GROWTH AND DEVELOPMENT

Transcription

1 CHAPTER IX Stages of Growth and Development VEGETATIVE GROWTH AND DEVELOPMENT 4/6/2008 DMA: Chapter 9 Hartmann's Plant 1

2 Shoot and Root Systems The root system and the shoot system tend to maintain a balance: As the top of the plant grows larger and larger, the leaf area increases and water loss through transpiration increases. This increased water loss is made up by water absorption from an increasing water system. The enlarging shoot system also requires greater amounts of mineral that are absorbed by the increasing root system 4/6/2008 DMA: Chapter 9 Hartmann's Plant 2

3 Definitions and Measurements Growth can be measured as increases in fresh weight or dry weight, or in volume, length, height, or surface area. Plant growth is a product of living cells, with all their myriad metabolic processes. We generally think of growth as an irreversible increase in volume or dry weight. Plant growth: size increase by cell division and enlargement, including synthesis of new cellular material and organization of subcellular organelles. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 3

4 Plant shoot growth can be classified as: 1. Determinate growth: after a certain period of vegetative growth, flower bud clusters form at the shoot terminals so that most shoot elongation stops. 2. Indeterminate growth: bear the flower clusters laterally along the stems in the axils of the leaves so that the shoot terminals remain vegetative and the shoot con-tinues to grow until it is stopped by senescence or some environmental influence. Example: grapevines. The determinate, bush-type plants produce much less vegetative growth than do the indeterminate type. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 4

5 Shoot Growth Patterns: Annuals, which are herbaceous (nonwoody) plants, complete their life cycle (seed to seed) in one growing season. Biennials, which are herbaceous plants, require two growing seasons (not necessarily two years) to complete their life cycle (seed to seed). Example: cabbage Perennials, which are herbaceous or woody plants, lives for more than two growing seasons. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 5

6 Annual Plants 4/6/2008 DMA: Chapter 9 Hartmann's Plant 6

7 Biennial Plants Stem growth is limited during the first growing season. The plants remain alive but dormant through the winter. Exposure to chilling temperatures triggers hormonal changes leading to stem elongation, flowering, fruit formation, and seed set during the second growing season. ** Most annual and biennial plants flower and fruit only once before dying. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 7

8 Perennial Plants In herbaceous perennials, the roots and shoots can remain alive indefinitely but the shoot system may be killed by frosts in coldwinter regions or by senescence-inducing factors. Shoot growth resumes each spring from latent or adventitious buds at the crown of the plant (Figure 9-5) 4/6/2008 DMA: Chapter 9 Hartmann's Plant 8

9 In woody perennials, both the shoot and root system remain alive indefinitely. Shoot growth of temperate zone plants takes place annually during the growing season, as indicated by Figure 9-6, 4/6/2008 DMA: Chapter 9 Hartmann's Plant 9

10 Root Growth Patterns In deciduous woody perennials, root growth peaks in the spring and again in late summer or early autumn. Reasons: 1. The spring flush of root growth re-sults from the accumulated foods stored in the tree the previous year. 2. When this source is depleted, root growth slows 3. Following gradual accumulation of carbohydrates from photosynthesis through the summer, root growth again increases in the autumn. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 10

11 Genetic Factors Affecting Plant Growth and Development The organism developing by cell division and elongation from the fertilized egg the zygote in every case is under the genetic control of the genes inherited from the parents at the time of fertilization. The genes direct the form and shape of the organism. At any given time, some of the organism's genes are transcriptionally active, while others are silent. The control of gene activity depends on : 1. The cell type, 2. Environmental conditions or, 3. The particular stage of development. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 11

12 Environmental Factors Influencing Plant Growth and Development Light The sun is the source of energy for photosynthesis and other plant processes, Light quality Wavelengths of 400 to 700 nm are commonly referred to as visible light or photosynthetic active radiation (PAR) (the most important to life on earth). 4/6/2008 DMA: Chapter 9 Hartmann's Plant 12

13 Photomorphogenesis describes several highly integrated processes: 1. Seed germination in light-sensitive seeds. 2. De-etiolation (the greening of young seedlings when they emerge from the soil), 3. Stem growth in plants competing for light with other plants. Most photomorphogenesis responses are regulated by the phytochrome pigment system 4/6/2008 DMA: Chapter 9 Hartmann's Plant 13

14 Phototropism Phototropism is a photomorphogenic response of plants to the direction of light. A blue light receptor called phototropin is responsible for sensing the direction of light. The bending in positive phototropic re-sponses is due to increased cell growth on the side away from the light source. It is believed that the plant hormone auxin accumulates on the shaded side, promoting cell expansion 4/6/2008 DMA: Chapter 9 Hartmann's Plant 14

15 Photoperiodism Photoperiodism is the photomorphogenic response to seasonal variations in the amount of daylight. Numerous aspects of plant growth and development are controlled by photoperiod including: 1. Flowering (which will be discussed in more detail later in this chapter); 2. Induction of bud dormancy in woody species 3. The formation of vegetative propagules such as bulbs, tubers, corms, and runners (stolons). 4/6/2008 DMA: Chapter 9 Hartmann's Plant 15

16 All photoperiodically controlled processes can be categorized into three basic response types: 1. Long-day plants (LDPs); 2. Short-day plants (SDPs); and 3. Day-neutral plants (DNPs); which are photoperiodically The designation as a long- or short-day plant is not based on the absolute length of the day, but rather if the photoperiodically controlled process is induced only at daylengths longer or shorter than specific daylength, called the critical day length (CDL). 4/6/2008 DMA: Chapter 9 Hartmann's Plant 16

17 Long-day plants : A plant with photoperiodically controlled process that is induced only when the day length is longer than the CDL. Short-day plants A plant with photoperiodically controlled process that is induced only when the day length is shorter than the CDL. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 17

18 No direct relationship exists between the response type and the absolute length of the CDL. For example: - Red clover is an LDP with a CDL of twelve hours, Garden chrysanthemum, which is an SDP with a CDL of fifteen hours, 4/6/2008 DMA: Chapter 9 Hartmann's Plant 18

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20 Temperature All plants have optimal temperatures for maximum vegetative growth and flowering. Most temperate-region plants grow between temperatures of 4 C and 50 C, but these are generally the limits of plant growth. - The high temperatures destroy the protoplasm of most cells; - At the low temperatures, most plants just fail to grow because of a lack of cell activity. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 20

21 Low, non-freezing temperatures (0 C to 10 C) are sometimes used by plants as cues to coordinate growth and development with the changing seasons. Examples of cold-induced processes include: 1. Seed germination. Some seeds require a period of time during which the seeds are imbibed at low temperatures (stratification) before germination is possible. 2. Flowering. The cold induction of flowering is called vernalization. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 21

22 3. Dormancy breakage. The duration required for complete loss of dormancy is called the chilling requirement. 4. Acquisition of cold and freeze tolerance. For many herbaceous perennials, the low, nonfreezing tempera-tures (0 C to 10 C) that frequently occur during autumn nights induce the physiological processes responsible for the ability to survive the freezing temperatures of winter. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 22

23 Water Most growing plants contain about 90 percent water. It is stored in various plant tissues and It is used as one of the raw materials for photosynthesis. When the light from the sun is strong, the leaves tend to lose large quantities of water by transpiration and, additionally, the soil loses water by evaporation. Substantially more soil water is lost from transpiration by plants than by evaporation from the soil. The total soil water loss by both means is called evapotranspiration. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 23

24 If more water is lost through the stomata than the roots can supply, the plant wilts. Wilted plants eventually die if they cannot recover enough soil water to regain their turgidity. Herbaceous plants may wilt slightly at midday or later on a bright sunny day but they usually recover during the night. Deciduous fruit plants often fail to recover from this water loss. While the plant is wilted, the stomates are closed, cutting off the intake of CO2 for photosynthesis, and thus reducing carbohydrate manufacture. The quality of soil water, as determined by the quantity of minerals and salts dissolved in the water, is very important to the growth of plants. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 24

25 Gases The two gases most important to the growth of green plants are 1. Oxygen (O 2 ) and 2. Carbon dioxide (CO 2 ). Carbon dioxide is the third most abundant gas in the atmosphere: Nitrogen is approximately 78% O2 is approximately 21% CO2 is approximately 0.035% 4/6/2008 DMA: Chapter 9 Hartmann's Plant 25

26 Plants do most of the work to keep our atmosphere in a favorable balance. However: Atmospheric CO 2 rose 20 to 30 ppm during the 1900s. This elevation is believed to be enough to contribute significantly to global warming. Global warming results from atmospheric gases, especially CO 2, which trap heat at the earth's surface. The phenomenon is sometimes referred to as the greenhouse effect. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 26

27 Phase Change: Juvenility, Maturation, Senescence A newly emerged seedling pass through: Embryonic growth Juvenility Phase change (transition phase) Maturity or adult phase; Senescence and Death. The juvenile phase is characterized by the inability to reproduce sexually; (cannot flower). 4/6/2008 DMA: Chapter 9 Hartmann's Plant 27

28 The duration of the juvenile phase varies from a week or two up to thirty or forty years in some tree species. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 28

29 Aging and Senescence The life spans of the different kinds of flowering plants differ greatly, ranging from a few months to thousands of years. Olive trees with huge trunks found in the eastern Mediterranean area are believed to be several thousand years old (see Fig. 9-9). 4/6/2008 DMA: Chapter 9 Hartmann's Plant 29

30 Senescence is considered to be a terminal, irreversible deteriorative change in living organisms, leading to cellular and tissue breakdown and death of 1. Annual plants (population senescence) and of 2. Individual plants (whole plant senescence), 3. Leaves, seeds, flowers, or fruits (organ senescence). 4/6/2008 DMA: Chapter 9 Hartmann's Plant 30

31 Senescence is usually considered to be due to: 1. Inherent physiological changes in the plant, or 2. Pathogenic attack or 3. Environmental stress. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 31

32 REPRODUCTIVE GROWTH AND DEVELOPMENT Fruit and seed production involves several phases: 1. Flower induction and initiation 2. Flower differentiation and development 3. Pollination 4. Fertilization 5. Fruit set and seed formation 6. Growth and maturation of fruit and seed 7. Fruit senescence 4/6/2008 DMA: Chapter 9 Hartmann's Plant 32

33 Flower Induction and Initiation In some species, the formation of flowers is influenced by: Daylength (photoperiodic effect) and/or Low temperatures (vernalization), Flowering in most trees have neither response to daylength nor cold temperatures. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 33

34 Photoperiodism (Daylength) Figure 9-10 demonstrates how a flash of light (or night break) of sufficient intensity or duration Inhibits flowering of a short-day plant (long-night plant) Induce flowering of a long-day plant (LDP). 4/6/2008 DMA: Chapter 9 Hartmann's Plant 34

35 This information has been useful to commercial chrysanthemum growers who grow these short-day plants on a year-round schedule When they want the young plants to reach a size adequate for flowering, the growers use fluorescent lamps over the chrysanthemum plants (near midnight), each night for one to four hours, depending on time of year and latitude. This night break inhibits flowering until the plants reach the desired height. Conversely, when the natural daylight of summer is too long for chrysanthemum plants to flower, they cover the plants of proper size with black cloth or plastic each evening about 6 P.M. and remove it in the morning at about 8 A.M. This shortens the plant's day (lengthens the night) enough to induce and fully develop the flowers.. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 35

36 Pollination Pollination is defined as the transfer of pollen from an anther to a stigma. The anther and stigma may be in: Same flower (self-pollination), Different flowers on the same plant (self-pollination), Different flowers on different plants of the same cultivar (self-pollination), Different flowers on plants of different cultivars (cross-pollination). 4/6/2008 DMA: Chapter 9 Hartmann's Plant 36

37 If a plant is self-fertile, it produces fruit and seed with its own pollen, without the transfer of pollen from another cultivar. If it is self-sterile, it cannot set fruit and seed with its own pollen, but instead requires pollen from another cultivar. Often this is due to incompatibility, where a plant's own pollen will not grow through the style into its embryo sac (see Figs and 9-13). 4/6/2008 DMA: Chapter 9 Hartmann's Plant 37

38 Parthenocarpy Parthenocarpy is the formation of fruit without the stimulation of pollination and fertilization. Washington Navel orange, and many fig cultivars. (Not all seedless fruits are parthenocarpic) 4/6/2008 DMA: Chapter 9 Hartmann's Plant 38

39 Fertilization In the angiosperms the pollen tube grows through the micropyle opening in the ovule into the embryo sac and discharges two sperm nuclei (IN each): 1. One unites with the egg (IN) to form the zygote (2N), which will become the embryo and eventually the new plant. 2. The other sperm nucleus unites with the two polar nuclei (IN each) in the embryo sac to form the endosperm (3N), which develops into food storage tissue. This process is termed double fertilization. The elapsed time between pollination and fertilization in most angiosperms is about 24 to 48 hours. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 39

40 THE LIFE CYCLE OF A CORN PLANT (A MONOCOT) When a corn seed is planted in moist soil, it imbibes (absorbs) water from the soil. Germination begins with the emergence of the radicle (the primary root) and the plumule (the primary shoot). 4/6/2008 DMA: Chapter 9 Hartmann's Plant 40

41 The radicle grows downward through a protective sheath, the coleorhiza, from which the primary root develops and the secondary roots branch. Adventitious roots (roots other than those that develop from the radicle) grow from the shoot axis just at or above the soil surface (Fig. 8-2): The emerging plumule is protected by a sheathlike leaf, the coleoptile, that envelops the main stem as it grows upward through the soil. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 41

42 Female flowers, known as pistillate flowers or ears, appear at the base (axil) of one or more sheath leaves. Male flowers, known as staminate flowers or tassels, develop at the top of the plant. Figure 8-3 shows both kinds of flowers. Blown by the wind, pollen grains from the tassels fall on and pollinate the long pistillate filaments (silks) and subsequently fertilize the ovaries, which become the individual corn kernels borne on a stalk (cob). 4/6/2008 DMA: Chapter 9 Hartmann's Plant 42

43 Each ovary develops into a fruit, called a caryopsis, that encloses the true seed. After the kernels mature and dry, the fruits (containing the seeds) are harvested and stored over the winter. The seeds can be sown when weather conditions are favorable for germination, and the life cycle repeats itself. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 43

44 THE LIFE CYCLE OF A BEAN PLANT (A DICOT) The radicle grows downward and the hook of the bean, known as the hypocotyl, emerges above the soil, carrying the two cotyledons with it. Between the cotyledons lies a growing point (apical or shoot meristem) flanked by two opposite primary foliage leaves. The stem region just above the cotyledons and the first trifoliate leaves is called the epicotyl (Fig. 8-4). 4/6/2008 DMA: Chapter 9 Hartmann's Plant 44

45 The shoot apical meristem rapidly produces two trifoliate leaves opposite each other on the stem. The plant's green leaves are now capable of manufacturing food for future growth of the seedling. Flowers begin to develop in the axils of about the fourth set of leaves and in each succeeding set. These flowers are self-pollinated; thus, fruits (pods) develop as long as environmental conditions are favorable. The seeds mature and dry within the pod, and they can be sown at once to produce another generation of bean plants 4/6/2008 DMA: Chapter 9 Hartmann's Plant 45

46 The difference in emergence The difference in emergence of the growing points of beans and corn from beneath the soil affects the tolerance of each crop to light frosts. A late frost would be more likely to se-verely damage a newly emerged bean seedling than a newly emerged corn seedling because the growing point of corn is below the soil and protected. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 46

47 PLANT GROWTH REGULATORS Plant hormone is a natural substance (produced by the plant itself) that acts to control plant activities. Plant growth regulators, include plant hormones and other, nonnutrient chemicals not found naturally in plants but that, when applied to plants, influence their growth and development. 4/6/2008 DMA: Chapter 9 Hartmann's Plant 47

48 There are five traditionally recognized groups of natural plant hormones: Plant Hormone Rule Auxins Cytokinins Gibberellins Abscisic acid Ethylene Promote both cell division and cell growth Promote cell division Involve in regulating cell elongation Inhibit cell division Control fruit ripening 4/6/2008 DMA: Chapter 9 Hartmann's Plant 48