🌦️ Temperature and Its Effects on Crop Growth

Temperature acts like a biological clock for crops. It determines how fast a plant germinates, grows leaves, flowers, fills grain, and reaches maturity. If temperature goes beyond the suitable range, growth slows down, stress appears, and yield can fall sharply.

Why Temperature Is So Important

Temperature influences nearly every plant process:

• seed germination,
• enzyme activity,
• photosynthesis and respiration,
• root and shoot growth,
• flowering and pollination,
• grain filling and ripening.

This is why temperature is one of the first weather elements considered in agrometeorology.

Cardinal Temperatures

Every crop has three important thermal limits called cardinal temperatures.

Examples of Cardinal Temperatures

Thermal Indices

Temperature does not just affect crops day by day; it also accumulates over time. This idea is used in thermal indices.

Growing Degree Days (GDD)

GDD or heat units measure the thermal time available for crop development.

GDD = Sum of [(Tmax + Tmin) / 2 - Tbase]

Where:

• Tmax = daily maximum temperature
• Tmin = daily minimum temperature
• Tbase = base temperature below which crop development does not occur

Photothermal Units (PTU)

PTU = GDD × day length

PTU is useful when both temperature and photoperiod influence crop development.

Why Thermal Indices Matter

• They help predict flowering and maturity.
• They support sowing-date decisions.
• They help compare crop performance in different seasons or locations.

For example, two wheat crops may be sown on different dates, but the one accumulating required heat units earlier will also reach maturity earlier.

Effects of Temperature on Crop Growth

Germination

• Soil temperature strongly affects seed germination.
• Many tropical crops germinate best around 25-30°C.
• Cool-season crops often germinate well around 15-20°C.

Vegetative Growth

• Cell division and elongation depend on favorable temperature.
• Enzyme reactions generally become faster with rising temperature within the suitable range.
• This is often described using the Q₁₀ effect, where reaction rate may roughly double for every 10°C rise within limits.

Reproductive Stage

This is usually the most temperature-sensitive stage.

• In rice, temperature above 35°C during flowering can cause spikelet sterility.
• In wheat, terminal heat stress during grain filling reduces grain size and final yield.

Vernalization

Some crops need exposure to low temperature before they can flower properly.

• Common examples: wheat, barley, and brassicas
• Typical low-temperature range: 0-5°C
• Winter wheat generally needs longer cold exposure than spring wheat

Temperature Stress in Practical Farming

High Temperature Stress

• accelerates crop duration,
• increases respiration losses,
• reduces pollen viability,
• increases evapotranspiration and water demand.

Low Temperature Stress

• delays germination and growth,
• causes chilling or frost injury in sensitive crops,
• reduces nutrient uptake and slows metabolism.

Summary Cheat Sheet