Plant Transport and Physiology

The directional growth movements in which direction of growth is determined by the direction of stimulus are known as tropic movements. Tropic movement of plants/plant parts in response to moisture or water is called as hydrotropism. This makes options A and B incorrect as the question is about the movement of plants in response to light stimulus. Growth movement of plants in response to light is known as phototropism. Growth movement of plants towards the stimulus is known as positive tropism while movement of plant/plant parts away from the stimulus is known as negative tropism. This makes option C incorrect and option D correct as the movement of root away from light is known as negative phototropism. The correct answer is D.

Phototropism is the growth of organisms in response to a light stimulus. It is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light have a chemical called auxin that reacts when phototropism occurs. This causes the plant to have elongated cells on the farthest side from the light. Phototropism is one of the many plant tropisms or movements which respond to external stimuli. Growth towards a light source is called positive phototropism, while growth away from light is called negative phototropism. Thus, option D is correct and other options are wrong.

• Plant cells change their shape by changing the amount of water in them, resulting in swelling or shrinking. 

• Hydrotropism is a plant's growth response to water concentrations. The response can be positive (towards the water) or negative (away from the water). Roots, for instance, are positively hydrotropic. 
• That means they grow towards moist soils to avoid drought stress. Once a root cap has sensed water, it bends and then the root grows towards it. 

The greater growth of roots in moist soil zones than in dry soil zones is not usually a result of hydrotropism. Hydrotropism requires a root to bend from a drier to a wetter soil zone. Roots require water to grow so roots that happen to be in moist soil will grow and branch much more than those in dry soil. 
Roots cannot sense water inside intact pipes via hydrotropism and break the pipes to obtain the water. 
Roots cannot sense water several feet away via hydrotropism and grow toward it. At best hydrotropism probably operates over distances of a couple millimeters.

A flexible hinge located at the base of the stalk of the leaf is called as the pulvinus and a sudden loss of turgor pressure in the motor cells of the pulvinus is called as a thigmonastic movement which is seen in Mimosa pudica.

The process of hydrotropism is started by the root cap sensing water and sending a signal to the elongating part of the root. Coconut roots usually grow to a depth of close to 0.80 m with 60-90% found in the top 0.5 m of the soil. In 10-year old dwarf coconut grown under rainfed conditions, the effective root zone of absorption was at 1.4 m from the trunk. Coconut has no root hairs but it produces plenty of roots with a large number of rootlets. Coconut tree roots growth is determined by a stimulus or gradient in water concentration. 

Tropic movement is growth movement shown by plant in response to a unidirectional external stimulus. It is specific for direction and depends on the direction of stimulus. Movement towards the direction of stimuli is called as positive tropism while movement away from stimuli is called as negative tropism. It results in positioning of the plant part in the direction of the stimulus.

Turgor movement is a reversible change in position of a plant part due to a change in turgidity of cells. It is mostly caused by contact. In plants, turgor changes occur in thin walled cells of pulvinus present on lower side and pulvinnules present on upper side. Both side has different concentration of potassium and calcium ions. Upon touching, potassium ion concentration changes in the cells which results in turgor movement.

Tropic movement is growth movement shown by plant in response to a unidirectional external stimulus. It is specific for direction and depends on the direction of stimulus. Movement towards the direction of stimuli is called as positive tropism while movement away from stimuli is called as negative tropism. Plant part shows tropic response in terms of cell elongation. For example, in phototropism, faster cell elongation is seen on the shaded side than on the illuminated side. 

Auxin is a plant hormone that plays important role in the growth of stem and roots to gravity. Geotropism is the plant response to the gravity. Root shows positive geotropism and shoot shows negative geotropism. In stems, auxin accumulates towards the lower side. It results in increase in cell expansion and results in the shoot growing upright. In roots, auxin accumulates towards the lower side. However, it inhibits cell expansion and roots grow downwards.

Phototropism is the growth of plant in response to the light stimuli. Drooping of tamarind leaves after sunset is due to phototropism. Phototaxis is the movement shown by organism in the direction of the external light stimuli. Photonasty is the nastic movement shown by organism in response to the external light stimuli. Chemotaxis is the movement shown by organism in the direction of the external chemical stimuli. 

Geotropism is the response of plant parts to the gravity. Negative geotropism is growth of part of plant against the gravity. Positive geotropism is growth of part of plant in the direction of gravity. Downward growth of roots is example of positive geotropism. Primary stems grows against the gravity in upward direction and thus shows negative geotropism. 

The movement or growth of any part of a plant in response to an external stimulus is called tropism or tropic movement because most of the time the root grows underground and stem grows up to the ground and bending of stem towards light are the good examples of negative and positive phototropism.

Chemotropism is the movement of plants in response to a chemical stimulus. A classic example of this type of movement is the growth of the pollen tube towards the ovule, during fertilization, in a flower.