Tillering patterns and wheat plant stresses


The main purpose of this talk is to give you some understanding of the developmental history of healthy cereal plants and the symptoms that develop under stress. Objectives Include:

• Learning the parts of the cereal plant.

• Learning the names of these parts.

• Becoming familiar with the arrangement of leaves, tillers, and roots on the stem.

• Learning to “read” plants, that is, to identify individual leaves and tillers.

• Using growing degree day concepts to track plant development.

• Discovering the expected tillering pattern for unstressed plants.

• Finding missing tillers and interpreting these absences in terms of plant stresses and when they occurred.

• Deciding on timing and placement of fertilizer and herbicide applications with plant development in mind.

The parts of the cereal plant

Cereal plants are made up of main stems, tillers, and roots. The main stem is the plant that emerges when you seed your wheat; it is the first and usually the strongest culm on the plant. There is one modified leaf-like structure on the main stem that is important in the emergence process. That structure is the coleoptile which elongates and pushes its way up in the soil and protects the first leaf on the plant from damage by the soil. Tillers are branches of the main stem; they appear some time after emergence and serve to increase the number of heads per acre that are present in the final crop. Tillers start off dependent on their parent culm for nutrients and water but later they grow their own root system and become more independent. There are two root systems on cereal plants; one is the seminal root system which develops from root buds or primordia that are present already in the seed and the other is the adventitious or crown root system that is produced at the crown level later in plant development. There are roots which come from the coleoptilar node in some plants and these are considered adventitious roots.

Names of plant parts

Each leaf on the main stem has been given a name according to its order of appearance. The first foliar leaf is called Leaf 1 or L1, the second, L2, the third L3, and so on. The point of attachment of a leaf is called a node; nodes of cereals appear later in development as “joints” but at the seedling stage they are not so obvious. Each node takes on the name of its leaf--that is, L1 is attached to N1, L2 to N2 and so on. The same system is used for tillers. Each leaf has in its axil a bud which can either remain dormant or grow to produce a tiller. When a plant does not tiller, these buds are still present but remain small. Tillers are also named for the leaf which subtends them; Tiller 1 in the axil of Leaf 1 and so on. The tiller at the coleoptilar node has the designation of T0; this tiller is especially useful in determining adequate seed and seedbed condition. Roots have also been named according to the node to which they are attached. Thus the crown roots associated with Node 1 are called lA, lB, lX, and lY, depending on the direction of growth with respect to the midrib of the leaf at the node.

 

Arrangement of plant parts

Leaves are arranged on opposite sides of the stem in cereals. For example, if you hold a wheat plant in your hand with Leaf 1 on your left side, then Leaf 2 and all of the other even numbered leaves and tillers will be on your right side, and Leaf 3 and all of the other odd numbered leaves and tillers will be on your left-hand-side. Tiller 0, being even numbered will be on your right-hand-side if it appears. For most seedings, there will be a sub-crown internode between the crown and the coleoptilar node; the length of this internode will determine the depth of the crown. The seminal roots and the roots from the coleoptilar node will be grouped together at the level of the seed. The crown roots will begin to appear at Node 1--that is, at the bottom of the crown. It is because these plant parts always appear in a set sequence and in set places that we are able to identify all of the parts of the plant even when some of them are missing--such as ower leaves being rotted off or tillers that were never produced.

“Reading” plants

Because cereal plants develop according to a set pattern, it is possible to pick up a strange plant and figure out how old it is, how deep it was planted, whether it has been stressed during early development, and whether it has stopped tillering. All of this information is available from the morphology or form of the plant. To read a plant, we identify all of the parts present, especially leaves and tillers. We can see a distinct line where the soil surface was because all of the below-ground parts will be light-cream to very light green and all of the above-ground parts will be dark-green. The depth of the crown can be measured from the soil-line to the bottom of the crown and the seed depth can be determined in like manner. We can see which tillers are present and how many leaves there are on the main stem. The number of leaves on the main stem is referred to as the Haun Stage, after a person who developed a system for counting the number of leaves on a plant by counting all of those developed and adding to that number the fraction of the next leaf that has appeared. In the case of cereals we determine the Haun stage by counting leaves and adding on the decimal fraction that represents the length of the newest developing leaf compared to the blade of the leaf immediately below the newest one. With the Haun stage, we know that the plant is old enough to have a certain number of leaves on it. We also know when there were stresses because tillers fail to appear when there are stresses. Whenever tillers did not appear is when the plants were stressed. The next section will help you to understand how actual dates can be put onto these events. We also have to remember that the below-ground parts of the plant develop according to a pattern and that our applications of fertilizer materials must take this fact into account.

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