21.1 Responses in Flowering Plants Plants change their growth and physiology in response to environmental stimuli. –Light –Gravity –Seasonal changes Plants also have hormones that help control plant responses to these stimuli.
Plant Hormones There are five recognized plant hormones. – Auxins – Gibberellins – Cytokinins – Abscisic acid – Ethylene There are also man-made analogs of these chemicals that can also regulate plant responses to the environment.
Auxins Auxins such as indoleacetic acid (IAA) are the most common plant hormone. Auxins are produced in the shoot apical meristem. Auxins are found in young tissues, including young leaves, flowers, and fruits.
Auxins (cont.) Auxins control aspects of plant growth and development. –Auxins play a key role in cell elongation. –Auxins are responsible for apical dominance. –Application of auxins can induce rooting of woody plant cuttings. –Auxin production by seeds enhances the maturation of fruit.
Gibberellins The primary effect of gibberellins (GAs) is a stimulation of stem elongation. GAs are involved in seed germination. –GAs reverse the dormancy of the embryo. –GAs also stimulate the production of amylase, which helps breakdown the stored endosperm to provide the embryo with sugars for energy.
Cytokinins Cytokinins are found in actively dividing tissues where they promote cell division. Cytokinins can also slow the process of senescence in leaves. Cytokinins interact with auxins to coordinate the development of plant roots and shoots.
Abscisic Acid Abscisic acid (ABA) is produced by all green plant tissues. ABA has two main functions in plants. –ABA is involved in seed and bud dormancy. –ABA contributes to the closure of stomata.
Abscisic Acid (cont.)
Ethylene Ethylene is a gaseous plant hormone. –A primary effect of the gaseous hormone ethylene is the abscission of leaves. –Ethylene is also involved in the ripening of fruits like apples and bananas.
Environmental Stimuli and Plant Responses Plant response are strongly influenced by environmental stimuli. –Light intensity and wavelength –Day length –Gravity –Touch The speed of the plants response to a stimuli varies from very rapid to slow changes in growth.
Plant Tropisms A tropism involves directional growth of a plant towards or away from a stimulus. – Phototropism involves movement in response to light. – Gravitropism is movement in response to gravity. – Thigmotropism is movement in response to touch. Tropisms typically involve differential patterns of growth induced by auxins.
Plant Tropisms (cont.)
Photoperiodism Flowering in plants is a response to seasonal changes in photoperiod. –Short-day/long-night plants flower when the night length is longer than the critical length. –Long-day/short-night plants flower when the night length is shorter than the critical length. –The flowering of day-neutral plants is not influenced by photoperiod.
Phytochrome and Plant Flowering Plants sense the photoperiod with a pigment called phytochrome. Besides its role in flowering, phytochrome plays other roles in plants. –Sensing light conditions during germination –Influencing leaf expansion and stem branching
21.2 Sexual Reproduction in Flowering Plants The plant cycle is an alternation of generation between different multicellular forms of the plant. –The diploid, spore-producing sporophyte –The haploid, gamete-producing gametophyte
Overview of the Plant Life Cycle For flowering plants, the sporophyte is the dominant, flowering-producing generation. Flowers produce two kinds of spores. – Microspores that develop into the male gametophyte (a pollen grain) – Megaspores that develop into the female gametophyte (an embryo sac) Flower pollen carries sperm to the flower egg in the embryo sac.
Overview of the Plant Life Cycle (cont.) After the sperm in the pollen has fertilized the egg, an embryo develops within the flower. The structure that houses the embryo becomes the seed.
Overview of the Plant Life Cycle (cont.)
Flowers Flowers serve several important functions. –Production of spores –Protection for gametophytes –Attraction of pollinators –Pollen dispersal Monocots have floral structures in multiples of three while eudicots have floral structures in multiples of four or five.
Flowers (cont.) Flowers are comprised of four whorls of modified leaves attached to a receptacle. –The sepals protect the developing flower. –The petals are colored to attract pollinators. –The male portion of the flower, consisting of the stamen, anther, and filament. –The female portion of the flower, or carpel, consisting of the stigma, style, ovary, and ovules.
From Spores to Fertilization The flower anther produces male microspores, which divide mitotically to form pollen. The pollen is released from the anther. Within the ovule, female megespores undergo mitosis to produce the egg.
From Spores to Fertilization (cont.) During pollination, a pollen grain is transported to the stigma. The pollen tube germinates and extends a pollen tube to the ovule. The pollen tube delivers two sperm to the egg to carry out double fertilization.
From Spores to Fertilization (cont.) The sperm are involved in two fusion events. –One sperm fuses with an egg to form a diploid zygote. –One sperm fuses with two other ovule cells to form the triploid endosperm. The ovule develops into the seed, bearing the embryo and the stored nutrients.
Development of the Seed in a Eudicot Eudicot seeds have three main parts. –The seed coat is a protective layer. –The endosperm provides a food reserve. –A plant embryo is present. As the seed matures, the embryo undergoes a specific series of developmental changes before the plant axis develops.
Development of the Seed in a Eudicot (cont.)
Fruit Types and Seed Dispersal There is great diversity in the types of fruits produced by plants. –Fruits can be dry or fleshy. –Fruits can be simple, as for cereal grains. –Nuts can have a hard shell that surrounds a single seed. –Legumes are fruits with several seeds.
Dispersal of Seeds Once produced, seeds must be dispersed in order to germinate. –Some seeds have hooks that allow the seed to cling to the fur of animals. –Some seeds must pass through the digestive tract of animals before they can germinate. –Some seeds are dispersed by wind or water. –Some seeds are dispersed in a projectile-like fashion.
Germination of Seeds Seed germination is a programmed developmental process during which the embryo breaks dormancy and continues its development. Seed germination only occurs when sufficient moisture, temperature, and oxygen is present to sustain growth.
Germination of Seeds (cont.)
21.3 Asexual Reproduction in Flowering Plants Because plants have nondifferentiated meristem tissues, they are totipotent, meaning that a single cell can be used to regenerate an entire plant in tissue culture. Specific combinations of plant hormones are needed to control this development. These totipotent cells or fragments or the plant can be used to vegetatively (asexually) propagate plants.
21.3 Asexual Reproduction in Flowering Plants (cont.)
Genetic Engineering of Plants Hybridization can occur if two plants from different species are successfully crossed to produce a hybrid. Genetic engineering can be used to transfer genetic information from another organism to plant cells, creating a transgenic plant.
Agricultural Plants with Improved Traits Genetic engineering has been used extensively to improve agricultural plants. –Increased pest resistance –Resistance to certain herbicides –Tolerance to toxic elements and salinity –Increased disease resistance –Increased nutritional content –Increase yield and productivity
Commercial Products Genetically engineered plants have also been created to produce a variety of specific products. –Human hormones –Clotting factors –Antibodies –Vaccines