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A Year in the Life of a Blueberry Bush
I want to talk about how blueberries grow. First, I want to review some basic plant physiology so that we are all on the same page and you can understand how I think about plants. My basic model of a plant is a leaf stuck in a straw. At the opposite end of the straw from the leaf is a cotton wick, the root. That is your basic plant: leaves, stems, and roots. Plants can make virtually everything they need from water and air with a few nutrients that the roots absorb from the soil. The plant uses sunlight to split water and make sugar from carbon dioxide in the air. It discards oxygen as a waste product. Both plants and people use the oxygen in the air to burn sugar and make energy to live. The blueberry leaf's sole purpose is to harvest light and make sugar. Water comes from the roots. Air enters the leaves through small holes in the leaves called stomates. In blueberries the stomates are located on the bottom of the leaves. When the stomates are open, they let air in but they also lose water through evaporation. This is an important point to remember because if those stomates close to conserve water, photosynthesis and sugar production stops. It does not matter how much sunlight you have, if the plants are drought stressed growth slows or stops! Water is the single most limiting factor in plant growth.
Take a moment to consider the structure of a blueberry leaf. It is a thick, waxy leaf. The thick, waxy cuticle reduces water loss. The leaf is thick and the edges are smooth, which means the surface area is small and evaporation is reduced. This leaf is designed to conserve water. If you showed me that leaf, I would say that it came from a plant where water was a limiting factor in growth.
Water is vital for growth. Plants grow in two ways, cell division and cell expansion. Cell division creates more cells and cell expansion is the increase in size of the individual cells. Cells grow by taking up water. Plant cells can be imagined as a wet cardboard box (the cell wall) with a water balloon (the plant cell) inside. The cell swells by taking up water and pushing against the inside of the box causing it to swell and increase in size. Young cells expand and produce new cell wall at the same time. Eventually, the cell wall becomes so thick that the cell cannot expand anymore and growth stops. If water is reduced during growth, the final cell size is reduced. This means fewer, smaller leaves, smaller fruit, shorter, thinner stems and fewer roots. Drought stress results in a smaller, weaker plant.
Roots
Just as we can look at leaves as organs that harvest light, roots are organs, which absorb water. This is their primary function. Most of the water is taken up at the root tips. There is a zone of active cell division at the root tip where new cells are formed. Directly behind the tip is a region known as the zone of elongation where the new cells grow by increasing in length. This elongation pushes the root into the soil. There are no barriers to water movement in these young tissues and water can move freely into and out of the root. Behind the zone of elongation is the zone of differentiation, where the cells develop into different cell types. In the center of the root is woody xylem tissue, which carries water from the roots to the stems. Around the xylem is a layer of cells called the endodermis. These cells have thick waxy barrier between them. This barrier is called the ‘Casparian Strip’ and prevents water loss. This barrier is not only effective in keeping water in the root but also prevents its easy entry into the plant. You can test this if you have any potted plants. When potted plants wilt you can still water them. You could also cut a stem off the plant and place the stem in a glass of water. The cut stem will return to normal much faster than the potted plant because it does not have to pull water through the roots. As the soil dries, root growth slows. The zone of elongation becomes shorter and the casparian strip reaches almost to the root tip. This reduces both water loss and water uptake by the root.
Many plants have root hairs on their roots. These are tiny rootlets increase the root’s surface area, which increases the absorption of water and nutrients. Blueberries are unusual in that they do not have root hairs. Therefore the root surface of a blueberry root system is smaller than in other plants. In most root systems, most of the roots are located in the uppermost soil layer. This is also true in blueberries. Blueberries seem to have a relatively small root system that does not spread far from the plant. This means that the blueberry is unable to draw water from a large volume of soil, but only the area in the immediate vicinity of the plant.
Water Movement in Plants
Water has to enter the plant cells to get into the roots, but plant cells do not actively take up water. The water moves into the cells because they are full of salts and sugars. Root cells receive sugars from the leaves and also actively absorb salts from the soil. This concentration of salts and sugars causes water from the soil to move into the cell. This water is then pulled to the xylem by the active movement of salt ions into the xylem and the water follows the salts. The stem of the plant is simply a plumbing system. The inner layer is the wood or xylem, which carries water from the roots up to the leaves.
Water in the xylem is drawn up the through the stem by suction from the leaves because water is evaporating through the stomates into the air. If water flow into the roots cannot keep up with evaporation from the leaves then the stomates begin to close. This usually happens every day. The stomates open at dawn in response to light and photosynthesis begins in the leaves. By the late morning, the roots are struggling to keep up with the demand for water from the leaves and the stomates begin to close and photosynthesis declines. Later in the day, in the mid- to late afternoon when the roots have caught up, the stomates open and photosynthesis increases. In the leaves most of the water is evaporated out to the air. This water is not really wasted because it cools the leaves as it evaporates. If you do not think the plant gets hot, go stand in the field all day in a dark green suit and see how much you sweat. Both you and the plant are cooling by evaporation. When the loss of water through evaporation exceeds the amount of water coming into the leaves from the roots, the leaves begin to wilt. To go back to my analogy of a plant cell as a wet cardboard box filled with a balloon when we let water out of the balloon the box will sag. This is what happens when plants wilt. The hydraulic pressure in the cells is no longer enough to hold the cells erect. The guard cells, around the stomates, close the stomates when they wilt. When the guard cells are plump and full, they hold the stomates open.
Annual Cycle of Growth
Now, let’s look at how the blueberry grows over the course of the year. Right now our blueberry bushes are dormant and ready for winter. We have had an unusually warm fall with few days below freezing. Many growers are concerned about next year's crop. The bushes are still waiting for winter. As long as cold weather does not arrive abruptly we have little to worry about. The varieties we grow could easily handle cold temperature down to -10 F. Our blueberries varieties require about a thousand hours of chilling. Chilling temperatures are above freezing but below about 45 F. We probably begin to accumulate chilling hours sometime in November. The plant will not grow until this chilling requirement is completed. If we have long periods of subfreezing weather this merely delays the completion of dormancy. After the chilling requirement is completed the plants will begin growth as soon as the conditions for growth are right.
In the early spring, as the ground begins to warm, the roots begin to grow. The roots use sugar, which was stored in the shoots and roots the previous year, for this growth. Many growers will put on fertilizer at this time so the roots can absorb it. But the roots take up very little because plants that have no leaves use very little water. As the buds begin to grow they use sugar that was stored in the buds. The new leaves do not have a waxy cuticle water is lost water fairly rapidly. Now the plant begins to take up water from the soil. The roots and shoots are growing at the same time. There is plenty of water and sugar to go around. The roots grow where the conditions are best. Most root growth takes place in the moist warm surface soil early in the year. If the soil is saturated with water, as in flooded fields, the roots may drown. As the soil dries, the roots grow deeper. If the soil remains wet, because of a wet spring, a high water table, or a low spot in the field, then the root system will remain shallow.
