We’ve already seen something of the damage caused by heat to plants (As much heat as light). Unsurprisingly, perhaps, low temperatures can be equally hard for plants to cope with. We are not just talking about what happens when the water in the soil or inside a plant freezes; chilling injuries can occur at temperatures well above freezing. Just as membranes become too fluid at high temperatures, they become too rigid at low temperatures – more like a solid gel – and the proteins embedded within them cannot function normally.
For plants to cope with cold they have to undergo a process known as cold hardening or acclimation. Shortening days signal the approach of winter to plants, just as they do to us, and gradual exposure to lower temperatures stops growth and prepares the plant for winter. In the case of deciduous trees, the most dramatic manifestation of this is the loss of leaves after a blaze of glorious autumn colour.
Banks of the River Wear, Durham, October 2012
This need for gradual preparation is the reason that a sudden cold snap early in the autumn is much more likely to kill plants in our gardens than lower temperatures later in the winter.
A mechanical sensor picks up the decrease in membrane fluidity which occurs as the temperature drops (Survila et al., 2009). In response, genes are activated which produce a range of signalling and anti-freeze molecules. Some of these genes are also induced by salinity and water shortage. Like the salt we put on our roads in winter to prevent ice formation, ‘compatible’ solute molecules such as the amino acid proline accumulate in plant cells and allow liquid inside the cells to supercool, i.e. remain liquid even at temperatures below 0 °C.
The temperature at which membranes start to solidify depends on the type of lipids of which they are composed. Lipids based on saturated or monounsaturated fatty acids solidify at higher temperatures than those with polyunsaturated fatty acids because the hydrocarbon chains of unsaturated fatty acids (with many carbon-carbon double bonds) cannot pack together so closely. This may be a familiar idea to anyone concerned about what the saturated fat in their diet might be doing to blood vessels!
When really cold weather comes, water inside the plants but outside the cells (in inter-cellular spaces and in the xylem vessels which transport water from the roots) freezes first. This has the effect of sucking water out of the cells but is not lethal in the short term to plants which have been properly cold hardened. The real damage occurs when ice crystals form inside cells – these shear cell membranes and organelles, causing irreversible damage. As well as producing solute molecules such as proline during cold hardening, plants also produce specific antifreeze proteins which bind to the surface of developing ice crystals and slow their growth in an attempt to prevent this damage.
Damage to plants by ice formation is no bad thing for some bacteria, however. Pseudomonas syringae uses organic molecules released from damaged plant cells for growth and makes use of the fact that, for ice crystals to start to form, some kind of nucleating centre is needed. P. syringae has an ice-nucleation protein (Ina) on its outer cell wall which encourages ice crystals to form at temperatures as high as -2 to -4 °C, injuring plant buds and leaves so they release the food it requires. In the process, it can cause substantial damage to crops. In fact, one of the first projects to genetically engineer bacteria in the 1980s produced a so called ‘ice-minus’ strain of P. syringae, without the Ina protein, to be marketed under the name Frostban. The idea was that spraying plants with this would allow it to outcompete the damaging natural bacteria. However concerns about the release of the engineered bacterium into the environment meant it was never marketed commercially.
Incidentally, the Ina protein is also used in the manufacture of artificial snow and airborne P. syringae are important condensation nuclei for cloud formation, for the same reason, but that is another story…
Survila M., Heino P. & Palva E.T. (2009) Genes and gene regulation for low temperature tolerance. In Genes for Plant Abiotic Stress, M.A. Jenks & A.J. Wood (Eds), Blackwell, pp 185-219.