculentus L.). Processing tomato (cv. HD 8892 and EMP 113) and nutsedge (locally collected biotypes) were grown in pots for 4 to 8 wk. Population ratios ranged from a tomato plant alone (0:1) to a nutsedge plant alone (1:0), and included 1:1, 2:1, and 3:1. Ozone exposures were to 12 h means of 19.8, 78.0, and 142.3 nL/L. Chlorophyll content of leaves of tomato and nutsedge was reduced with increasing O 3 Carbon assimilation was reduced in nutsedge but not in tomato. Root respiration was not affected in either species. Tomato main stem length, shoot, and root biomass declined at the highest O 3 concentration under all levels of nutsedge competition. Nutsedge was much less affected. In the absence of O 3 exposure, interspecific competition (all population ratios combined) reduced tomato and nutsedge shoot and root biomass. Tomato was more sensitive to O 3 than nutsedge, but nutsedge was more sensitive to competition than was tomato. Nutsedge allocated greater resources to reproductive structures (tubers) at the highest O 3 exposure. As nutsedge reduced tomato productivity under low and moderate O 3 concentrations, it may become even more difficult to control, exert greater competitiveness, and colonize fields more rapidly because of greater tuber production, in projected near‐future environments. Under conditions of greatly increasing ambient O 3 concentrations, nutsedge may become less competitive because of its sensitivity to O 3