Angiosperm Reproduction The Flower, Pollination, the Seed, and Photoperiodism
Sexual Reproduction in Angiosperms • Gametes are delicate single cells. For two plants to cross fertilize, there must be a mechanism for the two gametes to reach each other safely. • There must also be a mechanism to disperse their offspring far enough away from the parent so that they do not have to compete with the parent for light, water, and soil minerals. • The functions of the flower solve both of these problems.
Pollination • The act of pollen (male gamete) being transferred from anther to stigma • Usually accomplished by wind or animals
The Flower • The flower consists of 4 concentric whorls of tissue • a whorl of sepals(collectively called the calyx) • a whorl of petals (collectively called the corolla) • stamens in which the microsporangia (sperm) form • carpels in which the megasporangia (eggs) form.
Anatomy of the Flower Non-reproductive Structures • Sepals: protection • Petals: attracts pollinators • Pedicel: connects flower to stem
Anatomy of the Flower • Male structures • Anther: produces pollen • Filament: supports and lifts anther • Stamen = anther + filament • Female structures (collectively known as carpels) • Stigma: attachment site for pollen • Style: connects stigma to ovaries; has pollen tube • Ovary: location of ovules
Sperm vs. Egg • Male gamete • Haploid • Produced in anthers • Called microsporangium or pollen • Each pollen grain has two cells • Germ cell • Tube cell • Female gamete • Haploid • Produced in ovary • Called megasporangium or ovule • Each ovule consists of 8 nuclei
Fertilization: The fusion of a male gamete (pollen) with female gamete (ovule) to produce a zygote
Dicot Seed Anatomy • 5-15% water • Can remain dormant and viable up to 300 yrs(species dependent) • Seed Coat: • testa • Embryonic Leaf: • plumule • Embryonic Root: • radicle
Conditions of Germination For successful germination seeds require a combination of … • oxygen for aerobic respiration • water to metabolically activate the cells • temperature for optimal function of enzymes Each seed has its own particular combination of the above three factors.
In addition particular species need other more specialized conditions such as: • fire • freezing • passing through digestive system of a seed dispersing animal • washing to remove inhibitors (beans) • erosion of the seed coat (Poppy) The particular conditions required by a seed allows it to match germination to favourable conditions
Seed Germination • Rehydration of cells stimulates metabolism (enzymatic pathways startup) • Gibberellin (hormone) produced in cotyledon • Gibberellin stimulates amylase production • Starch is digested in maltose • Maltose is transported to radicle and plumule • Maltose converted to glucose, used for cellular respiration or cellulose production
Photoperiodism • Many plants have the ability to measure the length of the sun cycle. • This allows them to schedule their flowering period for a specific time of year • Experiments have proven that plants measure night length, not day
Photoperiodism • Long Day Plants (LDP) • Plants will only flower after a minimum amount of daylight is achieved. • Typical of plants living in higher altitudes • Usually during spring or summer • Short Day Plants (SDP) • Plants that only flower when the day is shorter than a maximum time period • Usually during fall or winter
Photoperiodism also explains why some plant species can be grown only in a certain latitude. • Spinach, a long-day plant, cannot flower in the tropics because the days never get long enough (14 hours) • Ragweed, a short-day plant, fails to thrive in northern Maine because by the time the days become short enough to initiate flowering, a killing frost in apt to occur before reproduction and the formation of seeds is completed.
Phytochrome System: • The receptor of photoperiod is located within the leaf. • The cellular location of the receptor is unclear. • The chemical nature of the receptor is a the molecule phytochrome. • Phytochrome can be converted from one form to another by different types of light.
Flowering in SDP Short day plants flower when the night period is long. • In day light or red light, Phytochrome Red (Pr) is converted to Phytochrome Far Red (Pfr). • The conversion actually only requires a brief exposure to white or red light. • In the dark, Pfr is slowly converted back to Pr. • A long night means that there is a long time for the conversion. • Under short day conditions (long night) at the end of the night period the concentration of Pfr is low. • In SDP, low Pfr concentration is the trigger for flowering.
Flowering in LDP: Long day plants flower when the night period is short. • In day light (white or red) the Pr is converted to Pfr. • During periods when the day light period is long but critically the dark period is short, Pfr does not have long to breakdown in the dark. • Consequently there remains a higher concentration of Pfr. • In LDP, high Pfr concentration is the trigger to flowering.