Regulation of Growth in Plants

Goals

1. To understand how plants time important events such as flowering and seed germination

2. To understand how hormones function in plants to allow them to respond properly to their environment

Outline

How do plants flower at the right time of the year?

The eukaryotic internal clock

Internal clock keeps circadian rhythms

Clock is set by cues from environment

Setting the clock in plants

Plants can respond to amount of light

phytochrome -- two forms

light intensity and P_r vs P_fr ratio -- growth in shade

day vs night and P_r vs P_fr ratio -- measuring the length of day and night

Long day plants

before the summer solstice days grow longer

P_fr stimulates flowering after a day length threshhold is reached

Short day plants

after the summer solstice days grow shorter

P_fr inhibits flowering until a day length threshhold is reached

How do plants germinate at the right time?

Light as an environmental cue

phytochrome detects if seed is buried

some plants germinate only in light

Water as an environmental cue

Imbibition

abscisic acid -- inhibits germination, removed by water

gibberellin -- stimulates breakdown of starch into glucose -- released by water

How do plants know when to branch?

hormones

auxin made in shoot apical meristem

cytokinin made in roots

apical dominance

axillary buds inhibited by high auxin/cytokinin ratio

ratio drops if stem lengthens or apical meristem is removed -- branches grow

 

Vocabulary

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