Re: Are fruit seeds kept dry and seperate from the juices of the fruit?

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.