GROWTH RESPONSES AND BIOLOGICAL RHYTHMS IN PLANTS CH 33 Tropisms orient plant growth toward or away from environmental stimuli Plants sense and respond to environmental changes – In a variety of ways

Response to Light – Phototropism - bending in response to light May result from auxin moving from the light side to the dark side of a stem Light Control Tip removed Tip covered by opaque cap Tip covered by trans- parent cap Base covered by opaque shield Tip separated by gelatin block Tip separated by mica Darwin and Darwin (1880) Boysen-Jensen (1913)

Response to Gravity – A response to gravity, or Gravitropism May be caused by the settling of special organelles on the low sides of shoots and roots This may trigger a change in the distribution of hormones Figure 33.9A

Response to Touch – Thigmotropism, a response to touch Is responsible for the coiling of tendrils and vines around objects Figure 33.9B

Plants mark the seasons by measuring photoperiod – Photoperiod Is the relative lengths of night and day – The timing of flowering Is one of the seasonal responses to photoperiod

– Plants whose flowering is triggered by photoperiod fall into two groups Short-day (long-night) plants Long-day (short-night) plants Short-day (long-night) plants Long-day (short-night) plants Darkness Flash of light Light Time (hr) Critical night length 24 0 Figure 33.11

Phytochrome is a light detector that may help set the biological clock – Phytochromes are proteins with a light-absorbing component That may help plants set their biological clock and monitor photoperiod

Results Short-day (long-night) plants Long-day (short-night) plants Darkness Flash of light Light Time (hr) Critical night length 24 0

Defenses Against Herbivores – Some plants recruit predatory animals To help defend them against certain herbivores 1 Damage to plant and chemical in caterpillar saliva 2 Signal transduction pathway 3 Synthesis and release of chemical attractants Plant cell 4 Recruitment of wasp 5 Wasp lays eggs Adapted from Edward Farmer, “Plant Biology: New Fatty Acid–Based Signals: A Lesson from the Plant World” Science 276 (1997), p  1997 American Association for the Advancement of Science. Figure 33.14A

PLANT DEFENSES Defenses against herbivores and infectious microbes have evolved in plants – Plants use chemicals To defend themselves from both herbivores and pathogens

– Local defenses include Microbe-killing chemicals and sealing off the infected area – Hormones Trigger generalized defense responses in other organs (systemic acquired resistance)