Investigation of the factors affecting the rate of transpiration from a shoot of Privet.

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Hector Guinness                03/05/2007

Biology planning exercise: investigation of the factors affecting the rate of transpiration from a shoot of Privet

Introduction

        Transpiration is the evaporation of water from a plant, primarily from the leaves. This is because the undersides of the leaves have many small holes, called stomata, which are necessary for the absorption of carbon dioxide. Inside each stoma is an air space, and the surface of all the cells bordering that are coated with a thin film of water, in order to maximise the absorption of carbon dioxide. This inevitably means that some of the water evaporates - this is called transpiration. I intend to measure the rate of transpiration in a privet shoot (see preliminary work). However, it would not be very easy to directly measure the amount of water lost directly – the only way would be to measure relative humidity with the plant in an enclosed space, but humidity affects the rate of transpiration. Therefore, I will have to measure the rate indirectly. To do this, I could measure the weight loss of a plant over a period of time, but then I would also have to take into account the amount of gasses exchanged, so the best way would be to measure the amount of water taken up. This can be done with a potometer.

Key Factors

        Transpiration is affected by the following factors, which will have to be taken into account during the experiment:

        Relative Humidity – if the air if humid, it has more water potential, so there is less evaporation.

        Temperature – An increase of temperature means that more molecules of water have the necessary energy to escape the liquid, so there is more evaporation.

        Light Intensity – More light means that the plant can photosynthesise faster, so absorbs more carbon dioxide, and therefore loses more water.

        Wind Speed – More wind means that the tiny areas of high humidity, just underneath the stomata, are blown away. This means that the water gradient from the stomata to the outside is much steeper, so evaporation happens faster.

        Water Supply & Conditions before the experiment – If the plant had been water stressed just before the experiment, then it would have to supply water to make all its cells turgid again.

        Surface area of plant – A greater surface area will mean more space for transpiration.

        The stomatal density on the leaves – Transpiration happens in the stomata, so more stomata means more transpiration.

        The species of plant – Some plants photosynthesise faster than others, and so need more carbon dioxide, so have more stomata, and lose more water through transpiration. Some plants are adapted to live in arid conditions so minimise water loss with systems such as the one Thyme uses with many small hairs covering the leaves to trap any transpired water.

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I intend to investigate only one factor, so to make it a fair test, all the other conditions must be kept constant. As I have decided that the best way to measure the rate of transpiration is with a potometer, and I want to get continuous results, the best factor to investigate would be one of temperature, light intensity, or wind speed. However, water is used up in photosynthesis, and both temperature and light intensity have a direct effect in the rate of photosynthesis. This would mean that the results would be showing the effect of light intensity on transpiration ...

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