Paweł Jedynak from the JU Department of Plant Physiology and Biochemistry talks about how plants deal with extreme heat and excess of water.
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When there is a lot of rainfall or the temperature is very high for an extended period of time, animals try to seek shelter from these conditions. Plants just have to endure them – they have no other way. Old trees seem so unfazed by the weather that it is easy to forget that they are living organisms which struggle to survive every day.
Sweltering summer
The plants' struggle for survival is difficult to notice – it requires patience and a unique perspective, as some of the most interesting processes happen on the molecular level. It was only after genetic and biochemical tests that the scientists could determine that plant are fairly quick to react to external stimuli. They receive nourishment from solar energy, which is possible thanks to chlorophyll. However, absorbing too much energy may prove fatal. The excess energy not used in the process of photosynthesis is taken over by oxygen, turning it into a highly reactive and detrimental variety of this element which attacks the plant's cellular structure. Nevertheless, the presence of oxygen radicals is quickly detected and defence mechanisms are set in motion. The plant produces substances (carotenoids, ascorbinian, tocopherol) that neutralise the danger. The excess of light also causes a decrease in the amount of chlorophyll and the number of light-harvesting antennas in the plant. Another interesting strategy is the simulation of anthocyanins – antioxidants which absorb light and UV rays. They form a kind of natural filter which prevents cell damage.
Too high temperature proves to be another problem for the plants. It leads to protein denaturation and cell destruction. Over the course of evolution, plants have developed a significantly higher number of mechanisms alerting them about steep temperature changes than animals. The role of one protein is particularly important – it is responsible for "biochemical memory." When temperature rises to approximately 37 °C, the HSP1 protein is released. It is a chemical alarm signal for the plant, and for the next four days – as evidenced by tests performed on thale cress (Arabidopsis thaliana), a common herb – it is more resistant to thermal shock. Normally, the plant would quickly wither if the temperature rose to 40 °C, but the HSP1 proteins allow it to last for up to an hour.
If the heat wave is prolonged, it may cause drought. Lack of water massively impedes plant growth and productivity and interferes with the process of photosynthesis. Not many people know how much water is required to produce one molecule of sucrose. Over half a litre of water is used in the production of one kilogram of sugar. This cost is significantly increased by the plants' transport system which uses transpiration, i.e. evaporation of water through leaves. In some cases, a tree can lose several hundred litres of water an hour. To minimise the losses, plants close stomata (microscopic pores in located in leaves) which are responsible for the process of transpiration. Thanks to epicuticular wax, the leaves become almost completely sealed. The plant's roots also have a part to play in such situations: they secrete a substance called abscisic acid, which puts the plant in a more alert state. The roots' cells also increase the synthesis of dehydrins (a type of protein) and amino acids that help the plant to acquire water more efficiently. Drought spurs the growth of plants, providing them with a greater chance of producing seeds, which are much more resistant to lack of water.
Spring storms and autumn drizzle
Frequent rains make the soil excessively wet, which in turn causes plants' roots to struggle when trying to absorb oxygen. The sugar in roots starts to ferment, which results in ethanol being produced. It is a more effective than lactic acid produced by animal cells, since it does not acidify cytoplasm, and an optimal pH level is crucial for enzymes to function correctly. Haemoglobin present in plant cells also helps them to survive hypoxia (oxygen deprivation).
Original text: www.nauka.uj.edu.pl
