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Environmental control of dormancy in weed seed banks in soil

The species composition and density of weed seed in the soil vary greatly and are closely linked to the cropping history of the land. Altering tillage practices changes weed seed depth in the soil, which plays a role in weed species shifts and affects efficacy of control practices. Crop rotation and weed control practices also affect the weed seedbank. Information on the influence of cropping practices on the weed seedbank should be a useful tool for integrated weed management. Decision aid models use information on the weed seedbank to estimate weed populations, crop yield loss, and recommend weed control tactics. Understanding the light requirements of weed seed may provide new approaches to weed management. Improving and applying our understanding of weed seedbank dynamics is essential to developing improved weed management systems. The principles of plant ecology must be integrated with the science of weed management to develop strategies that take advantage of basic plant responses in weed management systems for agronomic crops.

Implications of weed seedbank dynamics to weed management

Temperature is a major factor determining the natural distribution of plants, and the success and timing of agricultural crops. Habitats occupied by plants show dramatic differences in temperature during the period of active growth, ranging from just above freezing in polar or alpine areas to over 50 °C in the hottest deserts. Moreover, in many habitats the same individual plant is subjected to wide seasonal variations in temperature and even diurnal temperature fluctuations may be considerable.

Responses of macrophytes to temperature

Summary Arabidopsis thaliana was transformed with the codA gene for choline oxidase from Arthrobacter globiformis under control of the 35S RNA promoter of cauliflower mosaic virus. As a result, high levels of glycinebetaine accumulated in the seeds of transformed plants. Transformation with the codA gene significantly enhanced the tolerance to high temperatures during the imbibition and germination of seeds, as well as during growth of young seedlings. The extent of enhancement of the tolerance to high temperature was correlated with levels of choline oxidase expressed and of glycinebetine accumulated in the transformed plants. The induction of homologues of heat shock protein 70 at high temperature was less conspicuous in the transformed plants than in the wild-type plants, suggesting that the transformation alleviated the hightemperature stress.

Enhancement of the tolerance of Arabidopsis to high temperatures by genetic engineering of the synthesis of glycinebetaine

INTRODUCTION 331 OCCURRENCE OF DORMANCY 332 STRUCTURE AND ORGANIZATION 332 Seed Coat Efficts 332 G en eral and Mitoch ondrial Activity .... ... ... ... . .. .. ... ........ 333 Nu cleic Acids and Prot ein Syntheses . . . 334 HORMONE AND PHYTOCHROME ACTION . . . . . . . . . .. .. . . . . . . . . .. . . . . ..... . .. . . . .. .. .. . .. .. . .. .. . . . . .. .. . . 336 Abs cisic and Gibb er el ic Acids ... .. 336 Cyto kinins and Ethylene .... .... 339 Phytochrome Action ...... 341 METABOLIC REGULATION 342 APPLIED COMPOUNDS .. . . . . . . . . . .. . .. . . . . . . . . ... .... 345 TEMPERATURE EFFECTS 346 CONCLUSION 347

Dormancy in Seeds

Leakages of amino acids and/or fluorescent material as functions of temperature between 15 and 40 C are reported for imbibed seeds of Avena fatua L., Lactuca sativa L., Barbarea vulgaris R. Br., Amaranthus albus L., Abutilon theophrasti Medic., Lychnis alba Mill., Daucus carota L., Setaria faberi Herrm., Setaria viridis (L.) Beauv., and Datura stramonium L. The leakage indicates prominent increase in permeability of the plasmalemma in the 30 to 35 C range for 8 of the 10 kinds of seeds studied. Germination of the seeds at constant temperatures or with daily shifts in temperature is related to the membrane transition temperature for permeation by amino acids. Seeds of A. albus and A. theophrasti, which did not show membrane changes in the 25 to 40 C range, germinated best at 35 to 40 C; the other seeds germinated best below 30 C. Seeds of B. vulgaris showed rapid permeation of limiting membranes upon initial wetting with water, which was indicative of membrane disorder when dry. Leakage under anaerobiosis was observed for S. faberi seeds.

Variation in Germination and Amino Acid Leakage of Seeds with Temperature Related to Membrane Phase Change

Transitions at 28-32°C in membranes of many kinds of seeds influence their germination and subsequent growth Changes at 28-32°C in the rates of loss of endogenous amino acids from imbibed Barbarea verna and Lactuca sativa seeds indicate permeation of the plasma membrane A transition in the same temperature range is shown by the change in fluorescence with temperature of substituted naphthalenesulfonate probe molecules associated with membrane fragments isolated from the seeds Some other plant responses that appear to be causally connected with the membrane(s) transition(s) in the 28-32°C region are growth of maize seedlings and tomato roots, flowering of some bulbs, and control of seed and fern spore germinations by light Generality of the control is indicated

Dependence of thermal responses of seeds on membrane transitions

Seeds of giant foxtail (Setaria faberi Herm.) entered secondary dormancy after pretreatment in H2O at 35°C. Pretreatment in 0.1 m ethanol, or several other substances with anesthetic properties, prevented secondary dormancy induction. Pretreatment in 0.5 m ethanol inhibited germination in darkness, but germination could be stimulated by a red irradiation. Germination was initially insensitive to light. Two separate responses are indicated. The first, affected by a variety of substances and low (0.1 m or less) concentrations of ethanol, is related to anesthetic effects and prevention of secondary dormancy. The second, induction of response to red irradiation, is caused by 0.5 m ethanol and some closely related substances. The anesthetic effect is accomplished within the first 8 hours of imbibition while the phytochrome induction effect required treatment for more than 24 hours. Both responses were lost if the 35°C imbibition began in H2O. Involvement of cell membranes is suggested in the prevention of secondary dormancy by anesthetics.

Anesthetic Effects on Secondary Dormancy and Phytochrome Responses in Setaria faberi Seeds

Germination of Potentilla norvegica L. (rough cinquefoil) seeds stimulated by fluorescent irradiations of nearly 24 hours was inhibited by ethylene at

Ethylene Inhibition of Phytochrome-Induced Germination in Potentilla norvegica L. Seeds

Since the introduction of maleic hydrazide (MH) as a plant growth regulator by Schoene and Hoffman (17) there has been some speculation as to its role as an auxin antagonist. Many of the responses to MH of intact or isolated segments of plant tissue suggest some relationship to auxin processes. Thus, the cessation of growth, the prolonging of dormancy and the breaking of correlative inhibition have been related in some way to the auxin process and are also affected by MH. Cell elongation is to be considered similarly. Leopold and Klein (14) were among those who showed that MH had inhibitory effects on this phenomenon as measured in several common auxin bioassays. These workers, have indicated that MH is an antagonist of auxin action. Support for this aspect of MIH action comes from the work of Kulescha (13) and Gautheret (8). However, the experimental approach used in these studies was one in which antagonistic effects could readily occur while masking other significant actions. It is now generally accepted that a significant amount of the activity of auxins on the elongation of plant cells occurs through its influence on the cell wall. A loosening of cell wall constituents followed by an osmotic uptake of water is envisaged with a subsequent elongation of the cell (18). Increased aerobic metabolic activity has been shown to occur also during this period and a variety of cellular reactions have been followed in detail during these various stages under the influence of auxins. In so far as the results of the present paper are concerned, the initial phases of auxin action are most significant. Other workers have shown that Avena coleoptiles incubated in the presence of auxin and isotonic or slightly hypertonic solutions of mannitol, can display auxin-induced elongation after the sections have been transferred to solutions where elongation may proceed and which contain no auxin themselves (4). These responses have been referred to as residual effects of auxin on cell walls. Such responses may also be readily observed by pretreating potato

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Ray B. Taylorson

Papers

Variation in Germination and Amino Acid Leakage of Seeds with Temperature Related to Membrane Phase Change

Dependence of thermal responses of seeds on membrane transitions

Anesthetic Effects on Secondary Dormancy and Phytochrome Responses in Setaria faberi Seeds

Ethylene Inhibition of Phytochrome-Induced Germination in Potentilla norvegica L. Seeds

Maleic hydrazide-auxin interractions in water uptake of potato discs