RTA: temperature–assimilation balance

What it does

A 24-hour mean temperature rule that follows crop photosynthesis rather than light alone, and prices a warmer day in CO₂ or fruit load. It is advice for the grower. It does not move a climate computer.

When to use it

When you are setting the next 24-hour average for a fruiting greenhouse crop and you know the daily light sum, the CO₂, and the fruit load you are carrying. It is not an instantaneous thermostat and not a night/day split.

How it calculates

• Advisory only. This rule does not send setpoints or any other hardware commands.
• Control rule 101 / 105. RTR baseline: T24 = 0.3·PAR + 17, PAR in mol·m⁻²·d⁻¹. RTA fitted at 500 ppm: T24 = −0.0032·PAR² + 0.358·PAR + 17.0, which is the same balance as T24 = 0.265·A_crop + 17.2.
• Assimilate demand rises about 7.5% per °C (about 10% per °C if fruit load is not reduced). Full compensation is +100 ppm CO₂ per +1 °C; otherwise lower the weekly plant load. Plan B is about 20 ppm per °C and covers maintenance respiration only (Q10 = 2, about +7.2% per °C).
• CO₂ response of crop photosynthesis relative to 400 ppm: 500 → 1.11, 600 → 1.19, 700 → 1.25, 800 → 1.29. Points between those are linearly interpolated. Below 400 ppm and above 800 ppm the multiplier is held. Leaf saturation is about 700 ppm.
• Fruit growth duration is 168 °Cd above 10 °C (14 days at 22 °C). Leaf appearance is 6 leaves/week at 20 °C and ×1.6 from 18 °C to 25 °C. Vegetative minimum supply is about 75 g CH₂O·m⁻²·week⁻¹, about 7.5 mol PAR·d⁻¹. The recommended 24-h mean is held at 24 °C.
• Stated limits: One summer season in two 520 m² compartments with four cultivars; measured photosynthesis gain was +3% against an expected 7%, partly because daily mean CO₂ hides low CO₂ at high-light moments. Abortion is not predicted, and current growth models cannot predict it either (de Visser 2024).
• Daily PAR, CO₂, and the 24-h mean fill from facility sensors when the readings cover the day. Weekly plant load is always typed. Missing sensors leave the manual value in place.

What you enter

Daily PAR(mol/m²/d)

Daily light sum at the crop, mol·m⁻²·d⁻¹. Filled from facility PPFD when the day is covered; otherwise type it.

CO₂(ppm)

Daily mean CO₂. The PAR curve was fitted at 500 ppm. A daily mean can hide low CO₂ at high light.

Current 24-h mean temperature(°C)

The 24-hour average you are running now, not an instantaneous thermostat setpoint.

Weekly plant load(fruits/stem)

Fruit load you are carrying. The cut is a fraction of this number, so the same math applies if you enter g CH₂O·m⁻²·week⁻¹ instead. Not read from sensors.

Temperature rise to price(°C)

Degrees of 24-h mean to cost out. Full compensation is +100 ppm CO₂ per degree; otherwise weekly plant load falls so assimilate demand stays flat. 0 prices the amount your current mean already sits above the RTA target.

What you get

RTR target T24(°C)

RTA target at 500 ppm(°C)

RTA target at this CO₂(°C)

Recommended T24(°C)

Estimated A_crop(fitted units)

CO₂ to add for this rise(ppm)

CO₂ needed (full compensation)(ppm)

Plan B CO₂ to add(ppm)

Plant load reduction(%)

Reduced weekly plant load(same unit)

Fruit growth duration(d)

Leaf appearance(leaves/week)

Limit warnings

Reading

Stated limits

Reading the result

Follow the recommended 24-h mean, which is the RTA target held at 24 °C. The RTR line is the light-only comparison. If you want to run warmer, add about 100 ppm CO₂ per degree or accept the lower plant load; 20 ppm per degree only covers maintenance. A daily PAR under 7.5 mol is below the vegetative minimum. The trial behind the rule was one summer in two 520 m² compartments: measured photosynthesis rose about 3% against an expected 7%, and abortion is not predicted.

Related calculators

• Photosynthetic Rate Calculator
• VeGe Balance Analyzer
• Transpiration Calculator
• Weather-Adaptive Controls