Above a temperature of 57 degrees Centigrade fluids, mainly water, expand in the plant cell walls causing them to rupture. This causes the liquid contents to start to leak pushing gases out of the cells, by replacing gas with liquid. Less light is reflected and any treated area will take on a darker appearance. This can be seen on the image on the left with a half treated potato field 2 hours after burning.
If the whole plant has been exposed to a high level of heat an increased permeability of the plasma membrane leads to a loss of rigidity of all non woody tissues. This loss of turgor results in leaves hanging limply and flopping over, this is particularly obvious in fleshy broad leave plants where complete collapse is common after exposure to heat. If a treated plant does not have the energy reserves to refoliate it can not photosynthesis and will die.
This use of heat is particularly effective on small weeds below 4 true leaf stage as a pre emergence treatment in horticultural crops and on senescing crops such as potatoes to speed up tuber skin set.
Heat is also commonly used to remove the ‘green bridge’ between one crop and the next, with the high temperature speeding up the decomposing of waste plant matter as well as killing any weed seeds and disease spores that are present allowing for a clean start for the new crop. This technique has also been successfully taken up in the poultry industry where by heating the shed floor it has been found to control pathogens such as coccidiosis.