Back to places with a rather more extreme environment than Durham….
It’s not only bright light and a lack of water that can cause problems for plants in Ladakh; temperature is another key aspect of the environment. Because of the altitude, plants here have to cope not only with seasonal temperature changes but also with extreme differences between day and night-time temperature. In July, for example, daytime maximum temperatures in Leh average 25° C but it can drop almost to freezing at night. On exposed, south facing rock surface, temperatures may get much higher than this during the day.
So, what sort of problems do these high temperatures cause for plants? The ultimate problem with high temperatures for living organisms is that they causes proteins to denature; that is, to lose their precise three dimensional folding. This three dimensional structure is held together by much weaker bonds than those which link the amino acids making up the protein but is essential for the normal function of proteins, particularly enzymes. Enzymes fit their substrates rather like a key entering a lock, so any change in their shape means they will no longer be able to catalyse the reactions essential for all life. One solution adopted by many organisms is the use of so called ‘chaperone’ or Heat Shock Proteins, which help other proteins to fold up correctly and stabilise their 3D structure.
Denaturation happens at temperatures around 45 °C, so this shouldn’t normally be a problem for plants in Ladakh. However other difficulties occur at much lower temperatures, which means we need another diversion to look at the lipid membranes around and inside cells, which maintain their integrity. Cell membranes are made largely of phospholipid and sterol molecules. Phospholipds are intriguing molecules made of two very different components. One end (the hydrophilic or ‘water loving’ head in the picture below) carries a slight charge and so dissolves freely in water. However the largest part of the molecule is two long chain fatty acids (the hydrophobic or ‘water hating’ tail below). This cannot dissolve in water (think what happens when you try to mix oil and water) and will spontaneously arrange itself in the midst of other, similar lipid tails, which leads to formation of a so-called phospholipid bilayer.
“Phospholipid TvanBrussel”. Licensed under Copyrighted free use via Wikimedia Commons
When you shake up phospholipids with water in the correct proportions you end up with what are known as liposomes – tiny spheres of phospholipid bilayer, with a little water trapped inside, which give us an idea of how the earliest cells may have formed.
This phospholipid bilayer is the main component of the membranes which surround all cells, from bacteria through to the largest plants and animals. It also forms the membranes around organelles such as the nucleus, mitochondria and chloroplasts in more complex cells. The phospholipids may be mixed with other lipids to give the properties required in a given situation and the membrane will also have, embedded within it, protein molecules with specific functions; letting substances in and out of the cells, communicating between cells and producing energy, for example.
So what has this to do with plants and high temperatures? Because of the lipids, membranes become more fluid at higher temperatures, just like fat or oil. The membrane may become leaky and the proteins embedded in it may not fold properly, and so may lose their function. One way the plant can deal with this is by changing the composition of the fatty acid tails in the phospholipid molecules. Just as too many saturated fatty acids in our diet can cause problems by stiffening up the walls of blood vessels, so a membrane whose phospholipids have a higher proportion of saturated fatty acids will be more rigid and, therefore, more tolerant of high temperatures.
It’s not just the outer membrane of plant cells that can be damaged by increased temperatures. The internal membranes, particularly those inside the chloroplast, are also vulnerable. In fact, many chemical processes which involve a number of steps take place on membrane surfaces; locating key proteins and enzymes next to each other in the correct orientation makes it easy for small reactants to pass from one to the next seamlessly. Any disruption to the membrane of a chloroplast or mitochondrion will interfere with the key processes of photosynthesis (where sugars are made) or respiration (where they are broken down to release energy). Both are vital in plant cells but photosynthesis is affected at lower temperatures than respiration, so the plant will carry on burning up stored reserves to produce energy when it is unable to replace these by photosynthesis – not a sustainable situation.
Unsurprisingly, the effects of high temperatures are exacerbated by a shortage of water, partly because leaves normally rely on the cooling effect of water evaporating from open stomata. In our old friends the CAM plants, with the stomata closed during the day, this isn’t possible.