Plants alter their metabolic pathways in response to a variety of abiotic stresses, not least a lack of water. A specialised form of photosynthesis known as Crassulacean Acid Metabolism (CAM) is just one manifestation of this. Before we look at CAM, a small diversion is needed to consider how plants normally make the sugars they need for growth.
Sugars are made by photosynthesis, a two-step process which takes place in the tiny green chloroplasts inside plant cells, with water, carbon dioxide and energy from the sun as the key raw materials. The first stage, the so-called ‘light reactions’, (unsurprisingly) require light to make them happen; energy from the sun is used to split molecules of water into hydrogen ions, free electrons and that all-important by-product, oxygen. Free electrons and hydrogen ions are too reactive to have floating around a cell and so are used immediately to produce the cell’s energy currency, ATP, and other intermediates (such as NADPH, in the diagram below) which are needed to produce sugars. The second stage of the process, the Calvin or Calvin-Benson cycle, uses the ATP and NADPH to convert carbon dioxide into glucose, the simplest form of sugar. This stage doesn’t require any additional light.
In most temperate regions, plants have sufficient water to allow them to open their stomata during daylight hours and take up the carbon dioxide they need, because they can replace the water which leaves at the same time by transpiration. The light and dark reactions of photosynthesis can then take place more or less simultaneously – think of the plant as a factory which runs a daytime only operation, making sugar. Such plants are known as C3 plants, because the carbon dioxide molecules form an intermediate molecule with three carbon atoms (PGA in the diagram below) before sugars are finally produced.
C3 photosynthesis in a chloroplast. Light energy splits water molecules and the energy released is used to produce ATP and NADPH. These drive the production of sugars from CO2 in the Calvin cycle.
CAM plants, however, such as the Sempervivum species we saw growing in intense sunlight at the top of Chang La pass, cannot afford to open their stomata during the day. Water is in such short supply that any lost during the day cannot be replaced by the roots at night.
Sempervivum sp. at Chang La, 5360 m
To get around this problem, CAM plants operate a night as well as a day shift in their sugar factories. They open their stomata only in the cool of night, when water will not be lost so fast by transpiration. This forces them to add an extra step into the regular process; carbon dioxide which enters the leaf cells during the night shift has to be stored there overnight (as the C4 acid in the diagram below), then released and made available for use during the day shift when the light reactions take place.
Crassulacean acid metabolism. CO2 enters the leaves at night and is stored before being released into the Calvin cycle during the day when light is available to supply ATP and NADPH from the light reactions (with the stomata closed).
CAM metabolism means these plants are very efficient at making use of the small amounts of water available to them. They make around one gram of dry matter for each 125 g of water they use, which is three to five times better than a typical C3 plant.
What about the name? This behaviour was first recognised in plants belonging to the family Crassulaceae which includes some less exotic plants found in particularly arid environments much closer to home. Think about what it’s like living on top of a dry stone – a tiny desert-like microhabitat, often in the full glare of the sun.
Mossy stonecrop, Crassula tillaea, growing on rocks in NW Scotland
Whilst on the subject of Crassulaceae, if anyone knows the name of this pretty little plant growing on bare rocks beside the road, particularly as we descended from Ladakh into Kashmir, I’d be very grateful. I suspect it to be some kind of Crassula or Sedum species, but can’t find it in any of my books.
Why don’t all plants use this clever variant of photosynthesis? Because there is no such thing as a free lunch and the extra steps required use up some of the plant’s precious energy. Plants will only resort to CAM when they need to in order to survive. One of the consequences of this is that some plants can switch in and out of CAM mode – so called facultative CAM plants. They can make the extra enzymes required, along with the physiological changes required to reverse the pattern of stomatal opening, just when they need to.
I haven’t been able to track down any Himalayan plants known to do this, perhaps because no-one has really looked, but the tropical American genus Clusia has a number of examples.
Clusia lanceolata. http://thewildpapaya.com