9.3 Growth in Plants Chapter 9: Plant Biology Plants show growth throughout their lives (indeterminate growth), however, they do die Death occurs based on the plants life cycle Some plants are annuals they complete their life cycle in one year Biennials take 2 years to complete their life cycle Perennials live many years- usually die because of infections or other environmental factors Meristems Growth in plants in confined to regions known as meristems Meristem the undifferentiated tissues in plants (like stem cells!) https://www.youtube.com/watch?v=h9kCPO7oMf8 Apical Meristem Found at the tips of roots and stems, and the tips of buds and shoots Produces primary growth (i.e. longer stem or roots) Apical Meristems Root Meristem -responsible for the growth of the root Shoot meristem is responsible for Growth (lengthening) of stem Cells that will produce leaves and flowers Which each division, one cell remains in the meristem The other increases in size and differentiates as it is pushed away from the meristem region Each apical meristem can give rise to additional meristems: Protoderm epidermis Procambium vascular tissue Ground meristem pith Lateral Meristems Found in the cambium Allow for secondary growth (i.e. for the plant to get thicker) Auxin Auxins are hormones that have a broad range of function including initiating root growth, development of fruit, and regulating leaf development IAA (indole-3-acetic acid) is the most abundant auxin IAA is produced in the apical meristem and is transported down the stem to stimulate growth At high concentrations it can inhibit growth Axillary buds are shoots that form at a node (the junction of the stem and the base of a leaf). Auxin produced at the shoot apex inhibits growth at the nodes. This is known as apical dominance. As the plant grows the shoot apex is further away from the node This means less auxin means the node can now grow Now the axillary bud can produce a stem or a fruit Cytokinins are hormones produced in the roots that also promote axillary bud development Plant Tropisms A tropism is a growth in response to a stimulus Phototropism: the growth in response of light Gravitropism: growth in response to a gravitational force Phototropism and Auxin Plants require light for photosynthesis; thus seedlings must grow toward sunlight Auxins cause the positive phototropism of plant shoots and seedlings (so long as the plant cells have auxin receptors) Auxin increases the flexibility of plant cell walls enabling cell elongation on the side of the shoot necessary to cause growth towards the light. Auxin is actually concentrated on the side of the stem away from the light source The elongation of the cells on the side away from the light allows for curvature toward the light source. Root auxin inhibits shoot elongation Micropropagation of Plants This is an in vitro procedure to clone plants Tissues from the shoot apex of a plant with desirable features is sterilized and cut into pieces known as explants The explants are grown in a nutrient agar that also contains growth hormones. Once the roots and shoots are developed, the clones plant can be transferred to soil Micropropagation Useful for propagating virus-free plants, since meristematic tissues are likely virus-free. Can be used to clone endangered plants Some plants, like orchids, are difficult to germinate the traditional way. Micropropagation is high successful. The plantlets can be stored in liquid nitrogen - cryopreservation 9.4 Reproduction in Angiospermophytes Dicotyledonous Flower Parts FLOWER PARTFUNCTION SepalsProtect the developing flower while in the bud and at night when the buds close. PetalsModified leaves; often colourful to attract pollinators StamenThe male reproductive structure; made of anther and filament AntherProduces and releases pollen FilamentStalk of stamen that holds up anther CarpelThe female reproductive structure; made of ovary, style, and stigma PistilCan refer to a single carpel or a group of fused carpels StigmaSticky top of carpel which pollen lands on StyleSupports and holds up the stigma; gives the stigma exposure to pollen ovaryBase of carpel in which the female sex cells develop; if fertilization occurs, it will turn into a protective fruit OvulesFound in the ovary; contain female sex cells, eggs PollenContain male sex cells (sperm) Flowers occur in various colours, shapes and types reflective of their pollinator Complete flowers contain all four basic flower parts (sepals, petals, stamen, and carpel) Incomplete flowers lack at least one of these parts Staminate flowers have only stamens Carpellate flowers have only carpets Pollination the process in which pollen (which contains the male sex cells sperm) is placed on the female stigma. Can occur via a variety of vectors Wind water Insects Birds Bats Angiosperms and their pollinators have coevolved (supported by fossil evidence) The flowers colours, patterns, odours, shapes and even the time of day it blooms are designed to attract a specific pollinator Often, the flower provides the gift of food to the pollinator in exchange for the pollinator unintentionally transporting pollen to the stigma. A mutualistic relationship! Examples Red flowers pollinated by birds Yellow and orange flowers bees Heavily scented flowers nocturnal animals Inconspicuous, odourless flowers wind As the humming bird eats nectar at the base of the flower, it picks up pollen which it will transfer to the stigma of the next flower it visits These orchids look similar to a particular wasp species. A wasp will come to the orchid and a attempt to mate with it. In the process it will pick up pollen and transfer it to the next flower it attempts to mate with. Self Pollination when pollen from the anther of a plant falls on its own stigma A form of inbreeding thus less genetic variation Cross Pollination pollen lands on the stigma of a different plant. Increases variation and offspring with different fitness Fertilization When the male and female sex cells unite to form a diploid zygote. The female sex cells are in the ovules. The sperm from the pollen that has attached itself to the stigma must make its way to the ovules in the ovary. Pollen attaches to stigma and begins to grow a pollen tube through the style Within the growing pollen tube is the nucleus that will produce the sperm. The pollen tube completes growing by entering an opening at the bottom of the ovary The sperm moves from the tube to combine with the egg of ovule to form a zygote. The Seed and Seed Dispersal Once the zygote is formed, it develops with the surrounding tissue into the seed As the seed is developing, the ovary around the ovule mature into a fruit Seed dispersal can be aided by water, wind, animals SEED Is the means by which an embryo can be dispersed into to distant locations. It is a protective structure for the embryo Seed PartFunction testaTough, protective outer coat cotyledonsSeed leaves that function as nutrient storage structures microphyleScar of the opening where the pollen tube entered the ovule Embryo root and embryo shoot Become the new plant when germination occurs Pre-Germination Once seeds are formed, a maturation process follows. The seed dehydrates until the water content of the seeds is about % of its weight. At this point, the seed goes into a dormant period where there is low metabolism and no growth or development. Duration is variable for different types of seed It is an adaptation to environmental conditions FUN FACT In 1995, a team of biologists found some seeds in a dried-up lakebed. The seeds were from a type of lotus plant. After germinating some of the seeds, the biologists found them to be nearly 1300 years old!!! (They used radiometric dating to determine this age.) Fun Fact In 2005, a 2000 year old Judean Date palm (found in the ruins of Herod the Greats palace)was germinated Germination Conditions If conditions become favourable, the seed will germinated. GERMINATION is the development of the seed into a functional plant. There are several conditions that must be fulfilled for a seed to germinate. WATER Required to rehydrate the dried seed tissues Makes the seed swell As a result the seed coat will crack and hydrolytic enzymes are activated- they will start to catabolize large molecules (storage polysaccharides such as starch is converted into maltose) for cellular respiration. OXYGEN Required for the break down of those sugars in cellular respiration TEMPERATURE Appropriate temperature is required, that is varied among plants depending on their natural environment. Ex. Period of low temperature followed by high temps ensures that the seed does not geminate until the winter has passed. Temperature is important for enzyme activity Many plants have specific conditions other than these that must be met in order to germinate. Ex:Lodgepole Pine The emerging seedling is fragile and will be exposed to hard weather, parasites, predators, and other hazards. Many seeds will not produce a functional plant because of these threats To compensate, plants produce a large number of seeds Metabolic Processes during Germination of a Starchy Seed 1. Seed absorbs water (which leads to many metabolic changes) 2. Gibberellin is released after the uptake of water Gibberellin plant growth hormone 3. Gibberellin triggers the release of the enzyme amylase 4. Amylase causes the hydrolysis of the starch into maltose 5. Maltose is hydrolyzed into glucose which can be used for cellular respiration or converted into cellulose to build cell walls for new cells 6. Stored proteins and lipids will also be hydrolyzed to make proteins/enzymes and phospholipids and energy metabolism. Germination uses the food stored in cotyledons to grown until it reaches light when it starts to photosynthesize Control of Flowering Light important factor for growth and development Plants are able to detect the presence of light, its direction, wavelength, intensity PHOTOPERIODISM the plants response to light involving the relative lengths of day and night. *To ensure continued existence in an area, a plant must flower when pollinators are available and when necessary resources are plentiful PLANT TYPEFLOWERING AND LIGHTEXAMPLES LONG-DAY PLANTS Bloom when days are longest and nights the shortest (midsummer) - Require P fr Radishes, spinach, lettuce SHORT-DAY PLANTS Bloom in spring, late summer, and autumn when days are shorter - Inhibited by P fr Poinsettias, chrysanthemums, asters DAY-NEUTRAL PLANTS Flower without regard to day length Roses, dandelions, tomatoes It is actually the length of night that controls the flowering process. The control is brought about by a special blue-green pigment called phytochrome. Phytochrome Phytochrome is a photoreceptor and a pigment It absorbs light There are 2 forms of phytochrome P r absorbs red light P fr absorbs far-red light/darkness Far-Red Light Wavelengths between nm At the far end of the visible light spectrum (Between red light and infrared light) During the day, when there is light, red light (wavelength of 660nm) is present P r absorbs red light and is rapidly converted into P fr At the end of the day, after many hours of light, plants will have most of their phytochrome in the form of P fr During the night, when there isnt light (therefore no red light), P fr is slowly converted back into P r By morning, most of the phytochrome will be P r again If there is even a flash of light interrupting the darkness during the night, it will disrupt the process of P fr turning into P r P r P fr _____________________________________________________________ P fr P r Long-day plants, require P fr to flower Long day = short night! At the end of a short night, there will still be lots of P fr remaining. The remaining P fr at the end of a short night stimulates the plant to flower. In short-day plants, the P fr acts as an inhibitor for flowering. So after a short night, the remaining P fr will prevent the plant from flowering. If it was a long night, all the phytochrome will be in the form of P r (there will be no P fr ) so flowering CAN occur.