|MadSci Network: Botany|
Ultimately, when a plant flowers is determined by its genetics. However, its environment, particularly light and temperature, often can promote or delay flowering. Plant age also often has a major effect on the ability of a plant to flower. Applications of certain plant hormones promote flowering in a few species. Other factors that can promote flowering are low-nitrogen fertilizer and small pot size, termed a potbound or rootbound condition, of potted plants such as African violets. Many trees will not flower until they are 5 or 20 years old. Beech (Fagus sylvatica) may need to reach 40 years old before it will flower. The period during which a plant cannot flower is termed its juvenile phase. When it can flower, it is in its mature or adult phase. Some familiar houseplants, such as common philodendron and pothos, are in the juvenile phase and rarely enter their mature phase under home conditions. The century plant (Agave americana) doesn't actually take a century until it blooms. However, it takes at least 10 to 15 years, sometimes much longer, and each shoot blooms just once, then dies. The effect of daylength, actually nightlength, on flowering is termed photoperiodism. Plants are classified into three main photoperiod groups. Day neutral plants are not affected by daylength. Short day plants flower only when the daylength is less than a critical daylength for that variety or species. Long day plants flower only when the daylength is greater than a critical daylength value for that variety or species. Sunflower, pea and corn are day neutral. Chrysanthemums and poinsettia are short day plants. Spinach and radish are long day plants. Different varieties of the same species may be in different photoperiod groups. The plant pigment that senses daylength is phytochrome. The photoperiod category of a plant can change depending on the temperature. For example, poinsettia is a short day plant under warm conditions but long day plant under cold conditions. There are also several other photoperiod groups. For example, there are intermediate day plants, such as coleus, that flower when days are neither too short nor too long. Plants use the daylength as a calendar so they flower in the appropriate season. Plant flowering is often given a time classification for cultivation purposes. Seed catalogs and vegetable seed packages have a cropping time in days which can be an indication of how soon they flower, if the crop produces a fruit, such as tomato or corn. Even chrysanthemums and poinsettia varieties are classified by the number of weeks it takes them to flower once they are exposed to the required photoperiods. There are 10, 11 and 12 week varieties of chrysanthemums for example. Biennial plants grow from seed the first season, overwinter as a rosette of leaves, then flower in the spring of the second season. Carrot, some beets, cabbage, celery, hollyhock, foxglove, and money plant or honesty are biennials. They require a cold treatment before they will flower. Many also require long days after the cold treatment in order to flower. A cold treatment that promotes flowering is termed vernalization. Winter wheat is planted in fall or winter so it can be vernalized by the naturally cold temperatures allowing it to flower the next summer. Botanists long ago speculated that there was a flowering hormone that stimulated a plant to flower. They even gave it the name florigen. However, no chemical has yet been identified as florigen. Many books and websites incorrectly state that florigen actually exists but we are not really sure. There are grafting experiments that show that something that promotes flowering is transmitted across a graft union from a plant induced to flower to a vegetative plant. The transmitted chemical or chemicals stimulates the vegetative plant to flower. Therefore, there is evidence that a chemical or combination of chemicals acts as a flowering hormone. The frustrating part is that despite the evidence no specific chemical(s) has been identified as florigen. Some plant hormones do promote flowering in a few species. For example, the hormone ethylene promotes flowering in pineapple and other bromeliads. Home gardeners who grow pineapple tops often enclose the plant and apple in a plastic bag. The apple gives off ethylene. Treating a biennial rosette, such as carrot, with the plant hormone gibberellic acid stimulates it to flower. The gibberellic acid substitutes for vernalization. Application of gibberellic acid prevents flowering in other species, such as strawberry and fuchsia. References Photoperiodism and Phytochrome Agave americana Florigen How to grow a pineapple at home EFFECT OF GIBBERELLIC ACID (GA3) ON SEEDSTALK DEVELOPMENT AND FLOWERING IN CARROT (DAUCUS CAROTA L. VAR. SATIVA DC) Taiz, L. and Zeiger, E. 1991. Plant Physiology. New York: Benjamin/Cummings. Salisbury, F.B. and Ross, C.W. 1985. Plant Physiology. Belmont, CA: Wadsworth.
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