Plant Hormones A. Stimulus Response: A house plant on a windowsill grows light. If you rotate the plant, it reorients its growth until its leaves face the window again. The growth of a shoot towards light is called. In the diagram below, molecular observations show that cells on the side of seedling are than those on the bright side of the plant. These different growth rates allowed the shoot s (actively dividing tips of roots and shoots) to bend towards light. Early experiments showed this different rate of growth in experiments. When the of a grass shoot was removed, the shoot grew and wouldn t bend toward light. And the shoot grew straight if the tip was. This proved that the tip was responding to. Later experiments showed the same with gelatin and mica. Further experiments involved cutting the of grass seedlings and putting them on a block of. Then the chemical messenger or diffused from the tip into the agar. The agar was then put as a substitute onto the different tipless plants below and grown in the to test only the chemical and not the sunlight. This showed that when shoots grow towards light they do so because of a concentration of the growth promoting chemical on the side of the shoot which is the hormone.
Another -an growth response to an environmental stimulus is. This is the idea that if you place a seedling on its side, it will grow so the shoot bends and the root curves. This response to gravity ensures that roots go in the soil and shoots reach sunlight regardless of how the seed landed when it was planted. Botanists are still not sure what causes this, but one hypothesis is by the settling of -organelles containing heavy/ dense. An uneven distribution of organelles may in turn signal the cell to redistribute. And another tropism is called. This is growth movement in response to. This can be seen when and other climbing plants contact an object and wrap around it. This is considered to be a lasting change. We will compare short, reversible changes later. B. Plant Hormones: Recall a hormone is made in part of the organism and influences cells at part. They can pass through and alter plant physiology. The 5 main classes will now be described. like IAA (indoleacetic acid) promote by cell elongation primarily in the of plants (the apical meristem). However, IAA promotes growth only when at a certain concentration. When IAA gets too, this causes the plant to make another hormone called that inhibits stem elongation. Auxins makes plants cells elongate by the inside primary cell walls. These H + then activate that break bonds of the in the cell walls. The cell then with water and elongates. This is because the weakened cell wall no longer resists the cell s tendency to take up water. The cell then stays bigger by synthesizing more cell wall material and.
The promote cell division also called. They are found in actively growing tissues, especially in the, which then may be transported throughout the plant to other target tissues. Cytokinins also retard the of and so are used to keep flowers fresh. Also, cytokinins influence organ development called. For example, the amount of cytokinin and auxin together can influence whether a plant is or. The tall plant on the left below has its bud intact so traveled down the stem and allowed the stem to elongate. This the auxillary side branches. The short plant on the right below had its terminal bud removed early on and meant no was there to both elongate the stem and the side branches. This then allowed the transported from the roots to activate the buds and the plant grew bushy. So the length/ fullness of a plant is controlled by the interplay of auxins and cytokinins. The are plant hormones made at the tips of and. They produce a wide variety of effects. One main effect is the in stems and leaves. High concentrations of GA can cause the elongation of stems. This is called. This action enhances the effects of the auxins. Also, in combination with auxins, giberellins influence development. Spraying gibberellins on some fruits makes them develop fertilization giving us such fruits as grapes. GA are also important in seed and can cause a seed to sprout when sprayed onto seeds.
There are times, such as the onset of or severe, when it s adaptive for a plant to become. The hormone (ABA) produced in the signals the buds to form that will protect them from harsh conditions. So ABA is a growth. ABA also causes to remain dormant. This is especially important to plants in because germination without enough water would quickly kill the plants. Seeds will not germinate until a downpour of washes ABA out of the seeds, getting rid of this inhibitor. So the ratio of ABA to (which promotes germination) determines if a seed sprouts or remains dormant. ABA also acts as hormone in growing plants. This helps them cope with adverse conditions. For instance, if a plant is, ABA accumulates in the leaves causing the stomata to close. ABA is named because it was once believed to cause or breaking off of leaves from trees in the fall. This has never been proven. At one time and grapefruit were ripened in sheds with stoves. Fruit growers thought it was the heat from the stoves, but it was actually due to a gaseous by-product called. We now know plants make their own ethylene that triggers fruit. Fruits ripen by the of that cause the characteristic color changes (green to yellow or red). This gas is why the phrase One spoils the is reality. And is why growers try and the action of ethylene. They do this by flushing stored apples with that inhibits the action of ethylene. So a person can put a very piece of fruit in a bag and cause an piece of fruit to ripen quicker. Ethylene also plays a role in the fall changes in leaves and and promotes, the of leaves.
C. Internal Clocks: In humans, blood pressure, body temperature, alertness, rate of cell division, etc. all fluctuate rhythmically with the time of day. Plants also display rhythmic behavior like the and of stomata and movements. A biological cycle of about hours is called a. These rhythms persist even when an organism is from /external cues - like in constant dark or constant light. All research thus far indicates that circadian rhythms are controlled by timekeepers called. A biological clock continues to mark time in the absence of environmental cues, but to remain tuned to a period of exactly 24 hours, it requires signals from the environment. For example, if an organism is keep in a constant environment such as constant, its sleep movements slightly to a cycle of about hours. Just what is a biological clock? We aren t sure, but in humans the clock is a cluster of in the of the brain. In plants, we don t know what they are or where they are located. Unlike most metabolic processes, biological clocks and the circadian rhythms they control affected by shifts. This is important when you consider if a clock speed up or slowed down based on outside temperature, this would be a very timepiece. D. How plants distinguish the season: A biological clock not only times a plant s activities, it also influences events., seed, and the onset and ending of are all examples of stages in plant development that occur at specific times of the year. The environmental stimulus plants most often use to detect the time of year is called, the relative lengths of and. Plants whose is triggered by photoperiod fall into 2 groups: plants (long-night) and plants (shortnight). Short-day/long night plants flower in the late,, or winter when there is little or long. are an example of short-day plants. Long-day/short night plants flower in the late or early and include plants like, lettuce, iris, and many cereal grains. Spinach only flowers when exposed to hours of daylight. If this is interrupted by even a single of darkness, spinach will not flower.
We now know that flowering and other responses are not controlled by length, but actually by length. So the short-day plants are actually long-night plants, but this was discovered after they were named. Look at the figure below to understand when long-night and short-night plants will flower. There is no night length for when plants flower. The actual length of the varies from species to species. How does a plant actually measure photoperiod or night length? Plants do this with the aide of called -a colored protein that absorbs light. To understand this, look at the figure below. The letter on the light flash stands for light (wavelength 660 nm) and is effective at night length. stands for (wavelength 730 nm) and can correct the effects of the red light of the number of red light flashes so long as is the last flash the plant receives. Now let s consider the role of phytochrome. Phytochrome pigment molecule alternates between two forms, one absorbs light the other form absorbs light. The two forms are written as and. The from one form to the other happens quickly. However, plants only make. So if a plant is kept in the, the pigment remains as. Normally, each day at, the form is converted to and this sets a plant s. Then each day at, the form is converted to. The biological clock measure the time the conversion of these 2 forms, or the time from sunset to sunrise. So this pigment tells time of day and the and especially triggers responses such as seed,, and opening.