FACTORS ENVIRONMENTAL THAT AFFECT PLANT GROWTH
MG Manual Reference Ch. 1, pp. 30 - 33
Plant growth and distribution are limited by the environment. If any one environmental factor is less than ideal it will become a limiting factor in plant growth. Limiting factors are also responsible for the geography of plant distribution. For example, only plants adapted to limited amounts of water can live in deserts. Most plant problems are caused by environmental stress, either directly or indirectly. Therefore, it is important to understand the environmental aspects that affect plant growth. These factors are light, temperature, water (humidity), and nutrition.
Light quantity refers to the intensity or concentration of sunlight and varies with the season of the year. The maximum is present in the summer and the minimum in winter. The more sunlight a plant receives (up to a point), the better capacity it has to produce plant food through photosynthesis. As the sunlight quantity decreases the photosynthetic process decreases. Light quantity can be decreased in a garden or greenhouse by using shade-cloth or shading paint above the plants. It can be increased by surrounding plants with white or reflective material or supplemental lights. Light quality refers to the color or wavelength reaching the plant surface. Sunlight can be broken up by a prism into respective colors of red, orange, yellow, green, blue, indigo, and violet. On a rainy day, raindrops act as tiny prisms and break the sunlight into these colors producing a rainbow. Red and blue light have the greatest effect on plant growth. Green light is least effective to plants as most plants reflect green light and absorb very little. It is this reflected light that makes them appear green. Blue light is primarily responsible for vegetative growth or leaf growth. Red light when combined with blue light, encourages flowering in plants. Fluorescent or cool-white light is high in the blue range of light quality and is used to encourage leafy growth. These lights are excellent for starting seedlings. Incandescent light is high in the red or orange range but generally produces too much heat to be a valuable light source. Fluorescent "grow" lights have a mixture of red and blue colors that attempts to imitate sunlight as closely as possible. They are costly and generally not of any greater value than regular fluorescent lights. Light duration or photoperiod refers to the amount of time that a plant is exposed to sunlight. When the concept of photoperiod was first recognized it was thought that the length of periods of light triggered flowering. The various categories of response were named according to the light length (i.e., short-day and long-day). It was then discovered that it is not the length of the light period but the length of uninterrupted dark periods that is critical to floral development. The ability of many plants to flower is controlled by photoperiod. Plants can be classified into three categories, depending upon their flowering response to the duration of darkness. These are short-day, long-day, or dayneutral plants. Short-day, (long nights) plants form their flowers only when the day length is less than about 12 hours in duration. Short-day plants include many spring and fall flowering plants such as chrysanthemum and poinsettia.
Long-day, (short nights) plants form flowers only when day lengths exceed 12 hours. They include almost all of the summer-flowering plants, as well as many vegetables including beet, radish, lettuce, spinach, and potato. Day-neutral plants form flowers regardless of day length. Some plants do not really fit into any category but may be responsive to combinations of day lengths. The petunia will flower regardless of day length, but flowers earlier and more profusely under long daylight. Since chrysanthemums flower under the shortday conditions of spring or fall the method for manipulating the plant into experiencing short days is very simple. If long days are predominant, a black plastic sheet is drawn over the chrysanthemum for 12 hours daily to block out light until flower buds are initiated. To bring a long-day plant into flower when sunlight is not present longer than 12 hours artificial light is added until flower buds are initiated.
The percentages of reflected light at 645 nm and 735 nm (relative to incoming sunlight at the same wavelengths) differed over the various surface colors (Table I). The FR/R ratio over the white surface was the same as that of incoming sunlight. of All the other colors used in this study reflected FR/R ratios greater than occurred in incoming sunlight. Because of earlier studies with the Beltsville Spectrograph (13), as well as other investigations of plant responses to population density under field conditions (11, 12), we hypothesized that even small differences in the FR/R ratio over the variously colored mulch surfaces could have significant
801 Plant Physiol. Vol. 91, 1989
impact on plant development. If true, a higher FR/R ratio (as was found over the green surfaces, Table I) should signal the plants to develop longer stems, thinner leaves, and perhaps a more efficient light harvesting system. Effects on Plant Size Plants were shortest when grown over white surfaces (Table I) and almost as short when grown over yellow (note that white and yellow surfaces reflected the lowest FR/R ratios). These responses are consistent with previous observations with soybean (10) and southern pea [Vigna unguiculata (L.) Walp.] (15). The plant height differences in the present study were due to internode length since leaf number per plant did not differ significantly (data not shown). Leaves were thickest on plants grown over white and thinnest on plants grown over green surfaces, as shown by weight per leaf area (Table I). These results are consistent with earlier controlled environment studies in which a low FR/R ratio resulted in the development of thicker leaves than were developed
plants that received a higher FR/R ratio during development (12, 16). The data support the concept that the FR/R ratio in upwardly reflected light may influence plant development in the same manner as the FR/R ratio received in different plant population densities.
on
Chlorophyll, the main photosynthetic pigment of plants, absorbs mainly blue and red wavelengths from the Sun and reflects green ones, and it is this reflected light that gives plants their leafy color. Light can be split into red, green, and blue. Plants are green because they reflect the green portion of the spectrum, but consume the blue and red portions. Perhaps photosynthesis uses blue and red portions of spectrum, making a light spectrum that favors red and blue portions of the spectrum the best for plant growth.
rocedures
1. Gather materials and carefully poke three holes in the bottom of each of the 26 cups with a pencil for water drainage. 2. Use the measuring cup and put 100ml of seed starting potting soil in each Styrofoam cup. 3. Then place a string bean seed in the center of each cup and fill the cup with 50 more ml of soil. 4. After that, sprinkle 5ml of purified water in each cup and divide the 26 cups into four groups of six. 5. Set each group of cups into different water trays and label the cups A1, A2, B1, B2 and so on, then label the trays A-red, B-blue, C-yellow, and D-clear. 6. Put a sheet of plastic wrap over each tray and place in the compartment of the grow box that bulb matches the label (dont turn the light on). 7. Let the trays sit in the grow box with the cover down until the plants start to germinate and you can see the seeds sprouting. 8. Then you turn the light on at 7:00 in the morning and turn it off at 10:00 at night every day for 18 days. 9. But at 3:00 each day you need to measure each plant by setting the ruler vertically at the base of the soil and measure to the highest point of the plant. 10. Then record your data. Measure each plant everyday at around the same time. 11. Also, everyday you need to sprinkle 5ml of purified water on each plant. 12. At the end of the eighteen days you can throw the plants away and clean out the grow box and the materials. 13. To find the average growth of the plants you would add together the highest height of all the plants in one group and divide by six to find the average growth. 14. Find the average to all groups and find out which color of light bulb works the best.
Results
The original purpose of this experiment was to determine which colored light bulb would increase the growth rate of string beans the most after eighteen days of growing.
The results of this experiment were that the plants growing under the clear light bulb grew the highest out of the plants under the red, blue and yellow light bulb. The average growth for the clear light bulb was 32cm, the yellow came in second with 28cm and the red got third place with an average of 23cm. The plants under the blue colored light bulb grew the least; the average for the blue was 17cm. Therefore, the best light bulb to use is the clear light bulb.