(3)

Auxins are widely used as herbicides by the gardeners to prepare weed-free lawns. 2, 4-D, a synthetic auxin is widely used to kill dicotyledonous weeds but it does not affect mature monocotyledonous plants.

(2)

Spraying sugarcane crop with gibberellins increases the length of the stem and therefore increases the yield by as much as 20 tonnes per acre.

(1)

Phototropism is the movement of shoot towards the light. Auxins get accumulated towards the shady region of the stem which stimulate the cells on that side to grow longer. This results in the bending of stem towards the light. This was seen in the experiments conducted by F.W. Went on Avena coleoptile tips.

(2)

(1)

Ethylene promotes rapid internode/petiole elongation in deep water rice plants. It helps leaves/upper parts of the shoot to remain above water.

(4)

Gibberellins are one of the promotory plant growth regulators. Spraying juvenile conifers with gibberellic acids hastens the maturity period, thus leading to early seed production.

(2)

Ethylene is a gaseous plant growth regulator that promotes root growth and root hair formation, thus helping the plants to increase the absorption surface. Gibberellic acid (${\mathrm{GA}}_3$) is used to speed up the malting process in the brewing industry. Cytokinin helps to overcome apical dominance. Auxin kills dicotyledonous weeds in the field.

(3)

Ethylene is a gaseous hormone that promotes female flowers in cucumbers thereby increasing the yield.

(4)

Auxins like 2,4-D act as weedicides by being selectively harmful to broad-leaved dicot weeds.

(2)

(1)

Gibberellic acid is involved in growth promoting activities, such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation, and is called a plant growth promoter. It breaks seed dormancy, which is antagonistic to abscisic acid.

(2)

Synthetic auxins like NAA and 2, 4-D are often employed for inducing flowering in pineapple. Ethylene also stimulates flowering in pineapple and related plants. Hence, both auxin and ethylene can be applied artificially to induce flowering in pineapple plants throughout the year to increase yield.

(2)

In low concentrations, auxins such as 2,4-D (2,4-Dichlorophenoxy acetic acid) are useful in preventing pre-harvest fruit drop and leaf drop.

(2)

Cytokinin and auxin are two plant hormones that are supplied to the tissue culture medium in definite proportions. They bring about cell division and differentiation of callus. A low auxin to cytokinin ratio promotes shoot formation whereas a high auxin to cytokinin ratio promotes rooting of callus.

(1)

Avena curvature test is a test based on the experiments of Went (1928) which can measure auxin up to 300 mg/litre. Indole-3-acetic acid (IAA) is a universal natural auxin and Avena curvature test serves as an accurate bioassay for this plant hormone.

(3)

Auxin bioassay is a quantitative test as it measures concentration of auxin to produce the effect and the amount of the effect. Avena curvature is based upon experiments of Went (1928). 10° curvature is produced by auxin concentration of 150 mg/litre at 25°C and 90% relative humidity. The test can measure auxin up to 300 mg/litre. Auxin from a shoot tip or any other plant organ is allowed to diffuse in a standard size agar block (generally 2 × 2 × 1 mm). Auxin can also be dissolved directly in agar. 15–30 mm long oat coleoptile grown in dark is held vertically over water. 1 mm tip of coleoptile is removed without injuring the primary leaf. After 3 hours a second decapitation is carried out for a distance of 4 mm. Primary leaf is now pulled loose and agar block supported against it at the tip of decapitated coleoptile. After 90–110 minutes, the coleoptile is found to have bent. The curvature is measured. It can also be photographed and the curvature known from shadow graph.

(2)

Auxins induce cell elongation. In a differentially illuminated plant, they accumulate in the shaded part, causing elongation of the cells in the shaded part. This unequal elongation on two sides causes the plant to curve or bend towards the light source, i.e., phototropic curvature.

(4)

(1)

Abscisic acid prepares plants to cope with stress conditions like drought, etc. by inducing stomatal closure and other reactions. Hence, it is named stress hormone.

(4)

Cytokinins help to overcome apical dominance. They promote nutrient mobilisation which helps in the delay of leaf senescence.

(1)

ABA is a plant growth inhibitor and an inhibitor of plant metabolism.

Apical dominance promotes growth of apical bud.

Statements A, B and C are correct.