Flowering
Light perception is often involved in the control of flowering. When plants flower at a particular time of year it is usually because they respond to day-length. Not all plants are regulated in this way:"day neutral" plants will flower at any time that they are able to grow. So-called "free flowering" garden plants like Petunia and Impatiens have been selected for this feature. Spring- or fall-flowering plants like strawberry and chrysanthemum tend to be short-day plants, whereas summer-flowering plants like coneflower and many grasses tend to be long-day plants."Long and short day" are misleading terms on two counts:

• Both may flower with the same actual day-length. The long-day plant requires that the day should be longer than a critical period. The short day plant requires that it should be shorter than a critical period.
• It would be more accurate to call them"short night" or"long night" plants, since it it the length of night that is important rather than the length of day.

AboveStrawberry, a short-day (or"long-night"?) plant.
Right Long-day plants like Rudbeckia can often be induced to flower at the seedling stage by exposure to continuous light

In short-day plants flowering is inhibited by the Pfr form of phytochrome

The night needs to be long enough for Pfr accumulated during the day to revert to Pr or be broken down altogether. If the nights are interrupted by a brief period of low-intensity light Pr will be converted back to Pfr and the plant will not flower.

In long-day plants flowering is promoted by Pfr.

If the nights are too long Pfr disappears and flowering is not induced. In this situation"night breaks" of low-intensity light will restore Pfr and flowering can occur even though the plant is growing in short days.

Flowering is also regulated by temperature. In herbaceous perennials and biennials, trees and shrubs that grow in temperate areas, flower buds develop or become fully differentiated during the winter months. This cold requirement for flowering is exploited in vernalization treatments where plants like Easter lilies are exposed to low temperatures in order to induce flowering at a particular time. In some plants flowering can be induced either by long days or by cold exposure, whereas others require both for optimum flower development. Treatments with gibberellins can often be substituted for the photoperiodic (long or short day) or temperature requirements.

Fruit and seed development
Fruit development is usually dependent on a signal from the developing seeds, although some plants such as banana can develop parthenocarpic fruit that lack seeds. Parthenocarpic fruit set can be induced in many species by auxin, GA or cytokinin or some combination of these hormones.

Early seed development is associated with cell division and synthesis, so that immature seeds contain hormones associated with growth, auxin, GA and cytokinin. As seeds mature they usually begin to desiccate, abscisic acid increases and dormancy sets in.

Senescence
Although we may find it undesirable, senescence is a natural part of plant development, and, like other aspects of development it is under genetic and hormonal control. Patterns of senescence vary from one plant to another.

• In monocarpic senescence the whole plant dies after seed formation. This is frequently observed in annuals (therophytes in Raunkiaer's terminology) and biennial plants, but some perennials such as the century plant Agave americana show monocarpic senescence.
• Senescence of all above ground plant parts is a feature of many herbaceous perennials (or geophytes and hemicryptophytes). An underground storage organ of some kind persists in a dormant state and produces new above ground structures in the following season.
• Woody plants (phanerophytes) in temperate regions often show a deciduous pattern of senescence in which all of the leaves die at the same time as the meristems become dormant. The leaves are replaced by a whole new set at the end of dormancy in the spring.
• Sequential senescence is a feature of evergreen plants, particularly those that grow throughout the year. Leaves are continuously produced and shed in order of age.

The"century plant" Agave americana takes about 30 years (not 100) to grow to maturity and flower. It then dies after setting seed

However senescence occurs, the underlying changes are very similar. There is a switch from synthesis to breakdown of cell structure. Photosynthesis declines as the chloroplast becomes a chromoplast. Proteins and other polymers are broken down by digestive enzymes. In perennial plants some of the amino acids and other small molecules are withdrawn from the leaves before they are shed. In this way the plant recovers some of its investment.

Senescing plant parts often produce ethylene and senescence is generally promoted by ethylene or ethylene-releasing compounds. One of the major uses of ethephon is to promote senescence of tobacco leaves prior to harvest.

Abscission
Like senescence, abscission is a natural and necessary part of plant development. Plant parts like flower petals, mature fruits and senescent leaves are detached form the plant along a clearly defined abscission zone. This zone is differentiated early in development; usually it is a region of smaller cells at the base of the petiole, petal or pedicel. Prior to abscission cell-wall degrading enzymes are secreted into the zone; the middle lamella is broken down between cortical parenchyma cells. The secondary walls of xylem elements are resistant to these enzymes and these cells simply break when the others have separated. Ethylene produced by the senescing organ stimulates the synthesis of cellulase and pectinase; ethylene (or ethrel) treatment can be used to promote abscission. This is particularly useful for mechanically harvested crops like coffee or for thinning excess crop on fruit trees.

This cabbage was stored in the same room as apples, which produce large amounts of ethylene. The ethylene stimulated abscission of the leaves from the stalk, even though they were immature.

Dormancy
In perennial plants senescence of some of the plant structure is associated with dormancy in the over-wintering structure. Dormancy is a phase in which cell division and cell expansion are suspended. Plants can stop growing for various reasons without being really dormant; perhaps the temperature is too low or there is not enough water light or nutrients for growth. In these situations growth can be restored by correcting the environmental problem.

In true dormancy plants will not grow even if they are given optimum environmental conditions. Dormancy sets in at the end of the growing season as the days get shorter and temeperatures fall. The phytochrome system is a part of the control mechanism; low Pfr levels a decline in GA and rise in ABA are all associated with the onset of dormancy.

To emerge from dormancy plants generally require a period of low temperature, weeks in the case of herbaceous perennials to months for many trees. After this cold requirement has been met the plant will grow again when environmental conditions become favorable.

In temperate and colder regions dormancy is associated with cold-hardening of above ground structures. While dormancy is a prerequisite for hardening it is not necessarily associated with it. The overwintering structures of geophytes are protected by being underground and are not particularly cold hardy. Dormancy also occurs in some tropical plants as a means of survival during dry rather than cold seasons.

Copyright © Michael Knee,
The Ohio State University
All rights reserved.