Twenty-eight days

Identify anatomical features of plant seedlings.
Describe the ability of plants to survive injury.

Crop plants often encounter hardships during the early stages of their growth that result in the loss of anatomical parts.  Some of the more common problems include insect injury, wind, hail, frost, animal grazing, and damage from machines.  A plant's ability to survive these obstacles to further growth is determined by the plant species and its size or stage of development.  In cereal crops, like corn, the main growing point (apical bud) determines early growth of the stem and its leaves.  If the stem is severed below the apical bud or if it becomes damaged the plant will die.  In contrast, the stems of many dicotyledon plants, in addition to the apical bud, have lateral buds that allow the plant to regenerate new stems if the apical bud or main growing point is damaged.  Regrowth can occur in these plants as long as one or more of the buds remains and the remaining vegetative growth or stored materials in the plant can sustain growth.  Crops are highly vulnerable to injury during the early stages of their development because the number of buds is very low.

Crops scientists are often asked to make economic decisions concerning crop stands injured by weather phenomenon, insects, or animals.  Examples of common decisions include replanting the crop and payments made on insurance policies.  Making the right decision requires an understanding of the location and development of buds within the crops species with which the professional crop scientist works.  The following demonstration illustrates the regrowth capacity in young plants of two common crop species with quite different anatomy - sunflower and corn.  Before viewing the demonstration, you will want to study sunflower and corn seedling anatomy.

  (Click here to see materials needed)

Sunlight or artificial lighting for plant culture

Sunflower seeds

Sweet corn seeds

Nine (9) 6-inch (1 gallon) plastic pots

Nine (9) gallons (1.2 cu. ft., 34 L.) potting soil mix - greenhouse mix of sphagnum peat, vermiculite, perlite, and a wetting agent

Fertilizer (Click here for recommendations on fertilizer)

Pot stakes and a permanent marker

1. Plant 5 sunflower seeds in each of five pots and 5 sweet corn seeds in each of four pots.  Fertilize the pots by following the directions provided by the manufacturer.  Water the plants as needed for the 28-day period.
2. One day after emergence of the sunflower seeds perform the following treatments. (Label the treatments using the pot stakes and marker.)
   1. Remove one cotyledon
   2. Remove both cotyledons
   3. Remove the epicotyl (This may be difficult right after emergence and could be done a few days after emergence.)
   4. Clip the hypocotyl
   5. Control (no treatment)
3. Perform the following treatments to the sweet corn seedlings. (Label the treatments using the pot stakes and marker.)
   1. Clip the seedlings in one pot at the soil surface 7 days after emergence
   2. Clip the seedlings in a second pot at the soil surface 14 days after emergence
   3. Clip the seedlings in a third pot at the soil surface 21 days after emergence
   4. Control (no treatment)
4. At the end of the 28-day period measure the height of the plants in each treatment. Compare the height of the sunflower plants and try to explain the various treatment responses. Do the same for the sweet corn seedlings.

The most growth will occur in the plants that are not clipped.  The sunflower plants should survive in they are not clipped below the cotyledons.  So, clipping the hypocotyls will result in death, but plants will survive removal of the cotyledons or epicotyl.  Some stunting may occur when the cotyledons are removed.  When the epicotyl is clipped, regrowth will slowly occur from buds located on the stem where the cotyledons are attached (cotyledonary node).

The sweet corn plants clipped on days 7 and 14 should continue to live and resume regrowth after the clipping treatment.  Those clipped on day 21 may or may not regrow depending on whether or not the growing point was detached.  In the field or garden, the growing point will be above the soil surface in three to six weeks, depending upon soil temperature and other growing conditions.  Under ideal conditions, the growing point will likely be above the soil surface in less than three weeks, thus a 21-day clipping should kill the plants.  If wind, hail, insects, or animals cut off young grass plants at the soil surface, the plants are likely to regrow.  If these problems occur later when the growing point is above the soil surface, no regrowth will occur and the plants will die.

 
The growing point of corn can be viewed by cutting the stem longitudinally.