The seed embryo is a bundle of potential growth with a built-in set of instructions for top size and root shape "placed there" by plant breeders after more than one hundred years of careful selection and testing, beginning with the wild-beet types of little commercial value and ending with the present-day monogerm hy brids of high productivity and disease resistance.
Emerging from the soil, often as soon as five days after planting, the delicate primary rootlet is already thrusting itself deeply into the soil. Any temporary shortage of water or minor disease during the early period of growth means loss of the seedling. By the time the cotyledons have become fully developed, and the first true leaf becomes readily visible, the tap root has already grown many inches. See figure 1.
While the tap root continues to grow-along with numerous lateral fibrous roots-the leaf area expands. The first blades grow horizontally to cover the soil sur face and capture sunlight for photosynthesis. Succeeding leaves grow almost vertically, but remain effectively exposed to the sun. After 10 or 12 leaves have been formed, maximum leaf size is reached. The life expectancy of these vertical leaves increases, as average temperatures for day and night drop. In cool weather, a dense canopy of leaves develops when nutritional and growing conditions are favorable; in hot weather, the newly formed leaves develop spindly petioles and narrow blades. The older leaves die and rapidly dry up.
Storage root growth starts only when sugar needs for basic metabolism,
top growth, and fibrous root development have been met. This commonly occurs
during early leaf development, when the tops have nearly reached maximum
size for a given environment. Once storage root growth has started, the
growth rate of the storage root is determined primarily by the amount of
surplus sugar produced by the tops. Root size is determined, within limits,
by plant spacing when plants
have formed a full leaf cover. Total storage root weight per unit area
is approximately the same, regardless of the number of plants. Individual
roots, therefore, become smaller as the spacing between plants decreases.
See figure 2.
A sugar beet plant in a favorable environment grows indefinitely. Apparently, no internal mechanism for "ripening" "tells" the plant to develop a high sucrose content. In growth, the sugar beet plant produces new leaves at a uniform rate; the leaves enlarge, mature, and gradually die. At the same time, the storage root enlarges at a uniform rate, and once the sucrose concentration reaches the equilibrium characteristic for that environment and variety, the sucrose concentration in the storage root remains relatively constant. "Ripening" under these conditions does not occur. Only a change in environment, such as a lowered night temperature, a depletion of nitrogen, or a combination of these factors and others will increase the sucrose content of the beet root. Ripening is also favored by a small root size during nitrogen depletion, since small roots "fill up" with sugar faster than large roots under similar climatic conditions.
Ordinarily, the best conditions for ripening prevail during the fall
of the year. However, this normal ripening often has been upset by the
addition of more nitrogen than can be used effectively by the crop or by
nitrogen applied too late to allow for several weeks of nitrogen depletion
before crop harvest. It is only by an adjustment of the nitrogen program
to the needs of the crop that a maximum tonnage of roots of good sugar
quality can be harvested. Here, technical knowledge can be of greatest
help to the sugar beet grower.
Sulfur and, at times, phosphorus deficiency increases the sucrose concentration of the beet root; but allowing these deficiencies to occur as a means of enhancing the sucrose concentration of the beet root is not recommended. On the other hand, all other nutrient deficiencies tend to decrease sucrose concentrations of the storage root.
Any cultural practice that limits top growth nearly always leads to
limited root size and frequently to limited sucrose concentration in the
storage root as well. These losses can be costly to the sugar beet grower.
For micronutrient analyses, plant material must be free of dust. Leaves
should be washed for 30 seconds in a bath containing 0.1 normal HCI, followed
by two successive rinses in distilled water. Leaf material should then
be dried in a ventilated oven at 160' F and ground to pass through a 40-mesh
sieve in order to analyze for nutrient constituents by well established
methods.
