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A plant's 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.

Summary• We investigated the effect of a herbicide resistance-conferring mutation on fitness in Amaranthus powellii .• Morphological and histological observations were made. Growth and leaf appearance were recorded for six resistant and six susceptible populations. The competitiveness of a susceptible population was compared with that of a resistant population using a replacement series experiment.• Leaves of the resistant plants were distorted and much smaller than those of susceptible plants. Additionally, they exhibited an abnormal morphological and structural pattern consisting of a mosaic of heterogeneous areas in the same leaf blade. The roots and stems had similar structures in susceptible and resistant plants, but the former were up to four times more developed. The resistant plants were slower to develop and produced 67% less biomass and 58% lower leaf area than susceptible plants. Under competitive conditions, one susceptible population outperformed one resistant population by 7-15 times.• The Trp 574 Leu acetohydroxyacid synthase ( AHAS ) mutation appears to have considerable pleiotropic effects on the early growth and development of the plants which, in competitive conditions, greatly reduce fitness.

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ment of pigweed under the maize canopy was attributed to the reduced level of PPFD, a potential light-quality Light quantity (photosynthetic photon flux density, PPFD) and effect on pigweed development in the latter study should quality (red:far-red ratio, R:FR) may affect phenological development of weed species growing under a crop canopy. An indoor study was not be excluded. conducted to quantify the effects of incident PPFD and R:FR on Light transmitted through a crop canopy is enriched development and dry matter accumulation of redroot pigweed (Amain far-red radiation (730-740 nm) and depleted in red ranthus retroflexus L.). Pigweed was grown in growth cabinets from radiation (660-670 nm) due to selective absorptance of the one-leaf stage to the initiation of seed set under three different red light and transmittance and reflectance of far-red PPFD/R:FR treatments: (i) high PPFD (550 mol m Ϫ2 s Ϫ1 ) and high light by green leaves. Consequently, the R:FR ratio R:FR (1.4) (HH), (ii) low PPFD (180 mol m Ϫ2 s Ϫ1 ) and high R:FR (light quality) is reduced from ≈1.2 above the canopy (1.4) (LH), and (iii) low PPFD (180 mol m Ϫ2 s Ϫ1 ) and low R:FR to anywhere between 0.1 and 1.0 under the crop canopy (0.8) (LL). The experiment was undertaken at 12-and 16-h daylengths (Rousseaux et al., 1999). The R:FR ratio is known as with three replications. Rate of leaf appearance (RLA) was accelerphotomorphogenic light, and affects many plant morated with an increase in PPFD (HH vs. LH) at both daylengths. The FR enrichment (LL) negated the effect of low PPFD on RLA under phological characteristics (e.g., stem elongation, branchthe 12-h but not under the 16-h daylength. Low PPFD delayed the ing, apical dominance, and leaf area distribution) (Salisoccurrence of floral primordia, flowering and initiation of seed set. bury and Ross, 1991; Ballaré and Casal, 2000). Most Plant height was a result of the complementary effects of PPFD research into effects of light quality on plants has foand R:FR. Total dry matter accumulation and partitioning, with the cused on morphological changes and dry matter distriexception of dry matter accumulation to the stem, were influenced bution (see Ballaré and Casal, 2000, and references by PPFD only. Results of this study show that both light quality and therein). The few papers that discussed development quantity influence the phenology of pigweed.