Plant communication and behavior relies on phenotypic plasticity. Plants are unable to run away, they have to live with the consequences of their behaviors.

The various cues that a plant recieves can affect a number of things including, shoot phenotype and root development. How do plants respong to shading, shade from their own leaves and shade from others?

They either confront, avoid, or tolerate. When a plant confronts a competitor the plant will have an elongation response, and increase apical dominance. When a plant avoids a competitor the plant will grow away from the competitor or have dependent germination. When a plant tolerates a competitor it will just be tolerant to the shade morphologically and physiologically.

How do plants perceive light levels and wavelengths? They possess photoreceptors, phytochromes, crytochromes, phototropins, and some unidentified UVB sensitive to particular wavelengths.

Plants can tell the difference between being in the shade of a building or a rock, versus shade of another plant. This is because of the different wavelengths of light that are absorbed by other objects. Red light is absorbed by plant canopy, far-red isn’t. That means a plant with full sun is being exposed to different ratios of red and far-red wavelengths opposed to plants which are being shaded.

Shade avoidance is a collection of responses to vegetative shading. Plants can anticipate and respond to future shade. Plants elongate less when wearing a collar that filters out far-red light reflected from adjacent leaves. Another example of this is Portulaca oleracea, or common parsalin, grows and branches in a wa that minimizs self-shading. When lower red/far-red ratios occuring in one direction the plant grows away from it.

Shade tolerant species tend to be very slow growing. The differences in allocation cause long-lived shade-tolerant plants that invest relatively more resources into defense against herbivores and pathogens.

Ligh information is more than just quantity and spectrum. It is likely that plants respond to a richer range of light cues. Vertical, mid-day shade might indivate a very tall neighbor where the plant would not want to compete to grow larger. Horizontal and late-day low r:fr cues might indivate similarly sized neighbors where the plant would compete with its neighbor.

Somatic Competition happens when plants compete with themselves. There are many redundant organs that compete with each other. In “Somatice Competition” the plant can increase performance by putting more resources into more successful organs. The growth rate can vary. Experimental evidence supports the 3 types of responses; independant, cooperative, competitive.

 

 

Male Gametophytes:  Pollen

Female Gametophytes: Embryo Sac

Generative Cell will divid to become the two sperm. The tube cell makes up the pollen tube into the pistil of the plant.

One sperm fertilizes the egg, the other will fuse with the polar nucei. The fertilized egg will become the embryo. The two polar nuclei and sperm will be the endosperm that provides nutrients to the embryo. This is known as Double Fertilization.

Embryo Development

The last step of seed development is that they become desiccated, they only have 5-20% water. The seeds coat will harden.

Inside of the seed will be the seed coat, the radicle (embryonic root), Cotyledon, Epicotyl (above cotyledon), Hypocotyl (below cotyledon).

Cotyledon are seed leaves that are packed with nutrients. In monocots, the cotyledon will wrap to become the Coleoptile.

Plant Hormones

Five Main Classes of Plant Hormones

The five hormones: Auxin, Cytokinins, Gibberellins, Ethylene, Abscisic Acid

Other Plant Growth Requlators: Salicylic Acid, Jasmonic Acid, Systemin

Working with Coleoptiles Charles Darwin and son Francis studies phototrophism. They found that cutting the tip prevents it from bending. They put an opaque cap on the tip and it did not bend. If covered with a clear cap it did. If they covered the bottom with a cap it still bent. This means that there was a signal from the tip of the plant that must have been responsible for this phototrophism. Peter Boysen Jensen took it a step further and studied the chemical signal. Fritz went isolated the chemical responsible for the signal. This chemical was an AUXIN.

Auxin

This is a the first plant hormone discovered. It is the Greek word for increases. Natural auxins consist of things like indole acetic acid (IAA) and relate to molecultes. They are produced in the apical meristems of shoots. They are usually transported from the top down, basipetal transport. This gradient become really important.

The process in which auxin phototrophism occurs is the ACID GROWTH HYPOTHESIS. Auxin increases proton pump activity. The cell wall becomes more acidic, and unzips the hydrogen bonds. This makes the wall soft allowing turgor pressure to push cell wall outward.

Auxin is also involved in gravitrophism. The starch-statolith hypothesis is an explaination of gravitrophism. The amyloplasts, leucoplasts that store starch, are used to figure out what is up and what is down. Whe the weight of the amyloplasts hit it triggers auxin production. Auxin is produces in the shoots.

Auxin is really important for apical dominance, tissue differentiation, lateral roots, fruit development, leaf abscission, and herbicide. In tissue differentaitaion, if you damage the vascular tissues, auxin initiates formation of vascular tissue to heal the wound. Auxin stimulates pericycl cells to divide.  The auxin enhances fruit growth, developing seeds are the source of IAA. The absicssion, or dropping of leaves and other plant parts, are a cause of a drop in auxin that drops the part. Synthestic auxins are used as weed killers.

Cytokinins

Johannes van Overbeek discovered that coconut milk possessed factors that accelerated the growth of plant embryo and isolted tissues and cells in test tubes. 

They are found in actively dividing tissues, cytokinesis. The are shown to cause plant cells to grow in an undifferentiated mass of cells (callus). The combination of auxin from above and cytokinin from the root tell the tissue what to differentiate into. Cytokinen is produced from the roots. They can delay and reverse senescence (the process of ageing) in plants. They release buds from apical dominance.

Gibberellins

E. Kurosawa was working on the disease of rice that produced fast growing, pale-colored and sickly plants. This was caused by a fungus isolated from the rice and later bean plants. The bean plants showed gibberellins were being released from the plant itself instead of the fungus.

Gibberellins are all about stem elongation. Dwarf plants are a varity of mutations that cause a lack of gibberellins or the inability to process gibberellins. Bolting, or the sudden verticle growth associated with flowering and death, is mediated by gibberellins. There is a spike in the gibberellins concentration. They are also really important for germination.  Gibberellins will induce genes to make enzymes that break down starch in the remaining endosperm. The biggest current use of gibberellins today is in Thompson’s seedless grapes.

Abscisic Acid

Abscisic acid causes stomatal closure, and prevents premature germination of seeds. It is very important for water regulation.

Ethylene

Ethylene is a gas that was discovered by accident by citrus growers. They noticed that if they kept them in a warm shed they could ripen the fruit faster. Ethylene gas hastens the ripening of fruits. It is importatn in seed germination, fruit ripening, and absission.

 

 

 

What is coevolution?

Coevolution involves two or more species which exert selective pressures on each other and evolve in response to each other.

The key idea is because each species involves in response to each other the selective environments is constantly changing.

When does coevolution occur?

Selective pressure will be strongest when there is a close ecological relationship. Close relative in terms as species who are specialists rather than generalists.

How do we study Coevolution?

There are different levels that we look at coevolution. The study of adaptations of the individuals, interactions between species or broad evolutionary patterns.

Does coadaptation demonstrate coevolution?

Some biologist would say that having coadaptation does not give evidence to the parallel evolution between taxa. To convincingly show coevolution you must show a parallel evoltuion between taxa.

An example of this would be the Fig Trees and Fig Wasps.

The figs have a specialist relationship with the wasps. The wasps act as pollinators for the figs. Coadaptations happen with these species. The receptive figs produce scents that are specific to a particular pollinator species. The shape of the ostiole is specifically shapped to the head of the particular wasp species. The morphology of individuals flowers specialized to a particular wasp species.
The Fig starts to develope and the ostiole opens up. The female wasp crawls in and lays here eggs in the plant with pollen from the fig she was born from. The eggs hatch and the females fly out and find another fig.

Cophylogeny

Congruent phylogenies due to cospeciation is strong evidence for coevolution.

Seed dormancy means that seed is in a state of suspended growth and development. This is a reduction in the metabolic rate. This gives resistance to adverse environmental conditions.

The conditions that must be present to break dormancy differs by species. Some species germinate as soon as the environment is suitable for growth. Other species need something else such as; An intial drying period, exposure to a long period of cold, scarification of the seed coat, or intense heat.

There are possible selective advantages to these cues to break dormancy. The cue for long periods  of cold are advantagious for species because of long winters.

Different species of plants produce seeds that require different cues to grow. A seed bank is produced by the different speices’ seeds waiting for these cues. A seed bank is a collection of dormant seeds in the soil. Seed banks allow dispersal in time as well as space.

There is a broad inverse correlation between the lifespan of adult plant and longevity in the seed bank. Annual plants will tend to have seeds that last for a long time . The storage effect is a model that shows the environment varies over time, and different species respond to the environment each year. An environment good for one species may not be good for another species. As long as the species has atleast one tough life storage stage that can get you from one good year to another good year this prevents the population from crashing. Populations stores up the good years to make it through the bad ones.

Long lived seeds do not need long live matured plants, but long lived matured plants do not need long lived seeds.

 

Sexual Reproduction in Plants

Alternation of Generations
Alternation of generations is when you have life history that alternates between free-living multi-cellular diploid stage (sporophyte) and the free-living multi-cellular haploid stage (gametophyte).

Alternation of generation can be applied to green algae. In the case of Ulva, the gametophytes and the sporophytes look exactly the same. This is call isometric alternation of generation. (Iso = same)

The general trend is to have sporophytes be the more common stage of reproduction. However, with moss this has not happened.

Alternation of generation can be applied to mosses. The sporophyte grow above the female moss.

Alternation of generation can be applied to ferns. The spores are located under the fern leaf. Each spore becomes a heart shaped gametophytes. Some ferns are hetero- or homo-sporophytes. They create male and female or just female ferns.

Alternation of generation can be applied to gymnosperms. The tree is the sporophytes. The gametophyes are located in the cones of the trees. The male cones are only on the tree for 3-4 weeks before completing the reproductive cycle. The female cones can stay on the tree for 3-4 years.

Alternation of generation can be applied to angiosperms.

Flower Review

“A diagram of the parts of flowers” Photo credit to Science & Plants for Schools retrieved from saps.org.uk

Ovule contains the embryo sac surrounded by integuments. Three cells are located at the bottom, the center one is the egg and the two that surround it are synergids. The three cells located at the top are antipodals. Two haploid polar nuclei are located in the middle. Ovule is the female gametophyte.

Embryo Sac = Female Gametophyte
Pollen Grain = Male Gametophyte

Double Fertilization is unique to angiospersms. A double fertilization occurs when a female gametophyte (embryo sac) and two male gametophytes (sperm) encounter each other. The sperm make its way down the pollen tube . One sperm produces endosperm, and one produce embryo. The endosperm is a nutritive tissue in seeds to feed the embryo.

Plant reproduction is complex.
There are sexual and asexual reproduction, but there are also apomictic seeds, agamospermy. The agamospermy produce seeds that are clones of themselves. This occurs in dandelions.

Perfect Flowers vs. Stamenate or Pistillate Flowers

Monoecious vs. Dioecious Plants
Some

Androdiecy vs. Gynodiecy
Some individuals produce perfect flowers and some produce only male or female flowers.

Co-sexual
Some of these species are obligate outcrossers. These outcrosser needs pollen from another plant and cannot self pollinate
Some of these species are obligate selfers. These selfers can only self pollinate.
A lot are flexible enough for both outcrossing and selfing to happen.

How does mating system influence geographic range size for selfers vs. outcrossers?
Selfers have a bigger geographic range because they can spread even with low densities. They have reproductive assurance.