What if a greenhouse got too hot?

A greenhouse is a house made of glass or plastic, just for plants. Sunshine warms it up inside.

What if the sun made it much too hot? Would the plants still make food?

A greenhouse lets sunshine in, and the air inside warms up. On a sunny day it can get far too hot: the Royal Horticultural Society warns that greenhouses, whether of glass or plastic, can overheat in sunny weather.

A leaf does two things with sugar at the same time. It makes sugar by photosynthesis, and it burns some sugar for energy, which is called respiration. This lab asks: when a greenhouse overheats, what happens to the balance between the sugar a leaf makes and the sugar it burns?

Net carbon gain is gross photosynthesis minus respiration. Both depend on temperature, but differently: University of Arizona Cooperative Extension notes that higher temperatures lower photosynthesis and speed up respiration, and that heat alone can damage the photosynthetic system.

This lab follows a model leaf as its temperature climbs from 25 °C to 45 °C under steady light and carbon dioxide. It asks where the two curves cross, what can push that point higher, and how the model's generic curves compare with what extension scientists say about real heat stress.

In Olly's greenhouse, the leaf is nice and warm, at 25 degrees.

It makes lots of sugar and only burns a little. That is just right for this model leaf.

Our model leaf starts at 25 °C, with bright light (800 units) and today's carbon dioxide (420 ppm). It takes in carbon dioxide at a rate of 12.7, while respiration burns 1.0, so its net photosynthesis, what it keeps, is 11.7.

That makes about 1.27 grams of sugar per square meter of leaf every hour. In this model, 25 °C is the temperature at which the leaf's food-making works best.

At T = 25 °C, I = 800 µmol/m²/s and C = 420 ppm: gross 12.7, respiration 1.0, net 11.7 µmol CO₂/m²/s, or 1.27 g glucose/m²/h. The model's temperature factor exp(−((T − 25)/15)²) peaks here at 1, so CO₂ (factor 0.58) is the limiting factor.

Respiration follows R = 1 × 2^((T − 25)/10), so at 25 °C it is only about 8% of gross uptake.

Now the greenhouse gets very hot: 45 degrees!

The leaf makes much less sugar. And it burns sugar faster. Now it burns more than it makes.

Heat the leaf to 45 °C. Carbon dioxide taken in drops from 12.7 to just 2.2, while the sugar burned in respiration climbs from 1.0 to 4.0. Net photosynthesis becomes −1.8: the leaf is using up sugar faster than it makes it, about 0.20 grams per square meter every hour.

Step the temperature up one degree at a time to find the tipping point. At 42 °C net photosynthesis is still 0.3, but at 43 °C it is −0.5. Above 45 °C the model's food-making stops completely and 'Holding it back most' says 'too hot'.

Between 25 and 45 °C, gross uptake falls from 12.7 to 2.2 µmol/m²/s as the temperature factor drops to 0.17, while respiration quadruples from 1.0 to 4.0. Net falls through 10.0 (30 °C), 6.2 (35 °C) and 1.9 (40 °C), crosses zero between 42 °C (0.3) and 43 °C (−0.5), and reaches −1.8 at 45 °C (−0.20 g/m²/h). Above 45 °C the model sets gross uptake to zero.

Carbon dioxide shifts the crossing only a little: at 1,000 ppm, 43 °C still gives 0.5 but 44 °C gives −0.4. Even with full sun (2,000) and 1,200 ppm, 45 °C gives −0.5.

Heat speeds up the leaf's burning of sugar. But too much heat slows down its sugar-making.

So in very hot air, the leaf uses up its food.

In the model, the chemistry that builds sugar works best in the middle: slowly in the cold, fastest when warm, and badly when too hot. Respiration is different. It speeds up as it gets warmer, doubling with every 10 °C in this model. So as the greenhouse heats up, sugar-making falls while sugar-burning rises, and eventually the two lines cross.

The model is a simplification. Its temperature curve is a smooth bell shape that stands in for real plants; it is not measured for any one plant. And its leaf never closes its pores, though real plants close them in hot, dry air (OpenStax Biology).

The model multiplies light, CO₂ and a Gaussian temperature factor, then subtracts a Q10 = 2 respiration term. Because the factor falls off around 25 °C while respiration grows exponentially, net gain must cross zero; here it does so between 42 and 43 °C, and the cliff at 45 °C is the model's own cut-off for enzyme failure.

The model has clear limits: the bell curve is generic, not measured; stomata close in hot, dry air, cutting CO₂ supply and transpiration cooling; water and nutrients are assumed plentiful; and leaf temperature is a single set value, though a leaf whose stomata have closed can heat up to damaging levels (University of Delaware).

Real plants in hot weather can droop and wilt. Gardeners help them with water and shade.

Greenhouses open their windows on sunny days to let the heat out.

Real plants struggle in heat too. Gordon Johnson, a crop expert at the University of Delaware, writes that photosynthesis decreases quickly above 94 °F (about 34 °C), which limits yields in many vegetables, and plant tissue can die at around 115 °F (about 46 °C). Warm nights make plants burn more sugar, leaving less for fruits and seeds.

The University of Arizona Cooperative Extension adds that high temperatures, especially at night, speed up respiration and slow photosynthesis, using up a plant's stored food faster than it is replaced. Johnson names watering, misting and shade cloth as ways to cool plants, and the Royal Horticultural Society advises opening all greenhouse doors and vents on sunny days.

Gordon Johnson of University of Delaware Cooperative Extension notes that plant tissue dies at around 115 °F (46 °C), that photosynthesis rapidly decreases above 94 °F (34 °C), limiting yields in many vegetables, and that warm nights accelerate respiration, limiting the sugars stored in fruits and seeds. When stomata close, transpiration cooling is lost and leaf temperature rises; misting, mulches and shade cloth reduce heat load.

University of Arizona Cooperative Extension states that higher temperatures, especially at night, raise respiration and lower photosynthesis, depleting carbohydrate reserves faster than they are replenished, and that heat alone can damage the photosynthetic system. For greenhouses, the Royal Horticultural Society warns that plant damage can occur if temperatures build up past about 27 °C. The model's 25 °C optimum is a generic choice, and real crops can be stressed well before the model's crossing point near 42 °C.