Transcript

How plants make food

DR. VINCENT GAUCI

The fundamental material of all living things on our planet is carbon. Now this starts out as an inorganic, molecular gas in the atmosphere, carbon dioxide or CO2. So to get the carbon into an ecosystem you need the process of photosynthesis. It's a process that is unique to plants and certain micro-organisms, but it benefits almost every living thing on earth. Photosynthesis is how plants make their food, using a simple set of ingredients. Sir David Attenborough described it as the very basis of life. So let's leave it to him to explain how it works.

SIR DAVID ATTENBOROUGH

Air seeps into the leaves from pores on their surface. It circulates within them and reaches tiny granules that contain a green substance, chlorophyll. This is the key facilitator that uses the energy of the sun to bond carbon dioxide to hydrogen derived from water and produces carbohydrate - sugars and starches. These dissolved in sap are then carried from the leaf into the body of the plant, even during the night when the leaf factory has shut down. Come the dawn, the sun re-appears and the process starts up again.

DR. VINCENT GAUCI

So photosynthesis is the fundamental process driving the production of material in ecosystems. Light, water, nutrients, and CO2 are all key ingredients in driving that level of productivity. If you were to reduce any one of those key ingredients, that would result in a loss of productivity in a plant, like this tree here. Those four key ingredients, together with temperature, are known as environmental variables. Each one of them can affect photosynthesis and as they are unevenly spread through space and time, there can be dramatic differences in productivity across the globe and over different time scales. The availability of light varies throughout the day as the earth spins on its axis. At the Poles there can be almost constant daylight in the summer months, but most of the planet experiences a diurnal cycle - night and day, darkness and light.

PROFESSOR DAVID GOWING

Well, light is key to photosynthesis because it's the source of energy, and therefore it determines the rate of which photosynthesis can proceed. This means that productive ecosystems need to be in full sun, like a forest canopy. Beneath the canopy where light is attenuated by the canopy above it, then plants can operate at a much lower rate and take in much less carbon per unit time. Too much light can be a problem because if leaves can't access carbon dioxide quick enough to use that energy in photosynthesis, the excess energy becomes a problem for the leaf. It may even damage the photosynthetic apparatus. So this is an issue at the top of the forest canopy, where the leaves are in full sun. And plants have come up with a whole range of adaptations to cope with that, including pigments to try and absorb the excess light and photo respiration, where they actually respire the carbon they have just fixed to produce carbon dioxide to soak up that excess energy.

DR. VINCENT GAUCI

At the equator, light is available for twelve hours a day, all year around. That steady supply of energy, combined with high levels of rainfall, make the tropical rain forests highly productive. Temperatures are high all year, so water is available in liquid form. It doesn't get locked away as ice during the winter. In other parts of the word there is a different mix and cycle of environmental variables. In temperate regions, where the seasons are more pronounced, production takes place in spring and summer, when there's the most sunlight and the warmest temperatures. This cycle of production through the seasons can vary year by year. And we can see a record of variations in tree rings, with the wider rings showing warmer summers. In dry, arid areas of the tropics it's not light that's the issue. It's the availability of water that limits production. Perhaps the most powerful way to show how water limits production is to look at what happens when a desert gets wet. And that happens on a huge scale in the area of the Kalahari Desert known as the Okovango Delta. These remarkable images were filmed by the BBC programme "Plant Earth". As water flows into the delta, the landscape is transformed. With the water comes life. Plants can once again produce carbohydrates through photosynthesis, converting CO2 and water. What was once a barren desert is now a wildlife oasis. This is an extreme example of water limitation and the relief water can bring to a dry ecosystem. But to fully understand the effect of water on productivity, we need to understand the biochemistry of photosynthesis.

PROFESSOR DAVID GOWING

Water is essential for taking carbon dioxide out of the air because plants exchange water vapour for carbon dioxide when their stomata are open. And so a lack of water can really limit the amount of carbon a plant takes in. In environments where you have a real lack of water then plants have come up with alternative photosynthetic pathways to cope with the problem. A group called the C4 plants concentrate carbon dioxide by having specialist cells that take it in, store it, and pass it to photosynthetic cells which are located deeper within the leaf. And by doing that, they're supplying those specialist cells with enough carbon dioxide that they can utilise light to its full without the risk of excess light damaging the apparatus. C4 plants tend to occur in dry environments in high light, so savannahs would be the most typical natural environment for them. And plants that have evolved in that sort of savannah environment are quite often used as crop plants such as maize, and sugar cane, and sorghum. They all use the C4 photosynthetic pathway, which means they are extremely productive if supplied with light. Taking that one step further, an even more extreme adaptation is cacti and succulents that only take up carbon at night. And they store the carbon as organic acids in their big, fleshy cells, for later use during the following day where it's re-released as carbon dioxide and normal photosynthetic pathways take over.

DR. VINCENT GAUCI

So light and water can influence and limit productivity. The same is true of CO2, which is vital for photosynthesis. Now the concentration of CO2 doesn't vary over the planet. On a fine scale you can have concentration differences that are sufficient to affect productivity. In a dense canopy, CO2 can be used up faster than it's being replenished from the atmosphere. Manmade emissions can also have an effect. So light and water, carbon dioxide and nutrients, all directly influence the productivity of any one ecosystem. These limiting factors, together with temperature, are known as environmental variables. But that's only half of the cycle. In the next film we will look at the other half - decomposition.