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Transpiration Investigation

Extracts from this document...

Introduction

Investigation into the effect of varying light levels on the transpiration rate of the Cherry laurel, Prunus laurocerasus L. Introduction In vascular plants movement is water up the xylem from the roots to the leaves is an essential process, responsible for supplying the plants tissues with water and maintaining cell turgor (which has a role in preventing the plant wilting). Aside from the weak "push" effect of the difference in water potential between the soil and the roots, the main force for transpiration in water evaporating from the spongy mesophyll tissue in plant leaves. The rate of evaporation depends a number of factors- some directly affecting the rate at which water evaporates (such as temperature, wind speed, humidity) and others affecting the number of stomata that are open, such as light levels. The use of a gas pressure sensor is a well-documented way to measure the rate of a plant's transpiration. The plant cutting is inserted cut-end-on to a plastic tube full of water, the other end being connected to a gas pressure sensor. As the plant takes up the water through the transpiration stream the volume within the tubing deceases, and the pressure changes with it. So if the change in pressure is recorded over a period of time the transpiration rate can be calculated from these values Hypothesis There are two factors that may affect the rate of transpiration elative to temperature. Firstly is the simple physical effect- light radiation will warm the laves (and consequently the water within them), thus increasing the rate of evaporation, speeding the transpiration stream- as a result the pressure in the tubing would drop more rapidly. However, it is known that the stomata are light sensitive, usually closing during the night (when photosynthesis is impossible) - i.e. at low light levels, and then opening during the day for gaseous exchange for photosynthesis. It may be that at very high light levels some stomata may close (or all of them may partially close) ...read more.

Middle

Trial 2 average 0 103.3 102.9 103.1 2530 1856 2193 10 103.2 102.8 103.0 2553 1862 2207 20 103.2 102.7 103.0 2544 1879 2211 30 103.1 102.7 102.9 2544 1844 2194 40 103.1 102.6 102.8 2546 1826 2186 50 103.0 102.5 102.8 2552 1829 2190 60 102.9 102.4 102.6 2527 1862 2194 70 102.8 102.3 102.6 2535 1820 2178 80 102.8 102.2 102.5 2563 1882 2222 90 102.7 102.2 102.5 2551 1850 2201 100 102.7 102.1 102.4 2551 1844 2198 110 102.6 102.1 102.3 2572 1862 2217 120 102.6 102.0 102.3 2523 1823 2173 130 102.5 101.9 102.2 2509 1856 2182 140 102.5 101.9 102.2 2513 1856 2184 150 102.4 101.7 102.1 2520 1894 2207 160 102.4 101.6 102.0 2568 1862 2215 170 102.3 101.6 101.9 2588 1862 2225 180 102.3 101.5 101.9 2579 1850 2214 190 102.2 101.5 101.8 2560 1809 2184 200 102.2 101.4 101.8 2554 1815 2184 210 102.1 101.3 101.7 2561 1856 2209 220 102.1 101.2 101.7 2540 1867 2204 230 102.1 101.1 101.6 2508 1856 2182 240 102.0 101.1 101.6 2510 1850 2180 250 102.0 101.0 101.5 2523 1856 2189 260 102.0 100.9 101.5 2501 1856 2178 270 101.9 100.9 101.4 2549 1867 2208 280 101.9 100.8 101.3 2563 1850 2207 290 101.9 100.7 101.3 2554 1856 2205 300 101.8 100.6 101.2 2543 1850 2196 310 101.7 100.5 101.1 2547 1856 2201 320 101.7 100.5 101.1 2553 1894 2223 330 101.7 100.4 101.1 2558 1803 2181 340 101.7 100.3 101.0 2512 1809 2160 350 101.6 100.3 100.9 2549 1856 2202 360 101.6 100.2 100.9 2544 1862 2203 370 101.5 100.1 100.8 2540 1873 2207 380 101.5 100.1 100.8 2554 1867 2211 390 101.5 100.0 100.7 2559 1856 2207 400 101.4 99.9 100.6 2522 1923 2223 410 101.3 99.9 100.6 2493 1894 2193 420 101.3 99.8 100.5 2488 1862 2175 430 101.2 99.7 100.5 2461 1879 2170 440 101.2 99.6 100.4 2490 1873 2182 450 101.2 99.6 100.4 2470 1873 2172 460 101.1 99.5 100.3 2492 1856 2174 470 101.1 ...read more.

Conclusion

The reason I would propose is as follows. Though direct heating of the leaf, thereby increasing the rate of evaporation will have some effect, we must also consider the role of photosynthesis. This reaction, which is dependent on light levels, consumes water & carbon dioxide to produce glucose and oxygen. During the daytime, we can expect it to be an important sink of liquid water, and thus be an important factor in the rate of transpiration. Now, besides light levels photosynthesis is limited by the number of chloroplasts in any one leaf, and other factors. In short, it appears that after a light intensity of approximately 2000lux, other factors come into play that limit the rate off photosynthesis and of the rate of transpiration Evaluation: Weakness of the experiment, in descending order of importance Corresponding improvement The light source output was highly variable over the course of the experiment. This has been partially compensated for by using mean output (as oppose to distance) Use of a light source with a more reliable output, with highest radiation in the visible range, to minimise heating (i.e. with a low infra-red output) The actual change in pressure was very small, making it quite susceptible to error A greater time period of measurement, that the magnitude of measurement (i.e. the change in pressure) be larger. Alternatively, if a tube o0f smaller bore were used, the changes in pressure relative to the rate of transpiration would be higher. The data range was quite small A greater range of light levels, i.e. greater light intensity, and lower light intensity, down to (approx.) zero The gap between the light intensities tested was quite large A greater density of trials, for example at 25cm, 35cm, 45cm, etc. Only a single cutting was examined Multiple cuttings, of the same species but perhaps of different plants should be examined The protective layer (paper) that reduced heating of the tubing (with its effect on internal pressure) was only a partial protection Use of a more substantial protective layer, such as card ?? ?? ?? ?? ~ 1 ~ ...read more.

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