Seed coating germination

Procedure Germination conditions were 25 1°C under continuous fluorescent light of 25 mE m 2 sec-1. Seed germination was monitored by observing the seeds directly in the Petri dishes with a stereomicroscope. They were considered germinated, when the radicle had protruded through the seed coat. Seeds sampled at different times after the beginning of imbibition were used for microscopy studies. 

A chemical stimulant, a root exudate, is required to break seed dormancy of a parasitic weed and initiate seed germination. Upon receiving a signal for germination, a radicle emerges from the seed coat. However, an additional chemical signal is needed for the radicle to penetrate host roots and form a haustorium. Therefore several plants can serve as catch crops or trap crops for the reduction of the parasitic weed seed bank in infested soil. 

During imbibition of whole tea seeds (6 days) two purine alkaloids, caffeine and theobromine, did not decrease in the seed coats and there was no increase in the seeds. In parallel with and after the breaking seed coats there was a gradual release of caffeine from coats of germinating seeds. By contrast, when the seed was freed from the outer seed coat and soaked, imbibition of the seed required only two days and simultaneously caffeine was released from the inner seed coat. 

In contrast with the seed caffeine of Coffea species, relatively little attention has been paid to that of tea. This is in part because the fruit of tea, including the seeds, is of minor economic importance compared with that of coffee moreover earlier studies revealed little caffeine in the tea seed (13.141. Recently we (H) found that the pericarp contains the greatest concentrations of alkaloids in the dry fruit of tea, and that appreciable amounts occur in the seeds, especially in the coats. Thus, from physiological and ecological viewpoints, our concerns are the roles of purine alkaloids and seed coats of tea during fruit development (seed formation) and seed germination. Caffeine in Coffea arabica seed is synthesized in the pericarp, transported to the seed, and accumulated there during fruit... 

The seed coat of tea regulates imbibition of tea seed (Figure 2) and releases caffeine during germination (Figure 3). These processes may be dependent upon... 

Figure 3. Caffeine Leakage from Germinating Normal Tea Seeds ( ), Seed Coats (O), Seeds removed from the Outer Seed Coats prior to...

Figure 4. Levels of Caffeine (A,A) and Theobromine (O, ) in Seed Coats (A, ) and Cotyledons (A,0) of Tea during Imbibition and Early Germination.

Many seeds become dormant as a means of survival and are able to germinate after many years in the soil. Factors that influence seed germination and seed dormancy include temperature, moisture, oxygen, light, inhibitors (e.g., allelopathic effects), hardness or impermeability of seed coats, mechanically resistant seed coats, immature embryos, and after-ripening requirements (e.g., cool temperatures for several months). Weed seeds may survive and germinate due to several of these characteristics (Pareja et al, 1985). 

A common problem in a number of wild Helianthus species is the presence of a seed coat dormancy mechanism. Though at a much lower level, some inhibition can also be seen in cultivated sunflowers. Kamar and Sastry (1974) found that 20-day-old seeds had a substantially higher germination than did more mature seeds (i.e., 30 and 40 days old), indicating the presence of a dormancy mechanism. The onset of dormancy early in seed development is typical of many seed-... 

There are two important classes of allelochemicals synthesized by oxidative cleavages of tetraterpene carotenoids. One is the plant hormone abscisic acid (ABA, 31) that plays important roles in growth and development of plants, especially in seed development and dormancy.17 Dry dormant seeds contain relatively large amounts of ABA, particularly in the seed coats. ABA and phenolic allelochemicals in the seed coats are easily released into the environment when the seeds are imbibed, resulting in inhibition of seed germination and seedling growth of plants in the vicinity. Both ABA and the phenolic compounds are rapidly broken down in the soil, and therefore the inhibition is short-lived. 

During germination the volume flux density of water into a seed is often limited by a seed coat (testa) of thickness 8SC (Fig. 9-13). The seed coat is a complex, multilayered, hard, rather impervious tissue that is relatively thin compared to the radius of the seed. For the volume flux density at the seed surface (r = rs> ra = rs> and t b = rs - Ssc Fig. 9-13), Equation 9.9 becomes... 

If a similar longitudinal section of a soaked grain be mounted in dilute iodine solution, the contents of the aleurone cells will be colored yellow indicating their proteid nature, while the starch grains will take on a blue to violet coloration. The endosperm will be observed taking up most of the room within the seed coat. The contents of its cells are not baled out to the embryo until after germination begins. Indian Corn is therefore an albuminous seed. 

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