Judith M. Bradow

Volatile seed germination inhibitors from plant residues.

Volatile emissions from residues of the winter cover legumes, Berseem clover (Trifolium alexandrinum L.), hairy vetch [Vicia hirsuta (L.) S.F. Gray], and crimson clover (Trifolium incarnatum L.), inhibited germination and seedling development of onion, carrot, and tomato. Using GC-MS, 31 C2-C10 hydrocarbons, alcohols, aldehydes, ketones, esters, furans, and monoterpenes were identified in these residue emission mixtures. Mixtures of similar compounds were found in the volatiles released by herbicide-treated aerial and root residues of purple nutsedge (Cyperus rotundus L.) and the late-season woody stems and roots of cotton (Gossypium hirsutum L.). Vapor-phase onion, carrot, and tomato seed germination bioassays were used to determine the time- and concentration-dependent inhibition potential of 33 compounds that were either identified in the plant residue emissions or were structurally similar to identified compounds. Cumulative results of the bioassays showed that (E)-2-hexenal was the most inhibitory volatile tested, followed by nonanal, 3-methylbutanal, and ethyl 2-methylbutyrate. All the volatile mixtures examined contained at least one compound that greatly inhibited seed germination.

Inhibition and promotion of germination by several sesquiterpenes

The sesquiterpene lactones, burrodin, confertiflorin. desacetyl-confertiflorin, dihydroparthenolide, parthenin, and 7α-hydroxy-3-desoxyzaluzanin C, and the sesquiterpene ester guayulin A were assayed at concentrations of 1, 10, and 100 μM for effects on seed germination of 16 dicot and nine monocot species. Six of the dicot and two of the monocot species were affected by one or more of these compounds. Germination was both inhibited and promoted, depending on the compound and the specific species or cultivars, at concentrations as low as 1 μM. For example, guayulin A, which promoted the germination of lettuce at all concentrations tested, inhibited the germination of tomato. Confertiflorin stimulated germination of the lettuce cultivar Grand Rapids at 1 μM, but inhibited germination of a light-sensitive cultivar at all concentrations tested.

Quantitation of cotton fibre-quality variations arising from boll and plant growth environments

Abstract Crop growth simulation models used to manage cultural inputs and to improve yields of cotton, Gossypium hirsutum L., do not address fibre quality, a major determinant of cotton fibre price and end-use. Fibre maturation simulations require rapid, reproducible methods for fibre quality quantitation at the boll or locule level. Combination of fibre quality mapping by fruiting site with quality quantitation by an electron-optical particle sizer provided replicated, reproducible data suitable for use in predictive models and quantitative studies of fibre quality variations attributable to genotype and growth environment. The efficacy and potential of this unique fusion of agronomic and textile technologies were examined through comparisons of three 1992 fibre quality database subsets from the US Southeastern Coastal Plain and Mississippi Delta. Comparisons of ‘Pee Dee 3’ fibre quality, on a locule-by-locule basis at positions 1 and 2 on main-stem nodes 5 through 18, revealed that fibre length, cross-sectional area, and physical maturity varied among fruiting sites. Subsurface microirrigation applied during an early-season drought increased fibre yield by 40%, significantly increased fibre fineness, and decreased fibre maturity indicators. Fibre length variations were compared between ginning methods and among nine genotypes grown in the Coastal Plain. Irrigation-related reductions in physical fibre maturity, found in the Coastal Plain, were contrasted with chronological maturities of ‘DPL5415’ and ‘DES119’ fibre harvested 21, 28, 35, 42, or 56 days post-anthesis in the Mississippi Delta. Fibre-quality mapping with particle-sizing represents a powerful, new tool for constructing fibre development simulations essential for improving cotton fibre quality and processing outcome.

Relationships between chemical structure and inhibitory activity of C6 through C9 volatiles emitted by plant residues

Leaf, stem, flower, fruit, and root residues of a wide variety of plants have been reported to emit bioactiven-alkanes, 2-alkanols,n-alkanals, 2-alkenals, 2-alkanones, andn-alkanoic acids containing from six to nine carbon atoms. During a 72-hr exposure to the vapor phase of these compounds (6.9, 20.8 or 34.4μ M/liter), germination of onion, carrot, and tomato seeds was inhibited to varying degrees. The extent of inhibition caused by these plant residue volatiles depended upon the compound type and concentration, carbon-chain length, and the seed species tested. Tomato seeds recovered more fully from exposure to these volatile inhibitors than did those of carrot and onion. Degree of recovery in all three species depended on the type and concentration of volatile present. The C7 and C8 compounds were the most inhibitory among these 24 volatiles. Of the six classes of chemicals examined, the 2-alkenals were the most inhibitory, followed by the 2-alkanols,n-alkanals, and 2-alkanones, which were equally effective as seed germination inhibitors. The straight-chain alkanes and alkanoic acids were relatively noninhibitory. Tests of a C7 and C9 alkadienal indicated that the C7 compound was the more inhibitory.

Germination stimulation in wild oats (Avena fatua L.) by synthetic strigol analogs and gibberellic acid.

At concentrations of 0.01–1 mM, five synthetic multiring analogs of strigol were effective germination stimulants of intact and dehulled wild oat (Avena fatua L.) seeds. The effect was concentration-dependent and equaled or exceeded that produced by equimolar gibberellic acid (GA3). The most effective strigol analog treatments induced 55–80% germination within 7 days in intact wild oat seeds and resulted in 63–86% germination and normal seedling growth over 14 days. Intact wild oat controls germinated 14% after 14 days. The stimulation of wild oat germination by these synthetic strigol analogs demonstrates that these compounds, initially developed as germination stimulants for the seeds of the parasitic weed, witchweed (Striga asiatica L. Kuntz.), have bioregulatory activity in dormant seeds of monocots, as well as dicots. None of the compounds tested significantly affected the germination of nondormant cultivated oat seeds (Avena sativa L.). The commonly used dispersal agent, Tween 20 (0.1%), was found to inhibit germination of cultivated oats, alone and in the presence of 2% acetone.

Identification of volatile allelochemicals fromAmaranthus palmeri S. Wats.

Allelopathic volatiles associated with the weed Palmer amaranth (Amaranthus palmeri S. Wats.; AMAPA) were trapped on Tenax GC, thermally desorbed, and identified by gas chromatography-mass spectroscopy. Methyl ketones and alcohols (C4–C11) were the principal components of the volatiles mixture. Seedheads, stems, or roots were placed in a glass container and incubated at 31 °C (10 hr)/21 °C (14 hr) for three days prior to trapping the volatiles. Seedheads were rich in 2-heptanone which was consistently found, together with 2-heptanol, in all AMAPA tissues. Vapors of authentic 2-heptanone and (±)-2-heptanol at concentrations of 1 ppm or higher strongly inhibited the germination of onion and carrot and almost completely suppressed the germination of tomato and AMAPA seeds.

Comparison of the seed germination effects of synthetic analogs of strigol gibberellic acid, cytokinins, and other plant growth regulators

Four synthetic multiring analogs of strigol, a naturally occurring sesquiterpene lactone that promotes germination of dormant seeds ofStriga (witchweed), were found to stimulate germination of dormantLactuca (lettuce) seeds. The effects on light-sensitive and light-insensitive lettuce seeds were concentration-dependent and exceeded those produced by equimolar (0.1 mM) solutions of gibberellic acid. Strigol and epistrigol promoted lettuce seed germination to a lesser degree than did the synthetic analogs. The strigol group compounds had minimal effect on the germination of monocot seeds. The results indicate that the synthetic strigol analogs have plant growth regulatory activity in dormant seeds of genera beyondStriga in which germination stimulation by strigol and the synthetic analogs was first demonstrated.

Applications of AFIS fineness and maturity module and x-ray fluorescence spectroscopy in fiber maturity evaluation

Worldwide round-testing and calibration of the nep-counting and fiber-length mod ules of the Zellweger Uster advanced fiber information system (AFIS) are well advanced. Lack of appropriate quantitative calibration standards for fiber maturity has limited similar development of the prototypic AFIS fineness and maturity (F&M) module. A combination of calcium x-ray fluorescence Spectroscopy (Ca-XRF) and AFIS-F&M map ping of fiber quality from twenty-one days post anthesis (DPA) to boll opening (56 DPA) permits direct comparisons of AFIS-determined fiber physical maturity (as micronAFIS, circularity, and cross-sectional area) with fiber chronological maturity (as DPA) and physiochemical maturity (as Ca-XRF). The AFIS-F&M module is a powerful tool that makes possible quantitative comparisons of fiber maturity across time (during fiber development and in different crop years), across space (different boll and locule positions and different growing areas), within single bolls and locules, and between cotton varieties and species.

Allelochemicals from palmer amaranth,Amaranthus palmeri S. Wats

The presence of Palmer amaranth (AMAPA) residues in the soil reduced fresh weight accumulation in onions and carrots and markedly decreased seedling field establishment of carrots. Solid-phase separation techniques were used to isolate fractions containing water-soluble organic compounds from AMAPA residues and soil amended with such residues. At concentrations of 20–100 mg/liter most of the organic solids thus extracted were inactive in seed germination assays using onion, carrot, AMAPA, and tomato seeds. Extracts from the roots of AMAPA increased 72-hr germination percentages in carrot, AMAPA, and tomato. A time-study of AMAPA residue decomposition in soil showed an increase in extractable inhibitors of onion germination after 62 days but no other significant changes in the activity. The most active allelochemicals from AMAPA proved to be volatile compounds. Volatiles emitted by soil containing AMAPA residues and by the dried and partially rehydrated leaf and flower residues themselves reduced carrot and tomato seed germination to less than 7%. Freshly harvested aerial AMAPA inhibited only carrot seed. Germination of AMAPA and carrot seeds was retarded by exposure to volatiles from dried AMAPA residues. Residues from AMAPA grown in Texas and Louisiana exhibited comparable inhibitory activity after air-drying two weeks. Onion seeds were also inhibited by volatiles from AMAPA residues.

Germination and Seedling Development

The responses of cotton (Gossypium spp.) seeds to the germination environment depend upon (1) the point in the germination-through-emergence sequence at which conditions cease to promote germination and seedling development, (2) the magnitude and duration of the deviations from conditions promotive of germination, and (3) seedling development ‘success’ potentials determined by the genetic and seed vigor of a particular cotton seed lot and genotype. Suboptimal environmental factors, both abiotic and biotic, modulate, delay, or terminate cotton seed germination and seedling development during any of the four universal phases of seed germination: (1) imbibition, (2) mobilization of seed reserves (cotyledonary lipids and proteins in cotton), (3) radicle protrusion and elongation through resumption of cell division, and (4) hypocotyls and cotyledon emergence above the soil with the shift from metabolic dependence on seed storage compounds to photosynthetic autotrophy. In cotton and other oilseeds, cotyledonary lipid mobilization depends upon subcellular organelle-cooperativity and membrane-transport phenomena elucidated as the gluconeogenic glyoxylate cycle of oilseed species.