Control Systems in Plants

Chapter 26

I. Control of Plant DevelopmentA. plant hormones

1. chemical messengers within organisms2. each has a very specific function 3. auxins

a. promote growth at apical meristemsb. cause growth in different directions

4. gibberellins• promote stem elongation

5. cytokininsa. promote cell divisionb. prevent premature aging (senescence)

6. ethylenea. causes dropping of leaves and fruit (abscission)b. ripens fruit

7. abscisic acida. promotes seed and bud dormancyb. closes stomata

B. Plant Responses to the Environment1. tropisms

a. growth toward or away from a variable• positive vs. negative

b. causes growth in different directionsc. phototropism, chemo-, thigmo-, gravi-

An example of positive phototropism

Fig. 26.11 (a) Negative gravitropism in stem growth. (b) Positive gravitropism in root growth.

2. circadian rhythmsa. activities that occur in 24-hour cyclesb. biological clocks

i. internal (genetic) mechanism that maintains circadian rhythms

ii. strongly influenced by photoperiodc. sleep movements

Fig. 26.13 Coiling response is an example of thigmotropism.

Fig. 26.15 Examples of sleep movements in prayer plants and morning glories.

d. photoperiodismi. responses caused by changes in

day-lengthii. controlled by the pigment

phytochrome • acts as a photoreceptor

iii. best example is flowering• short-day vs. long-day vs.

day-neutral plants

The influence of photoperiod on flowering in long-day and short-day

plants.

Fig. 26.5 Graph of circadian rhythms in morning glories.

II. Plant Defenses A. like animals, plants have defenses against disease

1. bacterial, viral, and fungal infections2. much of their defense is based on genetics3. chemical attacks against pathogens

a. often localized in more vulnerable areasb. phytoalexins, alkaloids, glycosides, etc.

B. plants also protect themselves from herbivores1. structural defenses

• spines, thorns, hair layers, tough bark, etc. 2. chemical defenses

• toxins, inedible substances, sticky substances, etc.

Fig. 26.19 Examples of

plant herbivores

Fig. 26.19 Plant fungal infection