What if a plant lived in a dark cave?

Plants make their own food. They need light to do it.

Deep inside a cave, it is very dark. Could a plant live there?

Animals have to find food, but green plants make their own. In photosynthesis, a leaf uses the energy in light to turn water and carbon dioxide from the air into sugar, giving off oxygen.

Caves can be very dark. Near the entrance there is daylight, but deeper in there can be total darkness. So what would happen to a plant growing in a dim cave? This lab lets you dim a model leaf's light almost to nothing and find out.

Photosynthesis needs light, but a plant's cells also respire, breaking down sugar for energy, by day and by night. A leaf's net gain is the difference between the two.

This lab follows one model leaf from bright light into cave gloom and asks a precise question: is there a light level below which the leaf loses more sugar than it makes? It then compares the model with what cave scientists find in real caves, including the green growth that appears beside the lamps in show caves.

Outside, the sun is bright. Olly's leaf makes lots of sugar.

The leaf uses a tiny bit of sugar to stay alive. It keeps the rest to grow.

Our model leaf starts outdoors in bright light: 800 units of light on each square meter of leaf every second. Full summer sunshine is about 2,000 of these units. The air has today's carbon dioxide and the leaf is at a comfortable 25 °C.

Press play. The leaf takes in about 12.7 units of carbon dioxide and burns about 1 unit for its own energy, called respiration. That leaves a healthy profit: about 1.27 grams of sugar per hour for every square meter of leaf, or about 2.5 grams over the two-hour run.

At I = 800 µmol/m²/s, 420 ppm CO₂ and 25 °C, gross photosynthesis is 12.7 µmol CO₂/m²/s and respiration 1.0, so net photosynthesis is 11.7 µmol/m²/s, about 1.27 g glucose/m²/h. The model's limiting factor at this light is carbon dioxide, not light.

The 'break-even light' readout shows the model's light compensation point under these conditions: 18 µmol/m²/s, under 1 percent of full sunlight (about 2,000 µmol/m²/s). Above it the leaf gains sugar; below it the leaf loses sugar.

Now Olly's leaf goes into a dim cave. There is only a tiny bit of light.

The leaf makes less sugar than it uses. It is using up its food!

Move the leaf into the cave and turn the light down to 10, a tiny glow. The leaf still does a little photosynthesis, about 0.6 units of carbon dioxide. But it still burns about 1 unit for respiration.

So the net photosynthesis readout goes below zero, to about −0.4. Instead of adding sugar, the leaf is eating into its stores, losing about 0.09 grams over the two-hour run. In total darkness it would lose more than twice as fast. A plant that stays like this slowly uses up its food.

At I = 10 µmol/m²/s, gross photosynthesis falls to 0.6 µmol/m²/s while respiration stays at 1.0, so net photosynthesis is −0.4 µmol/m²/s, a loss of about 0.05 g glucose/m²/h; the two-hour run ends at −0.09 g/m². At I = 0 the loss is −1.0 µmol/m²/s.

Caves are often cool. At 12 °C the model leaf respires only 0.41 µmol/m²/s, and break-even light falls to 16, but at I = 10 it still loses sugar (net −0.1). A cooler leaf loses more slowly; it does not break even. Only with carbon dioxide at its 1,200 ppm maximum and the leaf near 17 °C does the model leaf at I = 10 just reach zero.

A plant needs light to make sugar. But it uses sugar all the time, even in the dark.

In a dark cave, the sugar goes out faster than it comes in.

A plant's cells are always busy, so they always need energy. They get it by breaking down sugar, day and night. Making new sugar, though, needs light.

In bright light, the sugar made is far bigger than the sugar used, so the plant can grow. As the light dims, the sugar made shrinks, but the sugar used does not. At the break-even light the two are equal. Below it, the plant runs at a loss, like spending more pocket money than you earn. The model shows one typical sunlit leaf with plenty of water. Real plants that grow in shade can manage on less light than this leaf.

Photosynthesis runs only in the light, while respiration continues day and night in every living cell, using the sugars that photosynthesis produced. Net photosynthesis is gross fixation minus respiration; the light compensation point is the light at which they balance.

In the model, gross photosynthesis follows I/(I + 300), so at very low light it is nearly proportional to light, while respiration is a fixed 1 × 2^((T − 25)/10). The model describes one generic sun leaf of a C3 plant with ample water and nutrients. Shade-adapted species work at lower light, real leaves acclimatize, and the model does not say how long a whole plant could live on its stored reserves.

At a cave's mouth, green plants grow in the daylight. Deep inside, it is totally dark, and no plants grow.

In some caves, people put up lamps. Then green slime grows near the lamps!

Scientists divide caves into zones. The entrance zone gets sunlight, and green plants grow there. Further in is the twilight zone, where little sunlight reaches; at Oregon Caves, mosses grow in these dim areas near the entrances. Deepest of all is the dark zone, in total darkness, where no plants can make food.

In caves that visitors tour, electric lights change things. At Carlsbad Caverns, algae grow in the dark zone near the artificial lights, and park staff keep them under control. Scientists call this lamp-loving growth lampenflora. It can stain and damage cave rock. Cave staff clean it off, and scientists are testing whether gentler lighting could help.

Missouri State Parks describe three cave zones: an entrance zone that receives sunlight and holds green plants; a twilight zone where little sunlight reaches and, typically, no plant life is found; and a dark zone in total darkness. The National Park Service notes that at Oregon Caves, mosses thrive in the twilight zones near entrances, so the boundary depends on the cave and the plant.

Artificial lighting creates new light where there was none. The NPS says algae grow near the lights in Carlsbad Cavern's dark zones and are kept under control by staff. A 2021 study of Carlsbad Cavern (Havlena and colleagues, Applied and Environmental Microbiology) describes these algae and cyanobacteria, called lampenflora, as unsightly and damaging to speleothems; bleach is the most common treatment, and changing light intensity and wavelength might be gentler.