CHAPTER 39 CONTROL SYSTEMS IN PLANTS I PLANT

CHAPTER 39 CONTROL SYSTEMS IN PLANTS

I. PLANT HORMONES A. PLANT HORMONES HELP COORDINATE GROWTH, DEVELOPMENT, AND RESPONSES TO ENVIRONMENTAL STIMULI 1. THE SITE OF ACTION, THE DEVELOPMENTAL STAGE OF THE PLANT, THE CONCENTRATION OF THE HORMONE, AND THE PRESENCE OF OTHER HORMONES ALL AFFECT REACTION TO A HORMONE. 2. EACH HORMONE IS PRODUCED PRIMARILY IN THE APICAL MERISTEM OF THE SHOOT.

LIST OF PLANT HORMONES • AUXIN: SIMULATES CELL ELONGATION IN DIFFERENT TARGET TISSUES. • CYTOKININS: PRODUCED IN ACTIVELY GROWING TISSUES SUCH AS ROOTS, EMBRYOS, AND FRUITS, STIMULATE CELL DIVISION. • GIBBERELLINS: PRODUCED IN ROOTS AND YOUNG LEAVES STIMULATE GROWTH IN LEAVES AND STEMS. • ABSCISIC ACID: SLOWS PLANT GROWTH AND FAVORS THE DORMANT STATE • ETHYLENE: HELPS CONTROL FRUIT RIPENING AND SENESCENCE OF PLANT CELLS AND ORGANS.

II. PLANT MOVEMENTS AS MODELS FOR STUDYING CONTROL SYSTEMS A. TROPISMS ORIENT THE GROWTH OF PLANT • THESE GROWTH RESPONSES INCLUDE: 1. PHOTOTROPISM: GROWTH RESPONSE TO LIGHT 2. GRAVITROPISM: GROWTH RESPONSE TO GRAVITY 3. THIGMOTROPISM: GROWTH RESPONSES TO TOUCH

PHOTOTROPISM (A CLOSER LOOK) • • HORMONE: AUXIN STEPS: 1. APICAL MERISTEM PRODUCES AUXIN 2. AUXIN MOVES TO ZONE OF ELONGATION 3. STIMULATION GENERATES GROWTH

GRAVITROPISM (A CLOSER LOOK) • HORMONES: AUXIN AND GIBERRELLINS • STEPS (NOT WELL DOCUMENTED): SCIENCE AS A PROCESS • SEE EARLY EXPERIMENTS ON PHOTOTROPISM (FIG. 39. 2)

III. CONTROL OF DAILY AND SEASONAL RESPONSES A. BIOLOGICAL CLOCKS CONTROL CIRCADIAN RHYTHMS • FREE-RUNNING CIRCADIAN CYCLES ARE APPROXIMATELY 24 HOURS LONG BUT ARE ENTRAINED TO EXACTLY 24 HOURS BY THE DAY/NIGHT CYCLE.

B. PHOTOPERIODISM SYNCHRONIZES MANY PLANT RESPONSES TO CHANGES OF SEASON • PHOTOPERIODISM: RESPONSE OF PLANTS TO CHANGES IN PHOTOPERIODS • CIRCADIAN RHYTHM: AN INTERNAL CLOCK THAT MEASURES THE LENGTH OF DAY AND NIGHT • PHYTOCHROMES: LIGHT ABSORBING PIGMENTS FOUND IN PLANTS

A CLOSER LOOK • Pfr: RESETS THE CIRCADIAN CLOCK • Pr: ACTIVATED IN LEAVES • SPECIAL NOTES – PHYTOCHROMES ARE IN EQUILLIBRIUM DURING DAYLIGHT – Pr IS ACCUMULATED AT NIGHT – SUNLIGHT CONVERTS Pr BACK TO Pfr TO REACH EQUILLIBRIUM ONCE AGAIN – NIGHTLENGTH RESETS THE CIRCADIANRHYTHM CLOCK

• SOME DEVELOPMENTAL PROCESSES, INCLUDING FLOWERING IN MANY PLANT SPECIES, REQUIRE A CERTAIN PHOTOPERIOD. • THE RELATIVE LENGTHS OF NIGHT AND DAY. – FOR EXAMPLE, A CRITICAL NIGHT LENGTH SETS A MINIMUM (IN SHORT-DAY PLANTS) OR MAXIMUM (IN LONG-DAY PLANTS) NUMBER OF HOURS OF DARKNESS REQUIRED FOR FLOWERING.

IV. PHYTOCHROMES A. • PHYTOCHROMES FUNCTION AS PHOTORECEPTORS IN MANY PLANT RESPONSES TO LIGHT AND PHOTOPERIOD PHYTOCHROMES EXIST IN TWO PHOTOREVERSIBLE STATES, WITH CONVERSION OF Pr TO Pfr, TRIGGERING MANY DEVELOPMENTAL RESPONSES.

B PHYTOCHROMES MAY HELP ENTRAIN THE BIOLOGICAL CLOCK • PHYTOCHROME CONVERSION MARKS SUNRISE AND SUNSET, PROVIDING THE CLOCK WITH ENVIRONMENTAL CUES.