|MadSci Network: Botany|
In an intact fleshy fruit, such as an apple or orange, seeds are often kept separate from the fruit juice. If you carefully cut apart an apple or orange you can see that the apple seeds are in separate, gas-filled chambers and the orange juice is contained in specialized cells. During development the seed gets moisture and nutrients through the vascular system, not by absorbing moisture through its seed coat or breaks in the seed coat as it does when in the soil. Even in a fleshy fruit, mature seeds can get fairly dry. It is possible that some fruit juices could be inhibitory to germination because of their low pH, chemical inhibitors, and high sugar or dissolved solids content. High sugar or dissolved solids reduces the ability of the seed to take up water via osmosis. Seeds germinating while still in the fruit is termed vivipary if the seedling breaks through the fruit wall or cryptovivipary if it does not. While not that common, vivipary and cryptovivipary do occur in a few species consistently, such as mangroves. In mangrove, vivipary seems to be an adaptation to its aquatic environment. The dagger-like seedling root allows the seedling to bury itself in the mud when it falls off the tree. Vivipary and crytovivipary also occur occasionally in species such as tomato, citrus, and grains and is considered undesirable by plant growers. Crytovivipary in tomato has been associated with a mycoplasm disease. Keep in mind that seeds of many species and especially cultivated flower, lawn and vegetable seeds lack dormancy and will germinate promptly when given the environmental conditions needed for germination. Many other seeds exhibit dormancy, meaning they will not germinate even when given ideal environmental conditions required for germination. These dormant seeds will not germinate while still in the fruit and not because of the fruit juice. For example, fresh seeds of temperate fruits, such as apples, peaches and pears, are dormant. To overcome their dormancy, they require a cold, moist period of several weeks, termed stratification. Some seeds, such as honeylocust and many other legumes, have a hard seed coat that prevents germination. The hard seed coat must be worn down either mechanically or by concentrated acid. Wearing down a hard seed coat to allow germination is termed scarification. Plant hormones are also involved in seed dormancy. Abscisic acid generally prevents seed germination and gibberellic acid often promotes seed germination. The first website cited below descibes an experiment where tomato seeds were prevented from germinating when placed in tomato juice. However, that may have been due to the low juice pH or high dissolved solids rather than hormones or chemical inhibitors. References Tomato Juice Inhibition of Seed Germination THE ROLE OF ABSCISIC ACID DURING SEED PRECOCIOUS GERMINATION Seed Vivipary Foxtail seed vivipary Limberk and Udrychova (1972) Vivipary in fruits of tomato plants infected with a mycoplasma disease --potato Witches' Broom. Phytopath. Z. 73:227-234. Vivipary in Corn Seed Hartmann, H.T. and Kester, D.E. 1983. Plant Propagation Principles and Practices. Englewood Cliffs, NJ: Prentice Hall.
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