In wetlands, dormancy may be a key functional trait enabling seeds to avoid underwater germination, which could be lethal for seedling establishment. Our objectives were to find out (i) if shallow dormant (i.e. conditionally dormant) Echinochloa crus-galli seeds from an anaerobic germination resistant accession can break dormancy under hypoxic submergence and (ii) if underwater germination can be restored in scarified, non-dormant seeds. Shallow dormant E. crus-galli seeds perceived diurnally alternating temperatures (AT) and red light (R) pulses (i.e. dormancy-breaking cues) under hypoxic submergence; however, an inhibitory far-red light pulse given at the end of the 4-d inundation period demonstrated that most of the seeds (85%) were unable to break dormancy. Scarified E. crus-galli seeds, which did not express dormancy under drained conditions, were unable to germinate under hypoxic submergence, despite being exposed to dormancy-breaking cues (AT + R). Lastly, the temporal window for germination sensitivity to the inhibitory action of hypoxia, once dormancy-breaking signals have been applied, is progressively lost and bounded to approximately 18 h for half of the seed lot. These results highlight the importance of dormancy as a trait enabling E. crus-galli seeds to avoid underwater germination, a risky scenario for seedling emergence and establishment in this facultative hydrophyte.

Trials were carried out to determine the degree of light transmittance through soil and its role in photo-induced germination of buried weed seeds. Transmittance was found to be strongly dependent on soil type and decreased with decreasing particle size and with an increase in darker components. In each soil examined, light penetration fell below 0.01% at a depth of no more than 4 mm. It was also noted that with increasing soil depth, light permeability was proportional to wavelength, leading to progressive decline in the red-far red ratio. Low light penetration played a crucial role in germination of jimsonweed seeds, which had became light-sensitive after prolonged burial. Although this effect is a function of soil type, it may represent an important ecological factor influencing seed emergence in the first few mm of upper soil layers.

This research was conducted to evaluate the light requirement for redroot and smooth pigweed germination in soil, and how this requirement is affected by germination temperature and seasonal periodicity in seed dormancy. Seed enclosed in nylon mesh bags was buried in the field in December 1993 and 1994 and was recovered throughout the spring and summer of the following year, respectively. Germination was highest with red light or at 30 C. The requirement for red light was more pronounced at 20 vs. 30 C. The saturating fluence of red light was as low as 3 μmol m−2 in buried seed and 1,000 μmol m−2 in unburied control seed, depending on germination temperature. The effect of light and germination temperature pigweed germination also changed throughout the growing season. Our results indicate that light may be a requirement for germination in only the most dormant weed seed in the soil seedbank.

Investigations were conducted to determine whether light, temperature, or their interaction stimulated primary basal-bulb formation on underground stems of yellow nutsedge (Cyperus esculentus L.) seedlings grown from tubers. Basal-bulb formation results when internodes shorten and leaves lengthen. When seedlings were grown without medium around the underground shoots, a temperature alternation of 10C stimulated basal-bulb formation; light did not affect the process. When seedlings were grown with medium around the underground shoots, either light or alternating temperature stimulated basal-bulb formation. Nine colors of light gave the same stimulus as white (fluorescent) light. Phytochrome did not appear to be the photoreceptor for the stimulus, as both red and far-red light responses were identical to that of white light.

Perception of light by phytochrome is a mechanism that triggers weed seed germination in response to soil disturbance. Photoconversion of phytochrome from the red light absorbing form to the active far-red absorbing form depends on hydration of phytochrome. This research was conducted to determine the soil water threshold for the photoinduction of germination by the brief exposure of light that occurs during soil disturbance, and to determine how this threshold is affected by the fluence of the light stimulus and fluence sensitivity of the seed population. Redroot pigweed seedling emergence and germination response to red light (R) was studied for a range of water potentials. Water potential gradients were established by incubating seeds in soils wetted to various water contents, or in polyethylene glycol 8000 (PEG) solutions. After imposing the light treatments, seeds were returned to a fully hydrated state. Seedling emergence in response to R increased as the volumetric water content (θv) of soils increased. At volumetric water contents of 4.0%, R-induced seedling emergence was inhibited 50% compared to photoinduced seedling emergence at the highest soil water contents tested. Attenuation of photoinduction was more pronounced at low vs. high R fluences in freshly imbibed seeds, but was unaffected in seeds that exhibited enhanced fluence sensitivity. In ecosystems where seasonal soil moisture extremes are prevalent, the photoinduction of seed germination may be limited in dry microsites such as surface crusts or under extreme drought conditions.

White light-grown seedlings of smallseed dodder were (a) provided with unilateral far-red (700 to 800 nm) at photon irradiances ranging from 20 to 110 μmol m−2 s−1 against a background of cool white light (400 to 700 nm) from above at 77 μmol m−2 s−1, or (b) transferred to darkness and provided with unilateral white light at 20 μmol m−2 s−1, unilateral blue light (400 to 500 nm) at 10 μmol m−2 s−1, unilateral red light (600 to 700 nm) at 10 μmol m−2 s−1, unilateral far-red at 50 μmol m−2 s−1, or (c) in experiments utilizing bilateral irradiations, provided with unilateral far-red perpendicular to unilateral white light. Positive phototropic curvature was induced by unilateral white light and by unilateral blue light in otherwise darkness and by unilateral far-red in a background of cool white light. Seedling vines were also phototropic toward unilateral far-red when provided with unilateral white light perpendicular to unilateral far-red. Phototropism to unilateral white light was inhibited in seedlings treated with 200 μM norflurazon and 50 mM potassium iodide. Norflurazon- and potassium iodide-treated seedlings remained phototropic toward unilateral far-red when provided with unilateral white light perpendicular to unilateral far-red. Seedling vines were not phototropic to unilateral red or to unilateral far-red in otherwise darkness, and seedlings in cool white light were neither skototropic (i.e., tropic toward unilateral darkness) nor tropic to or from infra-red (radiation with wavelengths greater than 900 nm). Phototropism toward regions of lowered red:far-red may aid smallseed dodder in chlorophyllous host location and attachment.

This research was conducted to determine the environmental conditions necessary for development and expression of very low fluence (VLFR) germination sensitivity, and to determine the roles of type I phytochrome, gibberellic acid (GA) biosynthesis, and nitrate in this process. Redroot pigweed seed was subjected to pregermination incubation regimes of 5, 12, and 23 C for up to 21 d in a −1.2 MPa polyethylene glycol solution in the dark. Development of VLFR sensitivity was most pronounced at 23 C, and induction of germination by the lowest fluence tested (3 μmol m–2) was greater at a germination temperature of 30 than 20 C. In dormant smooth pigweed seed, development of VLFR sensitivity occurred only when seed was subjected to a prechilling treatment to overcome primary dormancy. The instability of the fluence response of seed subjected to pregermination conditions favorable for development of VLFR was consistent with type I phytochrome accumulation and degradation after photoconversion. In redroot pigweed seed, inhibition of GA biosynthesis during pregermination incubation resulted in the partial attenuation of VLFR sensitivity. The addition of 75 ppm nitrate to pregermination and germination media enhanced VLFR sensitivity. Our results suggest that development of VLFR sensitivity is due to cooperative action between type I phytochrome and phytochrome-independent GA biosynthesis. In the soil seedbank, this sensitivity may be augmented in elevated soil nitrate environments.

Laboratory studies were conducted to determine the effect of various factors on germination of dogfennel and yankeeweed. Dogfennel seed were found to be strongly photoblastic with no germination in the dark. Yankeeweed seed were moderately photoblastic, with 12% germination occurring in the absence of light Germination for both species increased in response to red light (650 nm), indicating phytochrome regulation. At the soil surface, dogfennel and yankeeweed emergence was 40 and 48%, respectively, but declined rapidly with increasing soil depths where dogfennel germination was lower than that of yankeeweed. Yankeeweed germination was 60 to 75% from 10 to 30 C while dogfennel germination was 45 to 70% between 15 to 30 C. Both species germinated over a broad range of pH (6 to 10) with the highest germination occurring at pH 8. Yankeeweed and dogfennel were moderately tolerant to water stress, but yankeeweed tolerated higher water stress than dogfennel. Both species germinated over a wide range of conditions, possibly enhancing adaptation to many diverse environments.

Substances having anesthetic-like properties were examined for stimulation of redroot pigweed and witchgrass seed germination. Chemicals included the n-alcohols through C-5, 2-propanol, benzyl alcohol, diethyl ether, chloroform, and 2-bromo-2-chloro-1,1,1-trifluoroethanol. Redroot pigweed seeds required the far-red absorbing form of phytochrome (Pfr) for an anesthetic substance to increase germination, but in witchgrass seeds the active anesthetic substances stimulated germination without added Pfr. The correlation coefficient comparing relative activity (RA) versus the membrane/buffer partition coefficient (M/B) of the active substances was very poor (−0.37) for pigweed seeds but was −0.91 for witchgrass. Leakage of cellular materials, including electrolytes, substances absorbing at 280 nm, and amino acids from seeds treated with either active or inactive anesthetics, was not particularly indicative of effects on germination in both species. Application of increased pressure (0.965 MPa) during anesthetic treatment, which in animals prevents anesthesia, was found to act oppositely and increase seed response to ethanol in redroot pigweed seeds, but in witchgrass seeds pressure suppressed the stimulatory action of ethanol. In both species, pressure appeared to have an action of its own, but for this action to be expressed the presence of a stimulatory anesthetic was also required. The results do not appear to support a good relationship of anesthetic activity with seed membrane lipophilic components in redroot pigweed seeds, but the relationship seems likely for witchgrass seeds.

Weed seeds can require an exposure to light for induction of germination. Conducting tillage operations at night and thus preventing the photoinduction of germination has been proposed as a means to reduce weed emergence in agricultural systems. This research was conducted to evaluate night tillage as a weed management option and to determine which tillage operations have the greatest effect on light-mediated recruitment. Weed emergence was evaluated after conducting factorial combinations of day and night moldboard plowing and disking in the springtime from 1992 through 1995. The light environment during disking generally had a slightly greater effect on emergence than the light environment during plowing. Emergence of pigweed species and giant foxtail was, at most, 30 to 55% higher following day vs. night disking. Emergence of other weeds was not affected by the light environment during tillage. We conclude that night tillage may not be a viable approach to weed management due to insufficient reductions in weed emergence associated with night tillage and the high degree of variability in the recruitment response to light conditions during tillage.

The purpose of this study was to investigate the role of sunlight perceived by weed seeds during and after soil cultivation with moldboard and chisel plows on induction of seed germination. Daytime cultivation with a moldboard plow increased weed seedling emergence by as much as 200% above the levels recorded following nighttime cultivation; however, the magnitude of this promotion varied depending on season and field history, and occasionally it was nil. In contrast, when a chisel plow was used in primary tillage, no differences in seedling emergence were observed between daytime and nighttime cultivation. The absolute germination level in plots cultivated with a chisel plow was at least 200% greater than in plots cultivated with a moldboard plow. This observation suggests that microenvironmental factors other than light played an important role in promoting seed germination in chisel-plowed plots. Covering moldboard-plowed plots with opaque plastic immediately after cultivation did not reduce weed seedling emergence compared to plots that were either covered with transparent film or left uncovered. Our results suggest that only the light stimulus perceived by the seeds during soil cultivation is effective in triggering germination.

The utility, advantages, and disadvantages of several spectrofluorometric and in vivo spectrophotometric methods in plant science are reviewed, with emphasis on their use in weed science. Examples of the use of in vivo, dual-wavelength spectrophotometry to probe the effects of herbicides on cytochrome f oxidation/reduction, the P515 chromatic shift, phytochrome synthesis, and other processes are discussed. Use of in vivo spectrophotometry to examine effects of herbicides on phytylation of chlorophyllide, protochlorophyllide photoconversion, porphyrin accumulation, and other processes that result in spectral changes is discussed and examples are provided. Spectrofluorometric methods for pigment identification and quantification and for enzyme assays are illustrated with examples. Spectrofluorometric methods used in conjunction with high-pressure liquid chromatography (HPLC) and thin-layer chromatography (TLC) are discussed.

Caryopses (seeds) of fall panicum (Panicum dichotomiflorum Michx.) have a dormancy that is not easily overcome. Although accelerated after-ripening for 4 weeks was partially successful in overcoming dormancy, a dark imbibition at 35 C for about a week was more effective and also interacted strongly with an accelerated after-ripening. In either case, activation of phytochrome by red irradiation was required to induce germination. Similarly, alternating temperatures allowed germination of red-treated seeds but constant temperatures did not. More seeds germinated in sealed flasks than in dishes. The enhancement of germination in sealed flasks appeared to be related to increased CO2 levels. The data suggested a multiple dormancy in fall panicum seeds. One dormancy, the most intense, was relieved by high-temperature imbibition. The residual dormancy was relieved by active phytochrome and alternating temperatures.

Laboratory studies were conducted to determine the effect of various factors on germination of tropical soda apple. Tropical soda apple seed were found to be moderately photoblastic with 30% germination occurring in the dark. Germination increased in response to green (545 nm) and red light (650 nm) to 75 and 66%, respectively, indicating phytochrome regulation. No germination occurred in response to blue (450 nm) and far-red light (750 nm). Germination increased from 4 to 64% between 10 to 30 C, but no germination was found at 5 and 40 C. Maximum germination occurred at 30 C. A second degree polynomial (Y= −48.7 + 7.9X – 0.2X2, R2 = 0.76) best fit the data. Seed germinated over a broad range of pH (2 to 14) with the highest germination occurring at pH 8 and no germination occurring at pH 2 and 14 (Y = −63.3 + 30.5X – 1.9X2, R2 = 0.95). Seed germination declined as osmotic potential increased from 0 to −1.0 MPa (Y = 76.7 – 12.9X + 5.2X2, R2 = 0.87). Seedling emergence was maximum from depth of 3 to 6 cm, but no seedling emergence occurred when seed were planted below 12 cm (Y = 10.2 + 17.8X – 1.6X2, R2 = 0.93). Optimum germination occurred at a depth of 5.6 cm. Mechanical and sulfuric acid scarification increased the rate of germination but not the overall percentage. Tap water or hot water increased the rate of germination 26% and KNO3, GA3, or ethephon increased it 53%. Tropical soda apple seed germinated in response to variable environmental and edaphic conditions which could allow establishment in diverse ecosystems.

The conditions causing the onset of secondary dormancy in seed of winter oilseed rape cultivars Falcon and Libravo were investigated in a series of laboratory experiments to provide practical guidance on how to minimize the persistence of volunteer oilseed rape seed. The germination of recently harvested oilseed rape seed in moist conditions at 12 C was close to 100% in both darkness and under white light. Imbibition in water stress with an osmotic potential (ψ) of −1,500 kPa at 12 C in far-red light (FR) or dark for more than 5 d induced secondary dormancy in both cultivars. Treatment with FR was more effective than darkness at inducing secondary dormancy. Libravo produced more dormant seed than Falcon. Imbibition in FR and absence of water stress (ψ = 0 kPa) decreased germinability especially at the low temperature (6 C). These conditions resulted in 19.4 and 27.2% dormant seed for Libravo and Falcon, respectively. The results suggest that phytochrome is involved in the induction of secondary dormancy in oilseed rape. In practice, much oilseed rape seed is buried by postharvest cultivation in late summer (low hydric conditions) and exposed to water stress and darkness for some time. To avoid soil seed persistence and subsequent volunteer oilseed rape, seed must be retained at the soil surface when there is adequate moisture available. Therefore, incorporation of seed by tillage should be avoided or delayed as long as possible.

Germination of seeds of 10 grass and 33 broadleaved weed species was examined for response to ethylene. Germination was promoted in nine species, inhibited in two, and not affected in the remainder. Of the species promoted, common purslane (Portulaca oleracea L.), common lambsquarters (Chenopodium album L.), and several Amaranths, including redroot pigweed (Amaranthus retroflexus L.), were affected most. Transformation of phytochrome to the active form (Pfr) gave interactions that ranged from none to syntergistic with the applied ethylene. In subsequent tests seeds of purslane, redroot pigweed, and giant foxtail (Setaria faberi Herrm.), a species not responsive to ethylene, were examined for germination response to 14 low molecular weight hydrocarbon gases other than ethylene. Some stimulation by the olefins propylene and propadiene was found for purslane and pigweed. Propionaldehyde and butyraldehyde were slightly stimulatory to purslane only.

The effects of temperature and light on the dormancy of velvetleaf, common waterhemp, and giant foxtail seeds were studied under controlled growth chamber conditions. Seeds were either kept chilled at 4 C for 12 wk under wet conditions or nonchilled at 4 C in dry storage. Then, seeds were germinated under increasing and decreasing temperatures and under continuous red light (R) and far-red light (FR). In addition, chilled and nonchilled seeds were germinated in the dark after being exposed to alternating R and FR flashes. Velvetleaf germination was increased by exposure to high temperatures (36 C) immediately after exposure to low temperatures (4 C), but light had no effect. Chilling increased common waterhemp seed germination and sensitivity to light and temperature. R promoted common waterhemp seed germination, whereas FR inhibited germination and maintained dormancy. In addition, the effect of light was reversible. Therefore, common waterhemp dormancy was phytochrome regulated. However, high temperatures (36 C) promoted the germination of chilled seeds, even when exposed to FR. The germination of chilled giant foxtail seeds was reduced by FR. Giant foxtail seed dormancy was partially phytochrome regulated, but dormancy regulation was more dependent on mean temperature.

Increased reflection of far-red light among plants can lower the red:far-red ratio (R:FR) of horizontally-propagated light before substantial shading among plants occurs. Although previous research suggests that altered R:FR can affect corn growth, it is not understood how early-season low R:FR (associated with higher plant densities in corn–weed communities than in weed-free corn) affects corn productivity in the field. We conducted field experiments from 2005 to 2007 to determine the effects of reduced R:FR on corn growth and yield. Corn was established at 53,800 plants ha−1 for a control R:FR treatment (weed-free corn) and at 107,600 plants ha−1 for a low R:FR treatment (simulated corn–weed community). The low R:FR treatment was thinned to the control plant density at V7 corn (seven fully-expanded leaf collars), which simulated total weed removal. Before thinning, R:FR (645:735 nm) at V7 corn in the low R:FR treatment (0.23) was less than 50% of that in the control R:FR treatment (0.49) across years. In 2005, stalk diameter, stem length, stem mass, leaf mass, specific leaf area, and total plant mass were less for V5 corn in the low R:FR treatment than in the control R:FR treatment. In 2006, V5 corn was taller in the low R:FR treatment than in the control R:FR treatment, but other morphological characteristics were similar between treatments. The shoot:root ratio of V5 (five fully-expanded leaf collars) (2005) and V7 corn (2006) was not affected by R:FR treatment, nor was shoot mass and grain yield of R6 corn (physiological maturity). Measurements in 2006 indicated that soil moisture and nutrient availability were not limiting and did not differ between treatments. Early-season R:FR in simulated corn–weed communities was similar to that in in situ corn–weed communities. These results suggest that low R:FR typical of early-season corn–weed communities was not associated with corn grain yield loss under field conditions in which water and nutrients were not limiting resources.

The dormancy pattern of buried seeds of Datura ferox was studied by testing germination responses to red light (R), far-red light (FR) or darkness after exhumation at regular time intervals. The seeds were deeply dormant at the time of burial in winter. After three months (spring), exhumed seeds began to respond to an R pulse. Summer temperatures did not induce secondary dormancy. During winter of the second year, seed germination after an R pulse increased further; however, germination after an FR pulse or in darkness also increased and reached values close to R. Changes in seed germination after an R or an FR pulse and in darkness were studied in a factorial design experiment, combining different depths of burial (0.5, 5 and 10 cm), presence of vegetation (with or without vegetation canopies), and date of exhumation (4 or 7 months after burial). The seeds placed at 0.5 cm depth did not germinate in any light treatment. A very high sensitivity to light, indicated by the promotion of germination by an FR pulse compared to darkness, was exhibited only in seeds buried at 5 cm in a soil without vegetation. A typical R/FR reversible response was observed in seeds exhumed at 5 and 10 cm in almost all situations of burial; except in a soil without vegetation, at 10 cm depth, in late summer when even dark controls showed high germination. It is concluded that buried seeds of D. ferox are released from dormancy in two steps, the first step during the spring of the first year and the second step during the subsequent winter. This process is not obviously controlled by summer temperatures. The acquisition of a very high sensitivity to light depends on vegetation cover and depth of burial, and thus it is predicted to be affected by agricultural practices.