Unit6 - Subjective Questions

Classification of Plant Bio-Regulators:
Plant Bio-Regulators are broadly classified into two groups based on their effect on plant growth:

1. Growth Promoters: These substances enhance growth activities. Examples include:
   • Auxins (e.g., IAA, IBA, NAA)
   • Gibberellins (e.g., )
   • Cytokinins (e.g., Kinetin, Zeatin)
2. Growth Retardants/Inhibitors: These substances inhibit or retard growth. Examples include:
   • Ethylene (Ripening hormone)
   • Abscisic Acid (Stress hormone)
   • Growth retardants (e.g., Paclobutrazol, CCC)

Role of Auxins in Horticulture:

• Propagation: Synthetic auxins like IBA (Indole Butyric Acid) are widely used to induce rooting in hard-to-root cuttings.
• Prevention of Fruit Drop: Application of NAA (Naphthalene Acetic Acid) prevents pre-harvest fruit drop in crops like mango and citrus.
• Parthenocarpy: Auxins can induce fruit set without fertilization (seedless fruits), e.g., in tomatoes and grapes.
• Apical Dominance: They maintain apical dominance, which can be manipulated by pruning to encourage lateral branching.
• Thinning: NAA is used for fruit thinning in apples to ensure better fruit size and quality.

Drip Irrigation (Trickle Irrigation):
This is a method of watering plants drop by drop directly into the root zone through a network of valves, pipes, tubing, and emitters. It operates at low pressure and allows for precise water application.

Merits:

• Water Efficiency: It has a high water use efficiency (90-95%) as evaporation and runoff are minimized.
• Weed Control: Since water is applied only to the root zone, weed growth between rows is restricted.
• Fertigation: Soluble fertilizers can be applied through the system, ensuring efficient nutrient use.
• Salinity Management: It keeps salts diluted in the root zone, allowing cultivation in saline soils.
• Uniformity: Provides uniform water application to every plant.

Demerits:

• High Initial Cost: The installation cost of the system (filters, laterals, drippers) is high.
• Clogging: Emitters are prone to clogging by salt precipitates, algae, or dirt, requiring regular maintenance and acid treatment.
• Root Limitation: Roots may become concentrated only in the wetted zone, potentially affecting anchorage in tall trees during high winds.

Foliar Application:
Foliar application refers to the spraying of dilute fertilizer solutions directly onto the foliage (leaves) of the plants. The nutrients are absorbed through the stomata and cuticle.

Advantages:

• Rapid Response: It provides a quick remedy for acute nutrient deficiencies (e.g., Iron chlorosis).
• Micronutrients: It is the most efficient method for applying micronutrients (Fe, Zn, Mn, Cu) required in small quantities.
• Stress Conditions: Useful when root absorption is restricted due to drought, waterlogging, or soil fixation.
• Efficiency: Nutrient use efficiency is generally higher compared to soil application for certain elements.

Limitations:

• Concentration Limit: Only low concentrations can be used () to avoid leaf scorching (burning).
• Quantity: It cannot supply the total macronutrient () requirement of the crop due to the volume limit.
• Cost: Repeated applications are required, increasing labor and equipment costs.
• Wash-off: Rain immediately after application can wash off the nutrients.

Differences between Manures and Fertilizers:

Ring (Basin) Method:
In this method, a circular basin is formed around the trunk of the tree. The size of the ring typically corresponds to the canopy spread of the tree. Water is supplied to these rings through a connecting channel or furrow.

Mechanism:

1. A small bund is made around the tree trunk to prevent water from touching the trunk directly (to avoid collar rot diseases).
2. A larger ring is made at the drip line of the tree where the feeder roots are located.
3. Water fills the basin area between these two rings.

Suitability:

• Fruit Orchards: It is extensively used for fruit trees like Mango, Citrus, Guava, and Pomegranate.
• It is suitable for young trees as well as established orchards on level ground.
• It ensures water is applied where roots are most active, improving efficiency over flood irrigation.

Fertigation:
Fertigation is the precise application of water-soluble fertilizers, soil amendments, or other water-soluble products through an irrigation system (mostly Drip or Sprinkler).

Advantages:

• Nutrient Efficiency: Increases nutrient use efficiency (up to 90%) as nutrients are applied directly to the root zone.
• Reduced Losses: Minimizes leaching and volatilization losses compared to broadcasting.
• Labor Saving: Eliminates the manual labor required for fertilizer application.
• Flexibility: Nutrients can be applied frequently in small doses tailored to the crop's growth stage.
• Crop Quality: leads to better yield and quality due to the steady supply of water and nutrients.

Role of Gibberellins ():
Gibberellins are growth hormones primarily responsible for cell elongation and breaking dormancy.

• Breaking Dormancy: is used to break seed and bud dormancy in crops like potatoes and various flower seeds, ensuring uniform germination.
• Increasing Fruit Size: In grapes (e.g., Thompson Seedless), application leads to berry elongation and loosely packed bunches, significantly improving market value.
• Stem Elongation: It induces bolting (elongation of internodes) in rosette plants like cabbage, which is useful for seed production.
• Parthenocarpy: Like auxins, gibberellins can induce the development of seedless fruits in species like pear and guava.
• Delaying Senescence: Delays the ripening and aging of fruits (e.g., in citrus), allowing for a longer harvest window.

Green Manuring:
It is the practice of ploughing under or incorporating green plant tissues into the soil to improve soil fertility and physical structure.

Differentiation:

1. In-situ Green Manuring:

   • Green manure crops are grown in the same field and incorporated into the soil at the flowering stage.
   • Examples: Sunn hemp (Crotalaria juncea), Dhaincha (Sesbania aculeata).
   • Best for: Large field crops or during fallow periods in orchards.

2. Ex-situ (Green Leaf) Manuring:

   • Green leaves and tender twigs are collected from plants/trees grown on bunds, wastelands, or nearby forests and then incorporated into the main field.
   • Examples: Gliricidia, Pongamia, Neem leaves.
   • Best for: Intensive vegetable gardening or where the main crop covers the land continuously.

Sprinkler Irrigation:
Water is sprayed into the air and allowed to fall on the ground surface somewhat resembling rainfall. The spray is developed by the flow of water under pressure through small orifices or nozzles.

Preferred Conditions:

• Undulating Topography: It is highly suitable for uneven or rolling land where land leveling is too expensive or impossible.
• Sandy Soils: Suitable for soils with high infiltration rates where surface irrigation would lead to excessive deep percolation losses.
• Shallow Soils: Good for shallow soils where land leveling might remove the fertile topsoil.

Merits:

• Saves 30-50% water compared to surface methods.
• Can be used for frost protection (by creating a latent heat barrier).
• Soluble fertilizers and pesticides can be applied.

Demerits:

• Not suitable for heavy clay soils (slow infiltration leading to runoff).
• High wind velocity ( km/hr) disturbs distribution uniformity.
• Promotes fungal diseases in crops sensitive to leaf wetness.

Placement Methods:
Placement refers to applying fertilizers in the soil at a specific location (bands, pockets) relative to the seed or plant.

1. Plough-sole placement: Fertilizer is placed in a continuous band on the bottom of the furrow during ploughing. Useful for drylands.
2. Deep placement: Used for nitrogenous fertilizers (e.g., urea supergranules) in paddy fields to prevent volatilization.
3. Localized placement: Application of fertilizer into the soil close to the seed or plant (e.g., spot placement, ring placement).
4. Band placement: Applying fertilizer in bands to one or both sides of the row (side dressing).

Why Placement is better than Broadcasting:

• Reduced Fixation: Minimizes contact between soil and phosphatic/potassic fertilizers, reducing fixation (e.g., conversion of soluble to insoluble forms).
• Root Accessibility: Nutrients are placed in the root zone where they are easily accessible.
• Weed Control: Nutrients are available to the crop but less available to weeds growing between rows.
• Efficiency: Higher fertilizer use efficiency is achieved with lower quantities.

Biofertilizers:
Biofertilizers are preparations containing living or latent cells of efficient strains of microorganisms (bacteria, fungi, algae) that help in nutrient uptake by plants. They augment the availability of nutrients through nitrogen fixation, phosphate solubilization, or mobilization.

Importance:

• Eco-friendly and cost-effective.
• Reduce reliance on chemical fertilizers.
• Secrete growth-promoting substances.

Three Important Biofertilizers:

1. Rhizobium: Symbiotic nitrogen fixer, used for leguminous vegetables (Peas, Beans).
2. Azotobacter: Free-living nitrogen fixer, used for non-leguminous vegetables and flower crops.
3. PSB (Phosphate Solubilizing Bacteria): Solubilizes insoluble soil phosphorus, making it available to plants (e.g., Pseudomonas striata).

Physiological Role:
Ethylene () is a gaseous hormone known as the "ripening hormone." It triggers the ripening process in climacteric fruits by:

• Increasing the respiration rate (climacteric rise).
• Stimulating the breakdown of chlorophyll (loss of green color).
• Promoting the conversion of starch to sugars (sweetening).
• Softening fruit tissues through cell wall degradation.

Commercial Applications:

• Induced Ripening: Ethylene gas or Ethephon (a liquid that releases ethylene) is used to uniformly ripen bananas, mangoes, and tomatoes, ensuring they reach the market in ready-to-eat condition.
• Degreening: Used in citrus (oranges, lemons) to break down green chlorophyll in the peel, revealing the orange/yellow pigments, without necessarily affecting internal quality.
• Flowering: Induces flowering in pineapples and mangoes for synchronized harvesting.

Starter Solution:
A starter solution is a dilute solution of fertilizer (usually containing , , and in a 1:2:1 or 1:1:2 ratio) applied to young plants at the time of transplanting.

Preparation:
It is prepared by dissolving small quantities of high-analysis fertilizers (like Urea, SSP, MOP) or specialized water-soluble fertilizers in water. A typical concentration involves dissolving about 20g of fertilizer mix in 10 liters of water.

Benefits:

• Transplant Shock: It helps seedlings recover quickly from transplanting shock.
• Root Establishment: The immediate availability of Phosphorus promotes rapid root development.
• Early Growth: Accelerates early vegetative growth and ensures uniform stand establishment in crops like Chillies, Tomatoes, and Brinjals.

Comparison:

Integrated Nutrient Management (INM):
INM is a strategy that optimizes all aspects of nutrient cycling. It involves the combined use of mineral fertilizers, organic manures, biofertilizers, crop residues, and legumes in cropping systems to maintain soil fertility and plant nutrient supply at an optimum level.

Why it is Essential:

1. Soil Health: Continuous use of chemical fertilizers alone degrades soil structure and microbial life. INM restores organic carbon and biological activity.
2. Sustainability: It prevents nutrient mining and maintains long-term productivity.
3. Cost Reduction: Using locally available organic inputs and biofertilizers reduces the dependency on expensive chemical inputs.
4. Environmental Safety: Reduces groundwater pollution caused by nitrate leaching from excessive chemical fertilizer use.
5. Micronutrients: Organic components in INM supply micronutrients that are often missing in standard NPK fertilizers.

Water Use Efficiency (WUE):
WUE is defined as the yield of marketable crop produced per unit of water used in evapotranspiration. Mathematically, .

Practices to Improve WUE:

1. Mulching: Covering the soil surface with plastic or organic mulch (straw/leaves) drastically reduces water loss through evaporation and suppresses weeds that compete for water.
2. Modern Irrigation Methods: Adopting Drip or Micro-sprinkler irrigation systems minimizes conveyance and runoff losses, delivering water directly to roots.
3. Hydrogel Application: Using hydrogels (superabsorbent polymers) in the planting pit increases the soil's water-holding capacity and releases moisture slowly during dry spells.

Role of Growth Retardants:
Growth retardants suppress the synthesis of Gibberellins or interfere with cell division/elongation. They are used to control vegetative vigor and divert energy towards reproductive growth.

Applications:

• Creating dwarf potted plants.
• Controlling tree size for high-density planting.
• Inducing flowering.

Example: Paclobutrazol (Cultar):

• Mango: Paclobutrazol is widely used in Mango cultivation. When applied to the soil (collar drench), it inhibits vegetative flushing.
• Flowering: By suppressing vegetative growth, it promotes early and uniform flowering, especially in 'off' years or alternate bearing varieties.
• Canopy Management: It helps in keeping the tree canopy compact, facilitating easier harvesting and management.

Water Requirement Considerations:

• Fruit trees are perennial; their water needs vary significantly by season (dormancy vs. active growth).
• Deep-rooted trees can tap into lower soil moisture, but surface feeders (like citrus) need frequent irrigation.

Critical Stages:
Critical stages are physiological phases during crop growth where water stress results in irreversible yield or quality loss. Irrigation is mandatory during these phases.

Common Critical Stages in Fruit Crops:

1. Flowering/Fruit Set: Water stress here leads to massive flower and fruit drop (e.g., in Mango, Citrus).
2. Fruit Development/Enlargement: Lack of water results in small-sized fruits, cracking (e.g., Pomegranate), or poor juice content.
3. Root Growth: Active root flush periods require adequate moisture.
   Note: Irrigation is often withheld before flowering (in Bahar treatment) to induce stress and promote flower bud differentiation, but must be resumed once fruit set occurs.

Complex Fertilizers:
Complex fertilizers are commercial fertilizers that contain at least two or more of the primary nutrients (, , ) in chemical combination.

Examples:

• DAP (Di-Ammonium Phosphate): Contains 18% N and 46% .
• Nitrophosphates (Suphala): Various grades like 20:20:0 or 15:15:15 ().

Advantages:

• Convenience: Farmers can apply multiple nutrients in a single application, saving labor and time.
• Uniform Distribution: Each granule contains the nutrients in a fixed ratio, ensuring uniform distribution across the field compared to mixing straight fertilizers manually.
• Handling: They are usually granular and non-hygroscopic, making storage and handling easier.

Top Dressing:
Top dressing involves the application of fertilizers to the standing crop after it has established. It is usually done by broadcasting or side-dressing.

Primary Nutrient:
It is most commonly used for Nitrogen fertilizers (e.g., Urea).

Reasons:

1. Mobility: Nitrogen is highly mobile in the soil and easily leached by water. Applying the full dose at sowing (basal) would lead to significant losses before the plant matures.
2. Split Application: Nitrogen is applied in splits (part basal, part top dressing) to match the crop's uptake pattern during vegetative and reproductive stages.
3. Phosphorus/Potassium: These are immobile and usually placed deep (basal) at the time of planting; top dressing them is less effective as they won't move down to the roots easily.