Plant Growth Regulators for Greenhouse Crops
Cultural Practices for Controlling Plant Growth
Knowing how the cultural practices can affect plant growth will help in managing a crop’s growth. Interest in using cultural practices has grown because of the tighter controls placed on the use of agricultural chemicals, the public's negative perception of chemicals in general, and the cost in applying PGRs. There are a number of cultural practices that can be manipulated in the greenhouse to control plant growth as an alternative to PGR applications for some crops. The best management practice is often a combination of cultural practices and chemical control. Cultural alternatives to PGRs include container size, plant density, irrigation practices, nutrient management, mechanical conditioning, light quality and quantity, and temperature manipulation.
Container Size
Root restriction can be used to control plant growth by utilizing a small container or by increasing the number of plants per pot. This method works especially well when other production parameters such as ample light, i.e. wide spacing, and proper nutrition are provided.
Plant Density
The distance between plants is another important factor to consider. Spacing affects the amount of light, water, and nutrients that are available to individual plants. In general, plants grown at closer spacing grow taller and have smaller stem diameters than those grown further apart.
Irrigation Practices
Mild to moderate water stress can also be used to control plant growth. Mild to moderate water stress can be implemented by continuously allowing the root zone to dry and then irrigating right before the plants wilt or when early symptoms of wilt are observed.
Fertilization
Reducing or withholding fertilizer tends to slow overall plant growth. The nutrients which have the most effect on the size of greenhouse plants are nitrogen and phosphorus. The biggest effect of withholding a water-soluble fertilizer is nitrogen deficiency. Unfortunately if nitrogen deficiency conditions go on too long the plants will be too small. A phosphorous deficiency is somewhat more difficult to create than a nitrogen deficiency. However, if carefully managed a mild to moderate phosphorus deficiency will result in a desirable reduction in growth and no foliar symptoms.
Mechanical Conditioning
In addition to environmental and cultural height control options, there are also a few mechanical methods to reduce stem elongation. Pinching is a fairly common practice in many greenhouses, depending on the crop. Although pinching does reduce plant height, it is primarily used to promote branching and produce a more “fuller” plant. Brushing plants has been found to be a very effective way too in controlling plant height of many vegetable transplants or herbs.
Pinching
Pinching—removing the tip of the plant manually—is often used to increase branching, shape plants and reduce plant height. Pinching removes the apical dominance of the shoot which prevents branching. Apical dominance results from the production of auxin, a natural plant hormone by the terminal growing point and young leaves. Removing the terminal growing point and young leaves (pinching), removes the source of auxin and allows dormant buds below the pinch to grow. When and how a plant is pinched is important in relation to branching and time of flowering.
Brushing
Brushing in commercial operations is done using a boom draped with some sort of material, and then program the boom to run back and forth (See Figure 21.1). The brushing material or mechanism must be sufficiently strong and durable to manipulate the shoots in a high-density planting, especially after the treatment has caused the typical increase in stem stiffness and strength and the plants have attained some height. Generally, brushing reduces plant height, and commonly leaf area and dry weight, but increases stem and petiole strength. The goal with plant brushing is to make the plants bend without breaking leaves or stems or causing any other damage.
Light Quantity, Quality, and Photoperiod
Plant growth and developmental processes are regulated by light quantity, quality, and photoperiod. Together these three parameters strongly influence the growth of plants in the greenhouse, including plant stature, growth habits, the transition to flowering, and end point plant productivity.
Light Quantity
Light quantity refers to the intensity or concentration of sunlight and varies with the season of the year. One of the easiest ways to produce shorter plants in the greenhouse and reduce the need for PGR treatment is to maximize the amount of light plants receives to reduce stem elongation or “stretch.”
Light Quality
Light quality refers to the color or wavelength reaching the plant surface. The ratio of red to far-red light affects stretching and branching of many plant species.
Photoperiod
Photoperiod or light duration refers to the amount of time that a plant is exposed to sunlight. Natural photoperiods can be manipulated to create artificially long days or short days. Under natural long days, short days can be created by blocking out all light with black plastic or cloth.
Temperature Manipulation
Temperature is the one of the most easily and frequently modified environmental factors influencing plant growth. Day/Night Temperature (DIF) and daily average temperature are two practices often used to control plant growth in greenhouse operations of which are covered in Chapter 9, Managing Temperature in Greenhouse Crops.
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