Tuesday, December 25, 2012

Stages in Wheat Growth

If you really want to watch eyes glaze over at your next dinner party, try describing the stages of growth that our wheat is going through in Agoura Hills.  You might assume that only we small-grain foster parents would want to know all the particular details.  But it is fascinating how this most important food crop gets from small seeds in the ground, up into a tall grass, with fruit that eventually provides food for our table.

Basically, we’re talking about germination, plant growth, and final seed development.  But knowing the details and sub-steps within these major stages gives the grower an opportunity to adjust fertilizing and irrigation, and better estimate the eventual yield.  According to some sources, there are at least five scales to measure wheat growth, with the one called Feekes listing eleven stages of growth, and Zadok dividing the growth sequence into one hundred steps.  Fascinating.  How could this not intrigue even the most jaded guest?  

The first surprise is what goes on underground before the first green shoot emerges.  The initial (radicle) roots emerge from the seed (caryopsis)
to be followed by a whole different set of (coronal) roots.  A scholarly journal describes how these help support winter wheat.

"The anchorage of winter wheat, Triticum aestivum L., is provided by a cone of rigid coronal roots which emerge from around the stem base. During root lodging this cone rotates at its windward edge below the soil surface, the soil inside the cone moving as a block and compressing the soil beneath. A theoretical model of anchorage suggested that lodging resistance should be dependent on the diameter of the root-soil cone, coronal root bending strength and soil shear strength."  Those roots are no slouches.  Both kinds can grow at nearly an inch a day, and, by the time the wheat is mature, the root system can extend 4’ to 8’ laterally, and downward. 

Now, before your dinner guests can change the subject, tell them how wheat begins growing upward.  It starts within a stiff protective tube of greenery, called the coleoptile.    That will get their attention. 

This tube not only protects the tender stem on its way up toward the soil surface, but it also plays a part in delaying the first leaf until surface temperatures and conditions are acceptable.   Pretty smart plant.  The moment when the first leaf emerges from its tube is Zadok stage 10.

As the stem grows, a series of leaves appear.

At about leaf number four (we’re now at 14 on the Zadok scale), tillers begin forming.  These are side shoots that emerge from the main stem, put down roots, and grow upward in parallel with the primary stem.  A few of them eventually set seed.  The whole process of stem elongation then becomes relatively normal, taking us through Zadok 37.  And it is here that the flag leaf appears.  It is the final and uppermost leaf, quite different from the previous 8 or 9 leaves, because it contains the embryonic seed head or spike.


When the head is just a swollen area (making the flag leaf look as though it is expecting), the plant is entering the boot stage, and our wonderful little grains of wheat are beginning to take shape.  From Zadok 41 to 58, the spike gradually emerges from its wrapping in the flag leaf, and those individual, would-be grain kernels begin flowering. 

Although the plant has been growing now for several months, the flowering (inflorescence) stage takes only a few days. If you are beginning to lose your audience, here is another surprising thing about small grains:  the flowers open first at the center of the head and progress toward the top and the bottom of the spike.  This leads us to pollination (anthesis), which is when you can get the attention of those interested in sex.  Tell them, discretely, that most wheat is self-pollinating, and it is a very brief process, with pollination of individual florets completing in less than five minutes.  Thus it is quite rare that pollen from another plant enters the picture.  The process is completed for the entire crop in less than ten days, empowering a considerable percentage of the little grains along each stalk to begin maturation into tasty wheat berries (caryopsis once again), most of which are identical to the seed we planted.

The final stages of spike growth, Zadok numbers 71 through 100, are some months away for us in LABB.  As described in an article on the botany of the wheat plant, "the recently fertilized grain is creamy white in color and when squeezed exudes clear liquid. It grows rapidly, attaining its maximum length in about ten days, and becomes green in color as chlorophyll is formed. At this stage, the exudate becomes milky as starch grains are deposited. As growth proceeds, the endosperm becomes firmer (the soft dough and the hard dough stages) until, at physiological maturity, the green color is replaced by golden-yellow, which deepens as the grain desiccates to dead ripeness."  

So we can look forward to seeing the seeds swell, the colors deepen, and the individual berries be tested -- pressed between thumbnail and forefinger, as they progress from milk stage to dough stage to hard kernel stage.  They gradually firm up until Zadok stages 91 (“hard to split by thumbnail”), 92 (“cannot split by thumbnail”), and 93 (“kernel loosening in daytime”).  

And we all, including our dinner guests, will be pleased that harvest time has finally arrived.

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