Photothermal Quotient & Read Your Yield Potential

The photothermal quotient is the ratio of growth energy to development speed during a crop's grain-set window: PTQ = mean daily solar radiation ÷ (mean temperature − base temperature). It captures how much radiation the crop captures per unit of thermal development. A high PTQ means plenty of light per degree-day, which builds more carbohydrate per stage of development and lets more florets survive to set grain. Fischer (1985) showed grains per m² in wheat rises strongly with PTQ over the roughly 30 days before anthesis.

PTQ = radiation ÷ (mean temp − base temp). For example, wheat with a 4.5°C base temperature, 18 MJ/m²/day of radiation and a 17°C mean over the window gives 18 ÷ (17 − 4.5) = 18 ÷ 12.5 = 1.44 — a moderate-to-high value. The denominator is the thermal driving rate of development; the numerator is the energy available for growth. The tool does this live as you type and marks your value on the grains/m² response curve.

For wheat, a PTQ below about 1.0 is heat-capped (warm conditions burn through development fast with little radiation per degree-day), roughly 1.0–1.5 is a moderate, solid potential, and above 1.5 is high — plenty of radiation per degree-day supporting a high grain number. Typical good environments sit around 0.8–1.7. The figure to chase is a high PTQ during the critical window, not just warm, sunny weather overall.

Warmer temperatures speed development, so the crop spends fewer days in the critical pre-anthesis window. Fewer days means less total radiation captured per stage of floret development, so fewer florets survive to set grain. Because temperature is in the denominator of PTQ, higher heat directly pulls the quotient down — which is why a hot grain-set period caps yield even when total sunlight looks ample.

The base temperature is the threshold below which development effectively stops, so only degrees above it drive the crop forward. This tool uses 4.5°C for wheat and barley and 10°C for rice, in line with the crop-physiology literature. The denominator of PTQ is the mean temperature minus this base, which is why a cool-but-not-cold window with bright skies produces the highest quotients.

PTQ matters most over the crop's grain-number-setting window. For wheat that is roughly the 30 days before anthesis (Fischer's critical period); for barley it brackets awn emergence and anthesis; for rice it spans panicle initiation to heading, when spikelet number is set. Use the average radiation and average temperature over that window — not the whole season — because that is when grain number is decided.

When PTQ falls below about 1.0 the tool flags heat as the dominant limiter: development is racing and there is too little radiation per degree-day. When PTQ is at or above 1.0, radiation becomes the practical ceiling on how much more grain you could set. The widget shows both ingredients side by side so you can see which lever — cooler timing or brighter conditions — would move your potential most.

Grain number rises roughly linearly with PTQ over the relevant range: this tool uses grains/m² = intercept + slope × PTQ, with crop-specific coefficients drawn from the Fischer-type relationship (for wheat, around 2,000 + 13,800 × PTQ). So a wheat PTQ of 1.44 implies roughly 21,900 grains/m². Yield potential then follows grain number multiplied by a representative single-grain weight.

Yield potential = grains/m² × single-grain weight. The conversion is grains/m² × grain weight (mg) × 1e-5 to land in tonnes per hectare. For wheat at 38 mg per grain and ~21,900 grains/m², that is about 8.3 t/ha of potential — the ceiling the canopy could fill if water, nitrogen and frost do not intervene. It is a potential, not a guarantee.

Yes — the main lever is timing. Sowing earlier, or choosing an earlier-maturing variety, can move the grain-set window into cooler, brighter weather, lifting the quotient and the grain number it supports. Irrigation that cools the canopy helps in hot, dry environments. You cannot add radiation, but you can place the critical window where the radiation-to-temperature balance is most favourable.

Yes. The same radiation-to-thermal-time logic applies; only the base temperature, the window definition and the grain-number coefficients change. Barley uses a 4.5°C base around awn emergence; rice uses a 10°C base from panicle initiation to heading, where the spikelet number is set. Pick the crop and the tool applies the right coefficients and bands.

GDD sums heat only — it tells you how fast the crop is developing, not how much yield potential it has. PTQ adds the radiation term that GDD ignores, so it captures the radiation-per-degree-day that actually decides grain number. Two seasons with identical GDD can have very different PTQ and therefore very different yield potentials, which is exactly the gap this tool fills.

No — PTQ sets the grain-number potential, but realising it still needs non-limiting water and nitrogen through grain set and freedom from frost or disease during the window. Treat a high PTQ as permission to push agronomy: it tells you the ceiling is high, so protecting the crop pays off. A high PTQ wasted on a stressed crop still yields poorly.