https://pubag.nal.usda.gov/download/14449/PDF

Environmental control of dormancy in weed seed banks in soil

Preplant Physiological Seed Conditioning

Germination of lettuce (Lactuca sativa L. cv. 'Grand Rapids') seeds was inhibited at high temperatures (thermoinhibition). Thermoinhibition at 28 degrees C was prevented by the application of fluridone, an inhibitor of abscisic acid (ABA) biosynthesis. At 33 degrees C, the sensitivity of the seeds to ABA increased, and fluridone on its own was no longer effective. However, a combined application of fluridone and gibberellic acid (GA3) was able to restore the germination. Exogenous GA3 lowered endogenous ABA content in the seeds, enhancing catabolism of ABA and export of the catabolites from the intact seeds. The fluridone application also decreased the ABA content. Consequently, the combined application of fluridone and GA3 decreased the ABA content to a sufficiently low level to allow germination at 33 degrees C. There was no significant temperature-dependent change in endogenous GA1 contents. It is concluded that ABA is an important factor in the regulation of thermoinhibition of lettuce seed germination, and that GA affects the temperature responsiveness of the seeds through ABA metabolism.

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Abscisic acid in the thermoinhibition of lettuce seed germination and enhancement of its catabolism by gibberellin

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

The phototransformations of phytochrome involve several intermediates. The initial photoreaction of the chromophore is followed by a series of dark relaxation states of the chromophore and apoprotein. The nature of these intermediates is outlined and they are discussed in relationship to our knowledge of the tetrapyrrole chromophore.

Phototransformations of phytochrome.

Red light and gibberellic acid were about equally effective in promoting germination of Grand Rapids lettuce (Lactuca sativa L.) seeds. With initial far red light treatment more than 80% remained dormant in subsequent dark storage. After 2 days of dark storage, red light effectively promoted germination, while gibberellic acid action was weak. With between 2 and 10 days of dark storage, gibberellic acid had little effect, while promotion by red light decreased slowly and finally disappeared. After 10 days of dark storage, both gibberellic acid and red light were required for germination. The dark storage treatment interferes with phytochrome-independent germination processes and cannot be overcome by added gibberellic acid. However, storage may also decrease the effectiveness of endogenous gibberellins. Phytochrome-dependent germination seems to require only low levels of endogenous gibberellin activity or the addition of gibberellic acid. Gibberellins and red light appear to act on germination by regulation of sequential sites of a branched-looped pathway.

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Actions of Gibberellic Acid and Phytochrome on the Germination of Grand Rapids Lettuce Seeds

It is possible to distinguish two stages in the influence of light on the germination of Lactuca sativa var. Grand Rapids (lettuce) seeds. The first stage, which is represented by the photoactivation or transformation of phytochrome, requires only a relatively low seed water content. Slightly higher seed water content is required for maximum far-red light repression, than for red light stimulation of germination. The second stage is indicated by the well-known situation in which previous red irradiations of the seeds can enhance germination but this takes place only with relatively high seed water content. Phototransformed phytochrome appears to persist and to be susceptible to photoreversal for at least 24 h after irradiation in the seeds with relatively low water content.

Seed water content in relation to phytochrome-mediated germination of lettuce seeds (Lactuca sativa L. var. grand Rapids)

Dark reversion of the far red-absorbing form of phytochrome, which does not occur in dry lettuce (Lactuca sativa var. Grand Rapids) seeds, appears to take place in seeds stored in a water-saturated atmosphere. The water content (approximately 70% after 10 days) of such seeds is insufficient to support germination; however the treatment enhances germination in seeds stored for 1 to 5 days, but this enhancement subsequently disappears, and the effect of extended storage (up to 28 days) is inhibiting. The half-time for dark far red-absorbing phytochrome reversion is 7 to 8 days, and at this time it can be completely reversed by exposing the seeds to a flash of red light. Storage of more than 7 to 8 days decreases red light enhancement of germination.

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Dark Reversion of Phytochrome in Lettuce Seeds Stored in a Water-saturated Atmosphere.

Grand Rapids lettuce seeds (Lactuca sativa L.) with 6 to 8% water content show no light-induced germination responses, whereas in seeds with 15% or more water content, germination is promoted or retarded by red and far red light respectively. By adjusting seed water content, persistent potentiated responses to light are induced in the seeds at seed water levels much below that required for germination itself. Alternate moistening and drying of seeds in conjunction with red and far red irradiations show that potentiated responses may be phototransformed only with sufficient seed water. However, the process of drying and remoistening has no effect on potentiated responses.

Water Content and Phytochrome-induced Potential Germination Responses in Lettuce Seeds'

The effects of red or far-red light flashes on partially hydrated lettuce seeds were found to be undiminished by subsequent dry, dark storage for more than 1 year before germination. Upon rehydration of the seeds photoreversal of the latent response could be induced by a red or far-red light flash at any time during the entire period.

A. I-Hsiung Hsiao

Papers

Actions of Gibberellic Acid and Phytochrome on the Germination of Grand Rapids Lettuce Seeds

Seed water content in relation to phytochrome-mediated germination of lettuce seeds (Lactuca sativa L. var. grand Rapids)

Dark Reversion of Phytochrome in Lettuce Seeds Stored in a Water-saturated Atmosphere.

Water Content and Phytochrome-induced Potential Germination Responses in Lettuce Seeds'

Persistence of phytochrome-mediated germination control in lettuce seeds for 1 year following a single monochromatic light flash