Fruits
Botanist, salespeople, and even lawyers tried to define what a fruit is.  In 1893, the U.S. Supreme Court ruled that a tomato was a vegetable, not a fruit. However, from the botanical point of view, tomatoes, as well as squash, apples, watermelons, peanuts and wheat are all fruits, according to the  definition of a fruit as a mature ovary.  Inside the fruit there is a seed, developed from a mature fertilized ovule.

	There are three main categories of fruits: Simple fruits are formed from one ovary, and may be either fleshy (peach, cherry, tomato, citrus fruits, cucumbers) or dry when mature (legumes, sunflower "seeds", grass grains, etc.);
	Aggregate fruits develop from flowers that have more than one pistil. Raspberries (left) and blackberries are aggregate fruits consisting of many simple fruits.
	Multiple fruits develop from fused ovaries from different flowers like in pineapples (left) and mulberries.

 
 Fleshy Simple Fruits

Berry is a simple fruit that is fleshy throughout. It has one or several carpels with many seeds. A berry develops from an ovary consisting of several subunits.  According to this definition, tomato (right) and eggplant, rather than blackberries or strawberries are actually berries.

Drupe is a fleshy fruit, which can contain many carpels. However each carpel contains only one seed. The inner layer of the fruit wall is usually stony, like in olives, cherries, peaches and apricots (left). Pomes are characteristic of apples and pears (right). Their fleshy parts come mostly from the enlarged bases of stamens.

Kernels of grasses are also simple fruits. Caryopsis is the fruit of grasses. The caryopsis (also called a grain or a kernel) is a single-seeded fruit where the ovary wall and seed coat are fused into one layer.  Grains of rice, wheat, barley, and bermuda grass are examples of the caryopsis.

Anatomy of Grass Fruits

To the left are a diagram and a micrograph of a corn kernel cross-section. Let's study the diagram. Corn kernel is protected by a fruit wall called pericarp (1). The bulk of a mature kernel of any grass is endosperm (3). Endosperm is rich in starch which serves as the energy source for the germinating seed and seedling.  During germination, aleurone layer(2) produces a-amylase, an enzyme that breaks down starch and thus mobilizes energy for germination.

Unlike seeds of dicots, grass caryopsis has only one cotyledon (4). Unlike cotyledons of dicots, cotyledon of grasses does not serve as the storage tissue of the seed.
A radicle (8) (an embryonic root) is first to emerge when germination starts. It is covered by coleorhiza (9), a protective sheath.  Coleorhiza seals and prevents pathogens from entering the kernel through the rupture made by the elongating radicle during germination.  Radicle elongation is followed by elongating of the coleoptile(5) which acts as a protective sheath surrounding young leaves and apical meristem(7) of a plumule (6) (embryonic plant).   Rapidly elongating mesocotyl  pushes the growing coleoptile to the soil surface.  Upon emergence and exposure of the coleoptile to the sunlight, coleoptile and mesocotyl elongation stops and leaves start to elongate.
Black layer (10) forms near the tip of a corn kernel.  Once formed, it indicates physiological maturity.

Legume seed anatomy

The seed of a fava bean on the left is a true seed, covered by testa (1), the seed coat. The legume seeds are attached to the pod (legume fruit) by hilum (2), through which the seeds receive food during their growth and development. Hila may vary in color, thus providing means for identification. The embryo area is an area of the embryo axis which develops into the seedling and is in a very vulnerable position for mechanical damage.

An embryo consists of:

• an epicotyl (5), embryonic shoot and leaves. It contains the growing point and the first two unifoliate leaves;
• hypocotyl (6), the stem tissue between the epicotyl and radicle. In most legumes the hypocotyl elongates during germination to cause emergence of the seedling;
• radicle (4), embryonic root found in the lower portion of the embryo axis.

Energy for germination is stored in the two cotyledons (3).  Soybean cotyledons contain ~20% oil and 40% protein.  Nutrient and food reserves in the cotyledons supply the needs of the young plant during emergence and for about 7-10 days after emergence. Loss of one cotyledon has little effect on the young plantís growth rate, but loss of both cotyledons soon after emergence will reduce yield by  8-9%.

The main factors that determine seed quality are genetic and mechanical purity, germination, and vigor.  Genetic purity can be determined using molecular markers (regions of DNA conserved in the plants of the same variety).  When DNA testing is not readily available, genetic uniformity of a seed lot can be assesed visiually. For example, soybean varieties have characteristic colors of their seed coats (below, right) and hilum (left), these traits can help greatly in visual inspection of seed uniformity. Left: different hilum colors (black, imperfect black, gray, buff, yellow and brown) help identify soybean varieties (Seeds are labeled with the variety name and hilum color). Right: colors of seed coats (testa) and seed size can also help in identifying soybean varieties and their weedy relatives   With seeds of many crop species, however, visual assesment of genetic purity is impossible. Seed lots are also checked for phyiscal purity, i.e. lack of unwanted contaminants, debris and weed seeds.

During maturation, harvesting and storage, seeds inevitably deteriorate. The resulting loss in seed quality decreases stand establishment. Seed quality can be estimated by germination, and vigor tests.

Germination test
Germination tests are conducted under conditions that are optimal for seed germination.  Temperature, moisture, light, and pathogens are maintained at levels that are considered optimal for a particular species. Germination test results reflect the maximum potential of a seed lot to produce healthy mature plants under favorable conditions.

Vigor test
Seed vigor includes the seed properties that determine the potential for rapid uniform emergence and the development of normal seedlings under a wide range of stressful field conditions.  In vigor tests, seed samples are exposed to a stress (mechanical damage, storage at high temperature and/or humidity, soil pathogens, cold following imbibition).  Following the stress period, the seed sample is germinated under normal growth conditions.  The number of uniform well-developed seedlings that germinate indicates the relative seed vigor of the lot. 

Results of a seed vigor test that are similar to germination test results suggest that the stress treatment was insufficient to overcome seed vigor in that particular lot, and therefore the overall quality of the seed lot is high.  Seed vigor test results that are much lower than the germination test suggest the opposite, namely that the stress treatment was sufficient to overcome seed vigor in that lot, and therefore the quality of the seed lot is poor.

Tetrazoluim staining

Germination and vigor tests usually take a week or so. Tetrazolium staining allows relatively fast test of seed quality.
Living cells have active dehydrogenase enzyme that yields hydrogen ion (H+).  Tetrazolium is a compound that reacts with H+ and forms an insoluble red pigment.  This is how tetrazolium indicates living seed tissues.

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1. Slice a corn caryopsis, like on the figure above.

2. Open a legume fruit.  Slice open a legume seed.

3. View the unstained seeds under the dissecting microscope and  identify visible anatomical features.

4. Draw a dissected legume fruit and seed in the space provided. On the seed diagram, identify testa, hilum, cotyledons, epicotyl, hypocotyl and the radicle.

For the Seed Quality exercises, you will have a choice of pea, soybeans, corn, wheat or tall fescue.   Decide which crop your group would like to work with in this lab.  Develop a hypothesis to be tested. Suggested protocols are listed below.

Tetrazolium Staining Protocol

In your notebook, describe your visual obvservations of both seed lots.

Seeds and grains were imbibed overnight at room temperature, then soaked for an hour in 2% tetrazolium solution.

1.  View the seeds soaked in tetrazolium under the dissecting microscope.  Wherever the seed is stained red, there is a high concentration of living/respiring cells.

2. Shade the areas that stained red with tetrazolium on seed diagrams.

Rolled Towel Germination Test

1. Thoroughly moisten four sheets of the germination paper.

2. Fill a seed counter tray (one seed per hole) with seeds from lot A.

3. Release the seeds onto the germination paper so that the seeds are distributed fairly evenly over the paper.

4. Cover the seeds with another sheet of germination paper.

6. Fold up the bottom edge of the two sheets about 1 inch from the bottom of the sheets.  Then, loosely roll the sheets together from one side to the other.

7. Repeat steps 2 - 6 with lot B seeds.

8. Label the rolls with your team number and the lot number of the seeds.

9. Place both rolls in the plastic bag and give them to the laboratory instructor.  They will be stored in the lab at room temperature until you evaluate them the following week.

Cold Test for Seed Vigor

1. Label the tray with your team and lot number, and the crop you are working with.

2. Saturate a sheet of germination paper  with chilled water.

3. Place one sheet of the wet germination paper on the bottom of the tray.

4. Distribute 50 seeds of  lot A evenly over the left half of the paper and press the seeds gently into the paper.  Repeat the procedure with 50 seeds of lot B over the right half of the paper.

5. Cover the seeds in the tray with soil. Use a measuring stick to spread the soil evenly across the tray.

6. Cover the tray with plastic wrap.  Be sure to seal the trays to prevent drying in the cold chamber.
The trays will be kept in a dark chamber at 10o C for up to 72 hours.  The trays will be kept in the greenhouse for the remainder of the week (or up to 2 weeks depending on the conditions).

For more information, email us at maxtep@ufl.edu, mcmahon.43@osu.edu.