Core Concept: The Yield Gap

• The difference between the potential yield (genetically determined under ideal conditions) and the actual yield achieved by farmers is known as the yield gap. This gap exists due to physiological limits imposed by the environment and sub-optimal management.
• Potential Yield (Genetic Potential): The yield of a crop cultivar when grown in an environment to which it is adapted, with non-limiting nutrients and water, and with pests, diseases, weeds, and other stresses effectively controlled. (e.g., Yield in research plots).
• Attainable Yield: The yield achieved by a farmer using a good management package but under a specific set of agro-climatic conditions, which may include some stresses.
• Actual Yield: The average yield harvested by farmers in a region.

Physiological Limits of Crop Yield

Yield is limited by the factor that is most sub-optimal. These limits can be categorized based on the processes they affect.

Limits by Physiological Processes:

• Photosynthesis (Source Limitation):
  • Light Capture:Limited by leaf area index (LAI), leaf angle, canopy architecture, and photoperiod.
  • Light Conversion:Limited by the efficiency of the photosynthetic apparatus (RuBisCO efficiency, photorespiration in C₃ plants).
  • C₃ vs. C₄ Plants:C₄ plants (Maize, Sorghum) have a higher theoretical maximum yield due to their CO₂ concentrating mechanism which suppresses photorespiration, especially under high temperature and light.

2. Respiration (Loss Process):
   • Maintenance Respiration:Energy required to maintain living tissues. Increases with temperature.
   • Growth Respiration:Energy required to synthesize new biomass. A significant portion (up to 50%) of photosynthate is lost through respiration, reducing net biomass accumulation.

3. Partitioning (Sink Limitation):
   • The process of distributing photosynthates (assimilates) to different plant parts (roots, stems, leaves, grains).
   • Harvest Index (HI):The proportion of total biological dry matter allocated to the economically useful part (grain, tuber).
   • Limit:A poor HI means the plant is putting energy into vegetative parts instead of the harvestable yield. Modern cultivars have a higher HI (e.g., 0.4-0.5 for cereals).

4. Phenological Development:
   • The timing of key developmental stages (germination, flowering, maturity) must be synchronized with optimal environmental conditions (e.g., avoiding heat stress at flowering, frost at germination).

Limits by Environmental (Ecological) Factors:

Variability in Relation to Ecological Optima

• This refers to the fluctuation in yield caused by the interaction between the crop’s genetic potential and the variability of environmental conditions.
• Ecological Optima: The specific range for each environmental factor (light, water, temperature, soil) within which a crop genotype performs best and expresses its maximum attainable yield.

Causes of Variability (Why yields vary):

Temporal Variability (Within a location over time):

• • Rainfall variability:Drought or untimely rains during critical growth stages.
  • Temperature fluctuations:Unusual heatwaves or cold spells.
  • Weather extremes:Hailstorms, cyclones, unseasonal frost.

Spatial Variability (Between different locations/fields):

• • Soil heterogeneity:Differences in soil texture, depth, fertility, and pH across a field.
  • Topography:Slopes affect water drainage and erosion.
  • Microclimates:Variations in temperature, wind, and humidity within a region.

Genotypic Variability: Different cultivars have different tolerance ranges (ecological amplitude) for environmental stresses. A one-size-fits-all variety performs variably across different environments.

Management Strategies to Overcome Physiological Limits:

To Improve Photosynthesis & Light Capture:

• • Optimum Plant Population & Spacing:Ensures efficient light interception.
  • Optimum Planting Date:Synchronizes crop growth with periods of high solar radiation.
  • Intercropping:Maximizes seasonal light use efficiency.

To Manage Water Limits:

• • Irrigation Scheduling:Providing water at critical growth stages (e.g., crown root initiation, flowering in cereals).
  • Water Conservation:Mulching, conservation tillage.
  • Drought-Tolerant Varieties.

To Manage Temperature Stress:

• • Adjusting Sowing Time:To avoid heat stress at flowering (e.g., early sowing of wheat to avoid terminal heat).
  • Use of Mulches:To moderate soil temperature.

To Improve Partitioning (Sink Strength):

• • Balanced Fertilization:Especially K, which aids in assimilate translocation.
  • Use of Growth Regulators:g., Cytokinins to enhance sink strength.
  • Selection of High-HI Varieties.

To Manage Soil Variability:

• • Site-Specific Nutrient Management (SSNM):Precision agriculture to address spatial variability.
  • Soil Amendments:Gypsum for sodic soils, lime for acidic soils.
  • Integrated Nutrient Management (INM).

Important One-Liners for Quick Revision (ASRB NET Focus)

• The difference between potential and actual yield is the Yield Gap.
• The maximum possible yield under ideal conditions is Potential Yield.
• The proportion of total biomass allocated to the economic yield is the Harvest Index (HI).
• C₄ plantshave a higher theoretical yield potential than C₃ plants due to the absence of photorespiration.
• Photorespirationis a major yield-limiting factor in C₃ plants (e.g., Wheat, Rice).
• Heat stress at floweringcauses pollen sterility, a critical yield determinant in cereals.
• Water deficit causes stomatal closure, limiting CO₂ intake for photosynthesis.
• Temporal variabilityin yield is primarily caused by unpredictable weather patterns.
• Spatial variabilityin yield is often due to soil heterogeneity.
• Ecological optimarefer to the range of environmental conditions where a crop performs best.
• Agronomic management aims to minimize the yield gapby adjusting practices to meet ecological optima.
• Precision agriculture techniques help manage spatial variability within a field.