Review of Seed Germination and Control of Flowering Topic 9.3

Seed dispersal Way seeds are carried away from vicinity of parent plant Seeds are dispersed in various manners: Wind Water Animals The structure of seeds indicates their dispersal type

From: http://theseedsite.co.uk/seedparts.html Dicot seed structure The seed contains the embryo of the new plant, with a supply of food for the embryo until it has formed sufficient roots and leaves to obtain its own food. The food, endosperm, may be in the seed leaves or it may be outside the seed leaves and be absorbed when the seed germinates. To start germination, the seed leaves absorb water and swell, and the radicle emerges, followed by the plumule. From: http://theseedsite.co.uk/seedparts.html

Germination of Seeds Conditions required: Moisture Temperature Oxygen Needed so that metabolic action can take place and overcome dormacy Temperature Optimum range for enzyme action involved in mobilization of stored food reserves Oxygen Sustain aerobic cell respiration A period of dormancy Some conditions differ depending on seed and its environment

Germination of a dicot seed Water is absorbed by the seed, kickstarting the formation of the hormone gibberellin Gibberellin forms here Water enters

Germination of a dicot seed Gibberellin stimulates production of the enzyme amylase, which digests the starch in the seed into maltose Gibberellin Stimulates production of Amylase

Metabolic events of germination The maltose provides energy for the developing embryo to grow and develop.

Photoperiodism Response of plants to changing length of day Plants use light to determine when to flower, germinate seeds, break bud dormancy

Control of Flowering Key Concepts Phytochromes: Blue-green pigments that regulate plant response to light; found in low concentrations Large conjugated protein with a pigment molecule attached Receptors for photoperiods is in leaves, but location in plant cells is unknown Can be converted from one form to another with different wavelengths of light Critical night length: the minimum amount of night time required for regulation of plant growth Animation - Phytochrome Signaling

Plant Development and Phytochrome Image from: http://click4biology.info/c4b/9/plant9.3.htm#6 Pr is converted to Pfr in red light Pfr is gradually converted to Pr in darkness Action of Pfr promotes flowering in long-day plants (LDP) and acts as an inhibitor of short-day plants (SDP)

Short-day plants dark Critical night length If light exceeds or interrupts critical night length, no flowering happens. Phytochrome Red (Pr) is converted to Phytochrome Far Red (Pfr) This only requires a short exposure to white or red light At the end of the night time, Pfr is low, so low Pfr triggers flowering light

Long-day plants dark Critical night length Night can exceed critical night length or be interrupted, flowering can still happen. During the day, Pr is converted to Pfr During the night, the Pfr concentration does not drop significantly since the dark period is short High Pfr concentration triggers flowering light