What if plants received less light?

Plants cannot go to the store for food. They make their own!

They use light from the Sun. What if a plant got less light? Would it still make food?

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

But light is not always plentiful. Leaves sit in the shade of other leaves, under clouds or beneath tall trees. In this lab you can turn down the light on a model leaf and see how much food it still makes.

Photosynthesis uses light energy to reduce carbon dioxide to sugar, oxidizing water and releasing oxygen. Most plants use the C3 pathway, in which the Calvin cycle first makes a three-carbon compound that goes on to become glucose.

Light drives the first, light-dependent stage. This lab asks how the leaf's net sugar production responds when the light it receives falls from bright to shaded levels, and what sets the lowest light a leaf can live on.

Olly's leaf sits in bright light. It is busy making sugar.

The leaf takes in air and water. It makes sugar for food and gives out oxygen.

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

In these conditions the leaf takes in about 12.7 units of carbon dioxide, burns about 1 unit for its own energy, and keeps the rest, making about 1.27 grams of sugar per hour for every square meter of leaf.

Light is measured as photosynthetically active photon flux, in µmol of photons per square meter per second; full sunlight is about 2,000. At 800 µmol/m²/s, 420 ppm CO₂ and 25 °C, the model gives gross photosynthesis of 12.7 µmol CO₂/m²/s, respiration of 1.0 and net photosynthesis of 11.7.

Converted with six CO₂ per glucose (180.16 g/mol), that is about 1.27 g of sugar per square meter per hour. At this light the model reports carbon dioxide, not light, as the factor holding the rate back most.

Now Olly moves the leaf into the shade. It gets only a little light.

The leaf still makes sugar, but more slowly. In the dark, it cannot make any at all.

Cut the light to 200, a quarter of what it had. You might expect a quarter of the sugar, but the model leaf still makes about half: about 0.65 grams per hour instead of 1.27.

Keep dimming it and the sugar keeps falling. Below about 18 units of light, the leaf burns more sugar than it makes, and in total darkness it uses up its stored sugar instead of adding to it.

At 200 µmol/m²/s the model's gross rate falls to 7.0 and net photosynthesis to 6.0 µmol/m²/s: about 0.65 g sugar/m²/h, 51 percent of the 800-µmol value, from 25 percent of the light. Over the two-hour run that is 1.30 g/m² against 2.54.

The break-even (compensation) light is I_c = R·Ik/(Pmax·f_C·f_T − R) ≈ 18 µmol/m²/s. At 50 µmol (deep shade) net is 1.5; in darkness it is −1.0, a loss of about 0.11 g sugar/m²/h to respiration.

Light gives the leaf the energy to make sugar. Less light means less energy, so less sugar.

The leaf also uses a little sugar all the time, to stay alive. In the dark, it only uses sugar.

Light gives the leaf the energy for photosynthesis, so less light means less sugar. But in the model, each extra bit of light helps less than the one before, so taking some light away from a brightly lit leaf costs less than you might think. That is why a quarter of the light still gives about half the sugar.

A leaf also burns a little sugar all the time for its own energy, called respiration. In very dim light, that small cost is bigger than what the leaf makes. This is a model of one typical leaf; real plants differ a lot.

The model's light response is a rectangular hyperbola, I/(I + Ik) with Ik = 300 µmol/m²/s, multiplied by CO₂ and temperature factors. At high light the curve flattens (saturation), so the rate is far less than proportional to light; at low light it is nearly proportional.

Respiration is fixed at 1 µmol/m²/s at 25 °C, in light and dark. Limitations: the curves are generic for a sunlit C3 leaf; shade plants saturate at much lower light, plants acclimatize and can be damaged by strong light, water and nutrients are assumed plentiful, and one leaf says nothing about how much a whole plant grows.

In a forest, tall trees catch lots of sunlight. Little plants below get only a bit.

Some trees, like beech and hemlock, can still grow slowly in deep shade.

Under a forest canopy, young trees get only a sliver of the light. Foresters rank trees by shade tolerance. Eastern hemlock and American beech can survive on just 1 to 3 percent of full light, growing slowly and living long. Trembling aspen and red pine may need as much as 60 percent.

Shade-tolerant trees are not better at making sugar in dim light. They seem to burn less of it in respiration, so they keep a small profit. Sugar maple seedlings even use brief flecks of sunlight that flash through the leaves above.

Foresters rank species from very shade intolerant to very shade tolerant. Eastern hemlock and American beech persist on 1–3 percent of full light; trembling aspen and red pine may need up to 60 percent to stay competitive. Shade tolerance also changes with age: eastern white pine is more tolerant as a seedling.

The mechanism is the energy budget. Shade-tolerant trees do not photosynthesize much more in low light; they appear to respire less, keeping net gain positive. That matches the model's break-even idea: in the model, a typical sun leaf at 1 percent of full sun (about 20 µmol) only just breaks even.