Regulation of Plant Growth

Interacting Factors in Plant Development The development of a plant is regulated by four factors: Plants sense and respond to environmental cues. Plants use receptors, such as photoreceptors, which absorb light, to sense some environmental cues. Chemical messages, or hormones, mediate the effects of the environmental cues. Enzymes, which are encoded by the plant’s genome, catalyze the biochemical reactions of development.

An Overview of Plant Development As plants develop, environmental cues, photoreceptors, and hormones affect three fundamental processes: Cell division Cell expansion Cell differentiation 5 major classes of hormones in plants mediate growth and development.

Interacting Factors in Plant Development Auxins promote the elongation of cells in parts of the plant. Both phototropism (growth toward light) and gravitropism (directional growth with respect to gravity) is controled by auxins. Light and gravity are important environmental cues in plants. Phototropism: Plants grow toward light. Gravitropism: stems grow up and roots down.

Interacting Factors in Plant Development Hormones called Gibberellins control: Germination of seeds and sprouting of buds Elongation of stems Stimulation of flowering Development of fruit

Interacting Factors in Plant Development Photoreceptors are protein molecules that absorb light. Light is an environmental cue that acts directly on the photoreceptor. Onset of winter dormancy is controlled by the length of the light. Other environmental cues are light color, intensity, duration, and temperature. Light regulates many aspects of plant development.

An Overview of Plant Development Seed dormancy must be broken for a plant embryo to begin developing. As a seed begins to germinate it first takes up water. The embryo uses the seed reserves by digesting stored polymers into monomers. Germination is completed when the embryonic root emerges from the seed coat. At this point, the plant is called a seedling.

Figure 38.1 Patterns of Early Shoot Development (Part 1)

Figure 38.1 Patterns of Early Shoot Development (Part 2)

An Overview of Plant Development Some plants flower when they reach a certain size or age; others flower during certain times of the year. The latter have photoreceptors in the leaves that measure the length of night. When it is time for the plant to flower, a signal must be transmitted from the leaves to the place where flowers will form. Following fertilization a fruit develops and ripens under hormonal control.

An Overview of Plant Development Perennials continue to grow year after year, but some go into dormancy for part of the year. A hormone helps maintain this dormancy. In some species, leaves senesce and fall at the end of the growing season. This process is regulated by the interactions of hormones.

Ending Seed Dormancy and Beginning Germination Seed dormancy may last for weeks, months, years, or even centuries. The principal mechanisms of dormancy are: Exclusion of water and/or oxygen by the seed coat Mechanical restraint of the embryo Chemical inhibition of embryo development

Ending Seed Dormancy and Beginning Germination Seed dormancy must be broken before the embryo can germinate. There are many mechanisms for breaking dormancy. The seed coat may be abraded, broken, or chemically weakened to allow germination. The activity of microorganisms in soil probably help soften seed coats. A developing runner bean.

Figure 38.2 Fire and Seed Germination Seeds that must be scorched by fire avoid competition with other plants by germinating only in areas cleared by fires. Fire releases mechanical restraint in some species, or melts waterproof waxy coatings. Scorching by fire may break down chemical inhibitors.

Figure 38.3 Leaching of Germination Inhibitors Prolonged soaking in water can leach chemical inhibitors to break seed dormancy.

Ending Seed Dormancy and Beginning Germination Seed dormancy ensures that plants survive unfavorable conditions, germinate when conditions are likely to be favorable, and aids in plant dispersal. Some seeds require exposure to cold, ensuring that they don’t germinate until spring brings favorable conditions. Some seeds will not germinate until a certain amount of time has passed, which prevents germination while the seeds are still attached to the parent plant.

Ending Seed Dormancy and Beginning Germination The first step in seed germination uptake of water. The interior of the seed has a negative water potential, and water causes the seeds to swell substantially. Water uptake activates certain enzymes, and proteins are synthesized.

Ending Seed Dormancy and Beginning Germination Reserves stored in the endosperm provide energy and materials until the leaves begin photosynthesis. The energy storage molecules include starch, fats or oils, and proteins. These giant molecules must be digested to monomers by enzymes.