Irena Rajcan

Understanding maize–weed competition: resource competition, light quality and the whole plant

Abstract Although weed research in maize has broadened from an emphasis on herbicide technology to include studies of weed–maize competition, many studies only consider competition descriptively (e.g. defining the critical period for weed control). Furthermore, studies of the mechanisms of weed competition in maize have considered only competition for resources such as soil moisture, nutrients and light. Physiological ecologists have recently recognized the significance of early detection of neighbouring plants through the far-red/red (FR/R) signal as an important mechanism affecting plant–plant interactions. In this review, we have indicated the importance of integrating the concept of the mechanism of early detection of neighbours with the resource-limiting approach in reassessing weed competition in maize during the critical time for weed control. Hypothetical integration of early detection of neighbours into the existing concepts of critical time for weed control and weed thresholds led us to view maize–weed competition as a series of complex processes, which is triggered by the FR/R signal and followed by the development of shade avoidance characteristics accompanied by a reduction in the plant’s ability to absorb nutrients and water, and to photosynthesize. However, due to lack of research on effects of weeds on light quality impinging corn plants as well as corn response to an increase in FR light during critical time for weed control, our conclusions remain to be speculative. We believe that incorporation of early detection of neighbours through the FR/R ratio as a primary signal during the critical period for weed control would open a new approach for future studies on weed competition in maize. We recognize that resource limitation occurs in a maize–weed association, however, this may be more of an effect rather than a cause of competition.

Source : sink ratio and leaf senescence in maize: II. Nitrogen metabolism during grain filling

Leaf senescence in a recent maize (Zea mays L.) hybrid is delayed relative to that in an older maize hybrid and the trait is associated with an improvement of the ratio of assimilate supply (i.e., source) and demand (i.e., sink) during grain filling. This study examined whether effects of source : sink ratio of leaf longevity in an old and more recent hybrid are associated with changes in leaf nitrogen (N) concentration and N uptake during grain filling. A 3-year field study was conducted with maize hybrids Pride 5 (old) and Pioneer 3902 (recent) grown at two soil-N levels: 150 kg ˇ1 Nh a ˇ1 was broadcast in the high N treatment while none was added to the low N treatment. Four imposed source : sink treatments ranged from partial defoliation to no grain. Leaf N of the control treatments did not differ between the two hybrids, but the decline in leaf N from the control to the no-sink treatment was larger for Pioneer 3902 than for Pride 5. Total N uptake in above-ground portions was 10 and 18% greater in the new than in the old hybrid under low and high soil-N conditions, respectively. The difference in the total N uptake between the two hybrids could be attributed to post-silking N uptake. The proportion of N in the grain derived from post-silking N uptake was 60% for Pioneer 3902 and 40% for Pride 5 and this proportion was positively associated with the source : sink ratio. Higher rates of N uptake in Pioneer 3902 vs. Pride 5 appear to be, in part, the result of higher rates of dry matter accumulation of the newer hybrid during grain filling. # 1999 Elsevier Science B.V. All rights reserved.

Red–far-red ratio of reflected light: a hypothesis of why early-season weed control is important in corn

Abstract A plants ability to detect and adjust morphologically to changes in light quality (red–far-red [R:FR] ratio) is one mechanism by which a crop plant responds to weeds. To test this hypothesis, two experiments were conducted where corn was grown in growth cabinets under different light environments. First, to determine the effect of R:FR ratio on corn growth and development, treatments of high R:FR (1.37) and low R:FR (0.67) ratio were compared. These were established by planting corn in pots and then placing trays of either turface (a baked clay medium with high R:FR) or commercial grass sod (low R:FR) on each side of a row of corn pots. Grass sod was used to simulate low-growing weeds. The low R:FR sod treatment resulted in corn plants which were taller, had larger leaves, and greater shoot–root ratio than plants growing in the high R:FR turface treatment. In the second experiment, the effect of R:FR ratio on corn leaf azimuth position was examined. This was accomplished by adding a third treatment where each corn row had sod placed on one side and turface on the other. The proportion of leaves in four azimuthal classes was recorded. In the presence of sod, the proportion of leaves perpendicular to the corn row decreased, and this altered the proportion of leaves in other classes. Therefore, corn seedlings detected changes in light quality caused by the presence of sod (which simulated low-growing weeds) and responded by adjusting carbon allocation and leaf orientation to optimize the interception of light quantity and quality. These results support our hypothesis that low-lying vegetation can alter the growth of corn seedlings before competition for resources occurs. This change in growth may help explain the importance of early-season weed control in corn. Nomenclature: Corn, Zea mays L.

An Integrated Weed Management Strategy for Glufosinate-Resistant Corn (Zea mays)1

Abstract: Early canopy closure and manipulation of crop row spacing or density can reduce the amount and frequency of herbicide use in corn. Field studies were conducted at Woodstock, ON from 1996 to 1999 to evaluate the effect of corn row spacing, plant density, and frequency of glufosinate application on weed biomass and corn yield in glufosinate-resistant corn. Treatments included row width, corn density, and herbicide. The effect of row width and corn density on weed biomass was variable among years. In a wet year (1996), narrow (38 cm) rows provided greater weed suppression than wide (76 cm) rows regardless of crop density. In a dry year (1998), narrow-row high-density (100,000 plants/ha) corn had the lowest weed biomass. In other years, either narrow row or high density was equally successful in suppressing weeds. Effectiveness of herbicides in reducing weed biomass was not influenced by row width or corn density. Corn yield was influenced by row width or corn density. Although weed biomass was lowered by two applications of glufosinate in comparison with a single application, corn grain yields were similar between the two treatments. Planting corn at higher densities may help in reducing early-season weed competition, whereas narrow rows may help in controlling later-emerging species. Nomenclature: Glufosinate; glyphosate; metolachlor; dicamba; corn, Zea mays L. Pride X2650LL. Additional index words: Corn row spacing, corn density. Abbreviations: EPOST, early postemergence; IWM, integrated weed management; LPOST, late postemergence; POST, postemergence.

Effect of temperature and photoperiod on the phenological development of wild mustard (Sinapis arvensis L.)

The development of mechanistic weed models focuses on determining the outcome of weed‐crop interference. Phenological development is a major factor determining the outcome of weed and crop competition. The influence of temperature and photoperiod on phenological development of wild mustard (Sinapis arvensis L.) was studied in growth cabinets. The life cycle of wild mustard was defined in terms of biological days (Bd: chronological days at the optimum photoperiod and temperature). Wild mustard was a long-day species adapted to a wide temperature range of 1.5‐488C. Four phases of development of wild mustard were described: (1) a juvenile phase of 12.7 Bd; (2) a photoperiod-sensitive inductive phase of 6.2 Bd; (3) a photoperiod-sensitive post-inductive phase of 12.8 Bd; (4) a photoperiod-insensitive phase of 33.9 Bd. When effects of photoperiod on rate of development were normalized across phases of development, photoperiod sensitivity did not vary among phases of development. Interpretation of constant sensitivity to photoperiod will simplify simulation of weed phenology in mechanistic models. # 2001 Published by Elsevier Science B.V.