Early activities of seedlings

Seedlings need to develop forces necessary to push through the soil

Water potential gradients generate cell turgor pressure and drive cell expansion.

Seedlings need to orient themselves properly

Roots grow down into the soil to obtain water and nutrients

Shoots grow up to receive light and carbon dioxide

Seedlings must make the transition from heterotrophic growth to autotrophic growth before they run out of food reserves.

Heterotrophic - dependent on organic material for nourishment.

Autotrophic - can use inorganic material for nourishment - (self-feeding).

The transition to autotrophic growth is dependent on light for photomorphogenesis.

Questions to consider for seedling growth and plant growth in general include:

• What tells a cell to divide and in what direction?
• What tells a cell to enlarge and in what direction?
• How is cell growth coordinated so that highly organized structures like leaves are formed?
• How does a plant change itÕs growth to get the most out of itÕs environment?

Hormones are key players in seedling growth The ÒclassicÓ plant hormones include:

• Indole-3-acetic acid (IAA, also known as auxin)
• Gibberellic acid (GA)
• Cytokinin
• Abscisic acid (ABA)
• Ethylene

Other growth regulatory substances include:

• Brassinosteroids (BR)
• Salicylic acid
• Jasmonic acid
• oligosaccharides
• small peptides
• lipids

Factors involved in regulation of hormone responses include:

• Biosynthesis
• Degradation
• Transport
• Tissue and cell sensitivity

The steady-state level of a hormone is controlled by the rate of its formation and removal. Changes in the rates of any of the steps will effect the level of that hormone and, thus, the response.

Hormones are central to seed and bud dormancy.

Dormancy: Inability of seeds to germinate and buds to grow even when environmental conditions are favorable for growth.

Quiescence: When seeds and buds are able to grow but lack of favorable environment (e.g.; too cold, too dry -- garden seed in an envelope is quiescent, not dormant).

Dormancy is an adaptive response that evolved in regions with unfavorable seasons for growth (I.e., cool winters or dry seasons).

Dormancy is a means of deferring growth until the season is favorable.

Before becoming dormant, it is necessary that tissues become ÒhardenedÓ to withstand desiccation or freezing. In experimental conditions:

-some trees have survived -196 degrees C

-some seeds have survived temperatures a few degrees above absolute zero

During dormancy biosynthetic metabolism is greatly reduced and tissues partially desiccate.

Dormancy involves the development of special embryonic or meristematic tissues, with associated food reserves to be used when growth resumes.

Dormancy is an endogenous physiological attribute, genetically controlled, and frequently signaled by environmental changes, such as daylength and/or temperature that precede the onset of the harsh conditions of winter or drought.

Hardening to cold and drought is a characteristic of dormant tissues. Depending on species and the depth of dormancy, they can withstand desiccation and freezing. Hardening is promoted experimentally by gradually lowering temperatures and withholding H2O. Small cells with dense cytoplasm, characteristics of embryonic or meristematic cells, harden best.

General physiological changes associated with hardening include:

• Partial loss of water by controlled desiccation (dormant buds have about 40% H2O, vs. 80-90% for growing vegetative buds.
  
  
• Increase in solutes, especially sugars and some amino acids.
  
  
• Soluble proteins tend to increase.
  
  
• Cell membranes become more "fluid", showing lower transition temperatures (i.e. phase changes in molecular organization leading to leakiness) and greater stability to osmotic shrinkage. Fluid membranes have more unsaturated lipids relative to saturated lipids.
  
  
• Plants in which hardening is induced by chilling temperatures (e.g. winter wheat) show a number of changes in gene expression.
  
  
• There is evidence that in addiiton to ABA being required for dormancy, it is also required for induction of cold-hardiness.

  Specialized changes associated with prevention of ice damage Restricted ice development - in some plants, ice formation in hardened cells starts on the wall surface and ice crystals form in the intercellular spaces at the expense of water withdrawn from the protoplasts. The protoplast becomes partially desiccated and its solutes (sugars, organic acids, salts, etc.) are concentrated, lowering the freezing point. Damage occurs if the intracellular water finally crystallizes and ruptures membranes. The plasma membrane then looses its semipermeability and during thawing, solutes leak out and the cell dies.
  
  

  
  
  

  Ice recrystallization - some plants, like carrot, can basically freeze without being severely damaged by ice crystals. They accomplish this by producing proteins that inhibit ice recrystallization. Inhibition of ice recrystallization results in the production of tiny, rounded ice particles that are less likely to cause physical damage. In addition, these proteins can cause some thermal hysteresis in which the freezing temperature is lower than the melting temperature.

  Antifreeze compounds are produced in some plants to lower the freezing point of the cytoplasm. In plants antifreeze compounds include a variety of small molecules, including the amino acid proline, and in some cases antifreeze proteins are thought to be involved. Changing thermal hysteresis is a mechanism used by some antifreeze proteins.

  Supercooling: In many plants, protoplasmic water does not freeze (crystallize) until -35° to -40°C. In comparison, pure H₂O will supercool to -38°C before spontaneously crystallizing only in the absence of ice nucleating sites. Some plant cells are capable of deep supercooling presumably due to a lack of ice nucleation centers.

  Once seeds and bud tissues have become hardened, dormancy can be established.

  Mechanisms for controlling dormancy include:

  • Lock the inhibitor(s) in with cover that is impermeable to water and gasses.
    
    
  • Degradation of growth inhibitor after an appropriate signal is received.
    
    
  • Production of a germination (growth) hormone.

   

Seed dormancy can be primary or secondary

Seed that are released from a plant in a dormant state exhibit primary dormancy. In some plants, seeds may be nondormant when initially released but become dormant if environmental conditions become unfavorable. Such seeds exhibit secondary dormancy.

Stratification is the term used to describe the low temperature requirement for breaking dormancy. A general term that describes the process of breaking dormancy is after ripening.

One model for how low temperature can break dormancy is based on the effect of temperature on enzyme reactions.

-reaction rates generally increase with increasing temperature


-enzyme conformation and activity is also temperature dependent


-these properties combined determine an enzymes "optimal" temperature

If temperature affects the rate of biosynthesis of an inhibitor differently than the it affects the rate of degradation of an inhibitor, the level of the inhibitor will change accordingly.

If formation of P is slightly greater at low temperature than degradation of P, then the level of P will increase. If P is a growth promoting substance like GA, then after some time at low temperature, growth will be promoted (dormancy broken).

Similarly, if the formation of I is slightly slower at low temperature than the degradation of I, the level of I will decrease. If I is an inhibitor like ABA, then the growth inhibition will decrease and growth can resume (dormancy broken).

Other models to explain the low temperature requirement involve temperature effects on gene expression. One of these models suggests that during dormancy, there are proteins that repress transcription of genes that are necessary for growth. If low temperature causes a change in the confirmation of these repressor proteins, their binding properties may change and eventually result in modulation of transcription. Although a mechanism like this is almost certainly involved in some plants, there are no data available that directly test it.

With the repressor protein out of the way,a quiescent state may persist until the environmental conditions allow the basic machinery for gene expression to work.

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