Plant Physiology

1. MADRAS CHRITIAN COLLEGE, CHENNAI (AUTONOMOUS)
PRESENTATION ON
TROPIC MOVEMENT IN PLANTS
BY ALOK KUMAR
3RD BSC BOTANY

2.  IN HIGHER PLANTS WHICH ARE FIXED, THE MOVEMENTS ARE RESTRICTED ONLY TO BENDING OR
CURVATURE OF THEIR PARTS. SUCH MOVEMENTS ARE CALLED , CURVATURE MOVEMENTS.
 THERE CURVATURE MOVEMENTS CAN BE OF TWO TYPES-
AUTONOMIC (SPONTANEOUS) OR PARATONIC (INDUCED)
 MOVEMENTS IN HIGHER PLANTS CAN BE TROPIC MOVEMENT OR NASTIC MOVEMENT-
 TROPIC MOVEMENTS ARE GROWTH MOVEMENTS WHICH ARE PARATONIC IN NATURE. THESE
MOVEMENTS ARE PERFORMED IN RESPONSE TO DIRECTIONAL STIMULUS. THESE ARE IRREVERSIBLE
IN NATURE. THEY ARE PERMANENT.
 NASTIC MOVEMENTS ARE NOT GROWTH MOVEMENTS WHICH ARE AUTONOMIC IN NATURE.THESE
MOVEMENTS DO NOTREQUIRE ANY PARTICULAR DIRECTIONAL STIMULUS. THESE ARE REVERSIBLE
IN NATURE. THEY ARE TEMPORARY.

3. TROPIC MOVEMENTS
S.N. TYPE STIMULUS
1 PHOTOTROPISM LIGHT
2 GRAVITROPISM GRAVITY
3 HYDROTROPISM WATER
4 CHEMOTROPISM CHEMICAL
5 THIGMOTROPISM TOUCH

4. PHOTOTROPISM
 GENERALLY PHOTOTROPIC RESPONSE INCLUDES
BENDING OF GROWING STEM TOWARDS SUN.
 PHOTO-TROPIC MOVEMENT CAN BE POSITIVE
(TOWARDS SOURCE OF LIGHT) OR NEGATIVE (AWAY
FROM LIGHT).
 PLANT SHOOTS ARE POSITIVELY PHOTOTROPIC
WHILE ROOTS ARE NEGATIVELY PHOTOTROPIC.
 LEAVES ARE PLAGIOTROPIC IN NATURE.

5.  PHOTOTROPISM IS REGULATED BY AUXIN.
 WHEN THE SHOOT TIP RECEIVES UNIFORM
LIGHT FROM ALL DIRECTION, THE CONC. OF
AUXIN REMAINS UNIFORM IN SHOOT TIP AND
CELL ELONGATION OF ALL CELL HAPPEN
UNIFORM. SO SHOOT TIP IS UPRIGHT.
 WHEN TIP RECEIVES UNILATERAL LIGHT,
CONC. OF AUXIN COMES MORE IN SHADED SIDE
AND IT ELONGATES CELL OF DARK SIDES ONLY.
SO THE TIP REGION BENDS TOWARDS SOURCE OF
UNILATERAL LIGHT.

6.  UNILATERAL BLUE LIGHT IS KNOWN TO BE EFFECTIVE AN D ESSENTIAL IN CAUSING
PHOTOTROPIC CURVATURE.
 THE SINGULAR IMPACT OF PHOTOTROPISM, ON THE OTHER HAND, IS THAT IT ORIENTS
GROWTH AND LEAF ANGLE TOWARD INCIDENT LIGHT IN ORDER TO MAXIMIZE LIGHT
INTERCEPTION FOR PHOTOSYNTHESIS.
 THE BENDING OF COLEOPTILES AND HYPOCOTYLS IS ONLY THE MOST VISIBLE PART OF A
LARGER BLUE-LIGHT SYNDROME THAT PLANTS USE TO OPTIMIZE PHOTOSYNTHESIS.
 PLANTS ALSO USE BLUE LIGHT TO CONTROL STOMATAL OPENING AND FACILITATE GAS
EXCHANGE AS WELL AS TO RELOCATE CHLOROPLASTS WITHIN THE CELL.
 HOLOPROTEIN PHOTOTROPIN (PHOTROPIN 1 AND PHOTOTROPIN 2) ARE FOUND TO
EFFECTIVE IN THIS PROCESS OF CURVATURE. THEY ALSO AFFECT THE STOMATAL OPENING
AT HIGH LIGHT INTENSITIES.

7. GRAVITROPISM
 TROPIC MOVEMENT WHICH TAKE PLACE IN RESPONSE TO GRAVITY STIMULUS IS CALLED
GRAVITROPISM.
 IF THIS MOVEMENT IS IN DIRECTION OF GRAVITY , IT CALLED AS POSITIVE GRAVITROPISM.
IF THIS MOVEMENT IS IN OPPOSITE DIRECTION OF GRAVITY, THEN IT IS CALLED AS
NEGATIVE GRAVITROPISM.
 NORMAL PLANT ROOTS ARE POSITIVELY GRAVITROPIC, WHILE CORALLOID ROOTS AND
AERIAL ROOTS OF MANGROVES ARE NEGATIVELY GRAVITROPIC.
 THIS PARTICULAR EXPERIMENT CAN BE PERFORMED BY USING MAIZ SEEDLINGS.
 FOUR STAGES ARE SEEN HERE-
PLAGIOGRAVITROPIC, AGRAVITROPIC
ORTHOGRAVITROPIC, NORMAL GRAVITROPIC

9.  THE EFFECT OF UNILATERAL STIMULUS OF GRAVITY CAUSES UNEQUAL DISTRIBUTION OF
AUXIN IN ROOT TIP.
 THEREFORE MORE AUXIN AT LOWER SIDE THAN THAT OF UPPER SIDE. HENCE UPPER SIDE
CELLS ELONGATE TO GIVE POSITIVE GEOTROPISM.

10.  NEMEC AND HEBERLANDT PUT FORWARD ‘STARCH-STATOLITH HYPOTHESIS’.
 THIS STATES, SOME SPECIALIZED PLASTIDS CALLS AMYLOPLASTS CONTAINING A FEW
STARCH GRAINS INSIDE THEM ARE PRESENT IN COLUMELLA CELLS OF THE ROOT CAP.
 THESE CELLS ARE KNOWN AS STATOCYSTS
WHICH ARE GRAVITY SENSITIVE CELLS.
 WHEN ROOT TIP ALONG WITH ITS ROOT CAP
IS PLACED HORIZONTALLY, STATOLITH
SEDIMENT UNDER INFLUENCE OF GRAVITY
STIMULUS ON THE BASAL SIDES OF STATO-
CYSTS AND PROVIDE THE BASIC PERCEPTION
MECHANISM FOR GRAVITROPIC RESPONSE.

11. HYDROTROPISM
 THE TROPICAL MOVEMENTS OCCURRING IN RESPONSE TO WATER STIMULUS ARE CALLED
AS HYDROTROPIC MOVEMENTS.
 THESE ARE COMMONLY FOUND IN YOUNG ROOTS AND THIS CAN BE DEMONSTRATED BY
SIMPLE EXPERIMENTS.
 SOME SEEDS SOAKED IN WATER THE PREVIOUS NIGHT ARE KEPT IN A WIRE GAUGE
COVERED WITH SAW DUST. THE WIRE GAUGE IS THEN KEPT SLANTING IN HUMID
CONDITION. AFTER A FEW DAYS THE RADICLES WILL BE SEEN BENDING TOWARDS THE
MOIST SAW DUST.

12. CHEMOTROPISM
 CHEMOTROPIC MOVEMENTS OCCUR IN RESPONSE TO SOME CHEMICAL STIMULUS.
 THESE ARE BEST EXHIBITED BY FUNGAL HYPHAE AND POLLEN TUBES.
 POLLEN TUBE ATTRACTENTS SUCH
SOME POLYPEPTIDE CHAIN
PRESENT IN MICROPYLAR OPENING,
 THERE IS PROPER CHEMICAL SIGNA-
LING INVOLVED IN POLLEN TUBE
GERMINATION.

13. THIGMOTROPISM
 THESE MOVEMENTS TAKE PLACE IN RESPONSE OF TOUCH OR CONTACT STIMULUS AND ARE
VERY COMMON IN PLANTSWHICH CLIMB BY TENDRILS.
 THESE MOVEMENTS ARE VERY COMMON IN CUCUBITS, SOME MEMBERS OF FABACEAE ALSO.
 THE TIP OF TENDRIL IN THE BEGINNING MOVES
FREELY BUT AS SOON AS IT COMES IN CONTACT
WITH A SOLID OBJECT WHICH MAY PROVIDE
SUPPORT, IT TWINES AROUND THE OBJECT SO
THAT PLANT MAY CLIMP UPWARDS.