Plant Physiology Study Notes

Nondirectional, light-triggered development - Can result in complex changes in form, including flower production.
Directional growth responses to light - Compensate for the plant's inability to move.
Pigment-Containing Protein Family - Members: Multiple members exist within this family. - Domains: 2 main domains that serve distinct functions. - Chromophore: Sensitive to light and facilitates the expression of light-response genes. - Role of Phytochrome: Changes conformation in response to relative amounts of red and far-red light, affecting its ability to bind proteins involved in signaling. - Ubiquitination Site: Targets phytochrome for degradation. - Protein Kinase Domain: Responsible for signal transduction through phosphorylation.
Absorption Properties - Both phytochrome and chlorophyll absorb light in specific and narrow ranges of the electromagnetic spectrum. - Key Difference: - Chlorophyll uses light as a source of energy (photosynthesis). - Phytochrome conveys information about the relative amounts of red and far-red light, leading to developmental changes.
Phototropic Responses - Growing stems bend toward blue light; in general: - Stems: Positively phototropic (grow toward light source). - Individual Leaves: May also display phototropic responses; influence by plant hormones.

Gravitropism - Important during seed germination. - Shoot: Exhibits a negative gravitropic response (growth away from gravity). - Root: Exhibits a positive gravitropic response (growth toward gravity).
Mechanism of Gravitropism - Amyloplasts: Starch-containing plastids that sink toward the center of the gravitational field, providing a physiological signal. - Transduction of this signal occurs both inside the cell and to other cells. - Results in differential cell elongation, affecting cells on the “up” and “down” sides of the root or shoot.
Auxin Concentration - Increased auxin concentration on the lower side of the stem causes those cells to grow more than those on the upper side, resulting in a negative gravitropic response. - Gravity-sensing cells located in the root cap influence the growth symmetry.

General Responses - Include thigmomorphogenesis, which are reversible responses occurring in the short term, not classified as tropisms.
Thigmotropism - Directed growth of a plant or plant part in response to contact with another object or organism (e.g., winds or rubbing against another plant). - Result: Leads to thicker, shorter internodes, making the plant less likely to snap during storms.
Tendrils: - Specialized epidermal cells perceive contact and result in tendrils curling around an object within approximately 3 minutes. - Hormones involved include auxin and ethylene.
Reversible Responses - Many plants exhibit leaf movements in response to touch or other stimuli (e.g., Mimosa pudica). - Mechanism: Turgor pressure in specific cells changes, resulting in leaf leaflet folding upon touch. - Turgor movements may also be influenced by light, maintaining leaves at right angles to sunlight.

Life Cycle of a Flowering Plant - Consists of the sporophyte generation (2n) that produces haploid spores via meiosis and the gametophyte generation (n) producing a diploid embryo through fertilization. - Undergoes carefully regulated developmental changes.
Phase Change - Response to external or internal signals can trigger flower formation; can be morphologically obvious or subtle (e.g., distinct juvenile and adult phases in ivy).

Genetically Controlled Pathways - Four main pathways regulate flower formation: - Light-dependent - Temperature-dependent - Gibberellin-dependent - Autonomous
Light-dependent Pathway - Plants respond to seasonal changes in day length to optimize flowering when competition for resources is minimal. - Categories include short-day plants, long-day plants, and day-neutral plants.
Critical Day Lengths - Flowering is triggered: - When days become shorter than a critical length for short-day plants. - When days become longer than a critical length for long-day plants. - Day-neutral plants can flower regardless of day length as long as sufficient light for growth is available.
Temperature and Gibberellin Pathway - Cold temperatures can promote flowering in some species. - Lower levels of gibberellins can delay flowering, while the autonomous pathway is regulated by internal signals independent of external cues (apart from basic nutrition).

Evolution in Angiosperms - Floral organs are believed to have evolved from leaves, categorized into four distinct whorls: - Calyx - Corolla - Androecium - Gynoecium
Male Structures (Stamens) - Collective term for all stamens; specialized structures with filaments (stalks) that are slender and threadlike, ending with a swollen portion that contains 4 microsporangia.
Female Structures (Carpels) - Collective term for all female parts, consisting of a single carpel or fused carpels (known as a pistil): - Swollen lower portion connects to a style that narrows at the top into a stigma, which is pollen-receptive.

Pollen Grains and Embryo Sac - Formation involves diploid microspore mother cells in the anther, dividing by meiosis to form 4 haploid microspores, which develop into pollen grains via mitosis. - Each pollen grain contains a generative cell that divides to form two sperm cells. In the ovule, a single diploid megaspore mother cell divides to produce 4 haploid megaspores, generally resulting in one surviving megaspore that develops into an embryo sac with 8 nuclei.

Pollination - The transfer of pollen to the stigma of a flower can occur via air currents or animals; self-pollination is also possible.
Fertilization - Involves the germination of pollen, which carries the sperm nuclei to the stigma. One sperm fertilizes the egg to create a diploid embryo, while the second sperm fuses with two polar nuclei, leading to seed maturation within ripening fruit. Eventually, germination initiates another life cycle.