All environmental factors that influence plant growth potentially can affect the ability of weeds and crops to exploit the environmental resources for which plants compete. Stressful levels of environmental factors such as temperature, light, and water and nutrient availability influence weed/crop interactions directly and also may interfere with (or enhance) weed control. Weed and crop species differing in photosynthetic pathway (C3vs C4) are likely to respond differently to many of these factors. Long-term changes in the atmospheric concentrations of CO2and other radiatively-active “greenhouse gases” may exert direct physiological and indirect climatic effects on weed/crop interactions and influence weed management strategies. This review focuses on the effects of temperature, light, soil nutrients, water stress, and CO2concentration on weed/crop interactions with consideration of the potential impact of climate change.

Current and projected increases in the concentrations of CO2and other radiatively-active gases in the Earth's atmosphere lead to concern over possible impacts on agricultural pests. All pests would be affected by the global warming and consequent changes in precipitation, wind patterns, and frequencies of extreme weather events which may accompany the “greenhouse effect.” However, only weeds are likely to respond directly to the increasing CO2concentration. Higher CO2will stimulate photosynthesis and growth in C3weeds and reduce stomatal aperture and increase water use efficiency in both C3and C4weeds. Respiration, and photosynthate composition, concentration, and translocation may be affected. Perennial weeds may become more difficult to control, if increased photosynthesis stimulates greater production of rhizomes and other storage organs. Changes in leaf surface characteristics and excess starch accumulation in the leaves of C3weeds may interfere with herbicidal control. Global warming and other climatic changes will affect the growth, phenology, and geographical distribution of weeds. Aggressive species of tropical and subtropical origins, currently restricted to the southern U.S., may expand northward. Any direct or indirect consequences of the CO2increase that differentially affect the growth or fitness of weeds and crops will alter weed-crop competitive interactions, sometimes to the detriment of the crop and sometimes to its benefit.

Mathematical growth analysis techniques were used to evaluate the effects of CO2concentrations of 350, 600, and 1000 ppm (v/v) on growth and biomass partitioning in corn(Zea maysL. ‘Dekalb XL 395’), itchgrass (Rottboellia exaltataL. f.), soybean [Glycine max(L.) Merr. ‘Tracy’], and velvetleaf (Abutilon theophrastiMedic). Controlled environment chambers with day/night temperatures of 28/22 C and photosynthetic photon flux densities (PPFD) of 650 μE (microeinsteins) m-2s-1were used. Dry matter production in the two C3species (soybean and velvetleaf) was increased significantly by raising the CO2concentration above 350 ppm. In corn (a C4species), dry matter production was least at 1000 ppm CO2and did not differ between the 350 and 600 ppm treatments. In itchgrass (also C4), dry matter production was greatest at 600 ppm CO2and did not differ between the 350 and 1000 ppm treatments. Increasing the CO2concentration increased the rate of dry matter production per unit leaf area (net assimilation rate or NAR) in soybean and velvetleaf but either decreased or did not alter NAR in corn and itchgrass. At 45 days after planting, the weed/crop ratios for total dry matter production for velvetleaf/corn and itchgrass/corn were significantly greater at both 600 and 1000 ppm than at 350 ppm CO2. The weed/crop ratio for itchgrass/soybean was less at 1000 ppm than at 350 or 600 ppm CO2. Compared to the value at 350 ppm, the weed/crop ratio for velvetleaf/soybean was greater at 600 ppm and less at 1000 ppm CO2. We conclude that atmospheric CO2enrichment probably will make weeds with the C3photosynthetic pathway more competitive with crops having the C4pathway. Weeds with the C4pathway may become less competitive with crops having the C3pathway.

The effects of shade on the growth and photosynthetic capacity of the exotic noxious weed itchgrass (Rottboellia exaltataL. f.) were determined under controlled environment conditions. The plants were grown at day/night temperatures of 29/23 C under 100, 60, 25, and 2% sunlight in a climate-controlled greenhouse. Mathematical growth analysis techniques were used to evaluate the effects of shading on dry matter production and leaf area production. Infrared gas analysis and diffusion porometry techniques were used to evaluate the effects of shading on photosynthesis and stomatal resistance. Shading markedly reduced dry matter production. At 40 days after planting, plants grown in 2, 25, and 60% sunlight had 0.3, 16, and 55%, respectively, of the dry weight of the plants grown at 100% sunlight. Leaf area production was less severely retarded by shading; the plants grown at 2, 25, and 60% sunlight had, respectively, 1.7, 42, and 99% of the leaf area of the plants grown at 100% sunlight. Ambient photosynthetic rates of recently expanded, single, fully exposed leaves were 22.5, 51.6, and 65.5 mg CO2dm-2h-1in the 25, 60, and 100% sunlight treatments, respectively. Photosynthetic rates at saturating irradiance did not differ significantly in plants grown at 25, 60, or 100% sunlight and ranged from 76.4 to 78.0 mg CO2dm-2h-1. Stomatal resistances, ranging from 6.0 to 7.5 s cm-1, also did not differ significantly among these plants. In terms of dry matter production, itchgrass is a shade-intolerant plant. However, even when grown in shade, itchgrass maintains the capacity for high photosynthetic rates and high growth rates when subsequently exposed to high irradiance. These characteristics help explain its competitiveness with crop species.

Soybean plants (Glycine max var. Ransom) were grown at light intensities of 850 and 250 5Leinsteins m-2 sec-I of photosynthetically active radiation. A group of plants was shifted from each environment into the other environment 24 hours before the beginning of the experiment. Net photosynthetic rates and stomatdal conductances were measured at 2,000 and 100 zeinsteins m-2 sec-1 photosynthetically active radiation on the 1st, 2nd, and 5th days of the experiment to determine the time course of photosynthetic light adaptation. The (14). Plants were grown in a 1:1 vermiculite-gravel mixture and watered three times/day with modified half-strength Hoagland solution. Growth conditons were 12-hr thermoperiod with day and night temperatures of 26/ 20 C, and a daytime relative humidity of 80%. Full fluorescent and incandescent light was provided 12 hr/day. One hr of incandescent lights (50 ,ueinsteins m-2 sec-1) of photosynthetically active radiation was provided at the beginning and end of the full lighting period, and also in the middle of the night to delay flowering. Plants were grown at a light intensity of 750 ,ueinsteins m-2 sec-' until the first trifoliolate leaf was fully expanded, then 19 plants were exposed to 850 ,ueinsteins m-2 sec-1, and 18 plants exposed to 250 ,ueinsteins m-2 sec-'. These light intensities were achieved by raising plant height, and by shading with plastic screens, respectively. Leaf temperatures of plants in the high light environment were about 1 C higher than in the lower light environment as measured with thermocouples. After 10 days (during which time the second through the fourth trifolio-