14.4 The Life Cycle of Flowering Plants Biology 1001 November 28, 2005

Photoperiod and the Control of Flowering The events of the life cycle, such as flowering, seed germination and leaf growth, need to be appropriately timed with respect to season The environmental stimulus that plants use to detect the time of year is photoperiod, or the relative lengths of night and day A physiological response to photoperiod, such as flowering, is called photoperiodism

The Effect of Photoperiod on Flowering On the basis of photoperiod requirement for flowering there are three types of plants “Long-day” plants flower early in the season, in the spring or early summer, when the days are long  “Long-day” plants are really short night plants  Examples include radishes and irises “Short-day” plants flower late in the season, in late summer, fall or early winter, when the days are short  “Short-day” plants are really long night plants  Examples include poinsettias, chrysanthemums, and strawberries “Day-neutral” plants flower at a certain stage of maturity regardless of photoperiod  Examples include tomatoes and dandelions

How does interrupting the dark period with a brief exposure to light affect flowering? “Short-day” plants require a period of darkness greater than a critical period “Long-day” plants require a period of darkness less than the critical period A flash of light given in the night to a short-day plant inhibits flowering A flash of light given in the night to a long-day plant promotes flowering Figure 39.22!

Phytochrome is the pigment that intercepts light A unique characteristic of the pigment phytochrome is its reversibility in response to red and far-red light Red light is the most effective at interrupting night Because this effect is reversible by a flash of far- red light it is though that the pigment involved in photoperiod is phytochrome Figure 39.23!

Figure 39.24! – Is there a flowering hormone? It is leaves that detect photoperiod and transmit a signal to flower to buds The flowering signal is a hormone or combination of hormones, as yet unidentified, called florigen A bud’s meristem changes from a vegetative state to a flowering state – involves expression of a set of organ identity genes Flowering photoperiod Non-flowering photoperiod

Fruits Develop From Ovaries, Figure 38.9! A fruit is a mature ovary that protects the seed and aids in its dispersal The fruit begins to develop after pollination The wall of the ovary becomes the pericarp, the thickened wall of the fruit FertilizationFertilization clip Fruit developmentFruit development clip

FRUITS ~ Some variation in fruit structure ~~ Some variation in fruit dispersal ~

SEEDS Seeds are mature ovules that contain the multicellular plant embryo, the endosperm and a protective coat

SEED DORMANCY & GERMINATION As a seed matures, it dehydrates and enters dormancy Breaking dormancy requires an environmental stimulus such as fire, abrasion, rainfall, digestion Dormancy increases the chances that germination occurs under ideal conditions Germination begins with the imbibition of water which causes the seed coat to rupture and triggers the embryo tor resume growth Enzymes digest the storage material in the endosperm The embryonic root is the first organ to emerge, the the shoot breaks the soil surface Figure 38.10!!

Figure 30.10!,!! - The life cycle of an angiosperm

Figure 38.2!,!! – An Overview of Angiosperm Reproduction