Research Paper: Why Leaves Cease Photosynthesis in the Autumn

Introduction

Leaf senescence is considered the final stage of leaf development (Gan & Amasino, 1997). For deciduous trees, leaf senescence is more straightforward to observe as the leaf color changes to yellow before it falls in autumn. One vital sign of green leaves is that they use chlorophyll, a chemical component that helps plants to make photosynthesis happen, to make food from lights. According to Tanaka and Makino (2009), Photosynthesis is the process for plants, some protistans, and some bacteria use sunlight energy to produce glucose from water and Carbon dioxide. The glucose will be converted into pyruvate and release adenosine triphosphate (ATP) from cellular respiration” (Tanaka & Makino, 2009). Evergreen trees hold their leaves for two or three winter times because they can photosynthesize in the winter. Deciduous trees can only photosynthesize in spring and summer because the leaves will be losing in autumn time (A.F. Devi., S. C. Garkoti, 2013). In the wintertime, the plant does not have enough sunlight and water to support photosynthesis, so the green chlorophyll and color of leaves disappear (Tanaka and Makino, 2009). Therefore, is color changing a sign of leaf senescence? Moreover, when the Leaf senesces, does it stop photosynthesis? The purpose of this lab is to use different techniques to observe the physiological changes for photosynthesis, sugar production, and respiration. The lab will try to answer the questions: if the leaves stop photosynthesis first and then return yellow, or they start to turn yellow before they stop photosynthesis. On the other hand, the question of whether senescing leaves continue to be capable of photosynthesis until all their chlorophyll has disappeared will be tested in this lab. Three types of European beech (Fagus sylvatica L.) leaves used for the experiences. The hypothesis is that the yellow leaves will no longer be able to photosynthesize because the yellow ones have no chlorophyll since leaves use chlorophyll to capture light energy and convert energy into chemical forms like sugar (National Geographic Society, 2019).

Material and Mothed

At first, each group will obtain one leaf sample from the three categories: fully green, yellowish-green, and fully yellow. The first leave is fully green. The second leave is yellowish-green (the leaves should have both colors on it). The third leave is entirely yellow. A 1 cm wide strip of leaf tissue is cut from each leaf category by scissors.

Then, the carbon dioxide concentration will be measured. Each strip of leaf tissue is put in three18ml test tubes. The teste tubes are sealed by rubber serum cap and warped by aluminum foil. In this experiment, aluminum foil was used to exclude the sample from all light sources. Group name and leaf colors are used to label three test tubes. The test tubes should be kept away from the mouse to avoid extra CO2 get inside and influence the CO2 concentration of leaves. Then, 1ml of air from the test tube needs to be removed through the serum cap using a syringe. After that, the needle is plugged by stabbing it into a rubber bung. Finally, the test tube is immediately put in the water bath for dark incubation. The CO2 concentration (CO2INIT) of all three different categories of leaves are needed to be calculated before putting them in the water bath. Do not forget to record the time for 30 minutes. During the waiting time, the syringe is taken to the Infrared Gas Analyzer (IRGA) and the rubber bung is removed. The 1ml air is then quickly injected to the IRGA, and the peak height from the IRGA is recorded, as shown on the chart recorder in chart units (CU). The full scale is 100 CU. The starting Co2 concentration can calculate by CO¬2INIT = CU × Slope. In this lab, the slope is from the predetermined standard curve’s slope in the unit of μl CO¬2 L-1 air CU-1. The value of slope in formula CO¬2INIT = CU × Slope is 18.64 μl CO¬2 L-1 air CU-1(1CU=18.64 μl CO¬2 L-1 air CU-1).

After 30 minutes, three test tubes were removed from the water bath by the time order. At the same time, a second 1 ml air gotten from the test tube, which is CO¬2DARK. And, also need imminently plug the needle by stabbing it into a rubber bung. The second time is taken l ml air (CO¬2DARK) injected into the Infrared Gas Analyzer (IRGA) and recording the data. The Co2 concentrate (CO¬2DARK) calculated by the formula CO¬2INIT = CU × Slope (1CU=18.64 μl CO¬2 L-1 air CU-1) The aluminum foil need removing from the test tube. Then the test tube put back to the water bath for 15 minutes to make the sample react with the light. To making sure the leaf tissue is facing upwards and is not shad by anything like labels. A light meter was able to measure the PPFD (the photosynthetic photon flux density, given in μmol quanta m-2 s-1).

Fifteen minutes later, the test tube is removed from the light, and a third 1 ml air (CO¬2LIGHT) from the test tube is taken as soon as possible. The air took at the third time is also injected into the Infrared Gas Analyzer (IRGA), and the data is recorded. CO¬2INIT = CU × Slope (1CU=18.64 μl CO¬2 L-1 air CU-1) would use again to calculate the Co2 concentration CO¬2LIGHT).

Thirdly, for each leaf color, the leaf area of the specimen is calculated three times to reduce the error. The final leaf area of each tissue strip is the average of three data in square centimeters (cm2. The leaf area is converted to square meters (m2) to calculate the Leaf Mass per Area (LMA). After measuring the leaf area, retrieved the tissue, and determined the fresh mass using the balance, the chlorophyll meter is used to measure the chlorophyll content index (CCI). The three types of leave need further management and are left for lab associates to get dry mass data.

Finally, the repagination rate and photosynthetic rate are calculated by using formulas. The first step is to find the difference between CO2INIT and CO¬2DARK(CO2dark(uL/L)-CO2init(uL/L)). The second step is to estimate the difference between CO¬2DARK and CO¬2LIGHT (CO2light (uL/L) – CO2dark (uL/L)). To calculate the respiration and photosynthetic rate, these two formulas are used:

The formula for LMA is:

Results:

Table 1 shows the data of three steps CO2 concentrations, leaf area, fresh mass, dry mass, CCI, the time in the dark, and the time in the light. Table 1 also indicats that the yellow-green leaf has the highest CO2 concentration in CO2init and CO2dark. Yellow leaf has the highest CO2light level. The green leaf has the biggest leaf area and most massive fresh mass and dry mass. The yellow leaf has the lowest value of CCI.

Table 1 Averages for three colors leaves

Leaf color

CO2init (uL/L)

CO2dark (uL/L)

CO2light (uL/L)

Leaf area (cm2)

Fresh mass (mg)

Dry mass (mg)

CCI (unitless)

Time in dark (min)

Time in light (min)

Green

915.68

1868.09

590.05

10.23

252.67

97.83

19.28

30.04

79.83

Yellow

638.05

1183.25

1248.66

9.87

134.08

31.58

1.18

30.04

36.04

Yellow Green

432.60

1807.29

819.36

10.51

191.50

59.88

3.45

30.08

63.37

Table two tells the information about the difference between CO2dark and CO2init, the deference between CO2light and CO2dark, Co2 produced in the dark, Co2 consumed in dark respiration rate, and Photosynthetic rate. Table 2 shows that the yellow-green leaf has the highest respiration rate. The yellow leaf has the highest Photosynthetic rate. The green leaf has the maximum value for LMA. On the other hand, the difference between CO2light and CO2dark is a negative value, and the difference between CO2dark and CO2init is a positive value. Thus, the Photosynthetic rate is negative, and the respiration rate is positive.

Table 2 Photosynthetic and Respiration Rates.

Leaf color

CO2dark(uL/L)-CO2init(uL/L)

μL CO2 Produced (Dark)

Respiration Rate (μL CO2 cm -2 h-1)

CO2light (uL/L) – CO2dark (uL/L)

uL CO2Consumed (Dark)

Photosynthetic rate (μL CO2 cm -2 h-1)

LMA (g m-2)

Green

952.41

17.14

3.35

-1278.04

-23.00

-9.00

95.65

Yellow

545.20

9.81

1.99

65.41

1.18

0.48

32.00

Yellow Green

1374.69

24.74

4.71

-987.93

-17.78

-6.77

56.98

Figure 1 indicates that the green leaf has the highest CCI, and yellow leaf has the lowest CCI. The photosynthetic rate is shown in figure 2, the green leaf and yellow-green leaf have almost the same photosynthetic rate. However, the yellow-green leaf is 9.48 μL CO2 cm -2 h-1 bigger than the green leaf. Figure 2 shows that only the yellow leaf has the positive photosynthetic rate. The figure 3 shows the yellow-green leaf has the highest respiration rate. The green leaf has the highest LMA value was shown in figure 4.

Figure 1 CCI

Figure 2 Photosynthetic rate

Figure 3 respiration rate

Figure 4 LMA

Discussion:

The experiment tested the photosynthetic rates of senescing leaves by analyzing their CO2 exchange rate, leaf area, fresh mass, and chlorophyll content. In this lab, the specimens include three different leaves containing different amounts of chlorophyll. These leaves were used to observe the photosynthetic process to determine whether the process is stopped before leaves lose chlorophyll or in the opposite way. Figure 2 and table 2 show that photosynthetic rate of the green leaf is bigger than yellow-green leaf while the yellow leaf has the lowest photosynthetic rate. This phenomenon indicates that the capability of photosynthesis for leaves reduces when chlorophyll decrease in the leaves. In table 2 and figure 2, the photosynthetic rate of the yellow leaf is the positive, which is nearly 0; in this case, it can prove that the photosynthetic rate depends on the amount of chlorophyll in the leaves. This theory proves the hypothesis that the leaf stops photosynthetic when the leaf is turning to yellow. Comparing figure 2, figure 3, and the data in table 2, the respiration rate is lower than the photosynthetic rate. Table one shows that the green leaf has the most significant difference between the fresh mass and dry mass. The yellow leaf has the lowest difference between the fresh mass and dry mass, which means the yellow leaf does not contain much water.

The leaf mass per area (LMA) is shown in Figure 4, and the chlorophyll content index (CCI) is shown in Figure 1. These two graphs indicate similar results. The green leaf has the highest leaf mass per area (LMA) and the chlorophyll content index (CCI). The yellow leaf has the lowest leaf mass per area (LMA) and the chlorophyll content index (CCI).

Differences factors might influence the result of the experiment. First, the CO2 concentration might be affected by people who put the test tube near the mouth because humans will produce the amount of CO2 in one minute. If the tester put the test tube near their mouth, the CO2 produced by human will get into the test tube and syringe. That will make CO2 concentration higher. Second, the result data was taken from the average. It is hard to make sure every group does not have an experiment error during the experiment time. Thus, if one group has a significant error that will influence the final average result. Thirdly, some groups might not wrap the test tube completely to exclude light using the aluminum foil. That will impact the respiration rate and photosynthetic rate. Finally, the CO2 concentration in the lab is higher than the natural environment because there are too many people in the lab, which will provide the amount of Co2 in the air.

In conclusion, the results of the experiment fully support the hypothesis: the yellow leaves will no longer be able to photosynthesis because the yellow leaves have lost enough chlorophyll. Moreover, the results indicate that there is a positive relationship between respiration, photosynthetic, and chlorophyll. However, this lab also could be improved in different ways, such as experimenting in a bigger room and using more accurate equipment.

References:

1. A. F. Devi, S. C. Garkoti, (2013). Variation in evergreen and deciduous species leaf phenology in assam, india. Trees, 27(4), 1. http://dx.doi.org.ezproxy.library.ubc.ca/10.1007/s00468-013-0850-8
2. Gan, S., & Amasino, R. M. (1997). Making sense of senescence (molecular genetic regulation and manipulation of leaf senescence). Plant physiology, 113(2), 313.
3. National Geographic Society. (2019). Chlorophyll. [online] Available at: https://www.nationalgeographic.org/encyclopedia/chlorophyll/ [Accessed 17 Nov. 2019].
4. Tanaka, A. and Makino, A. (2009). Photosynthetic Research in Plant Science. Plant and Cell Physiology, [online] Volume 50(Issue 4), pp.Pages 681–683. Available at: https://doi.org/10.1093/pcp/pcp040 [Accessed 17 Nov. 2019].

Appendixes:

Green leaf:

1. CO2dark(uL/L)-CO2init(uL/L) = 1868.09(uL/L)-915.68(uL/L) = 952.41 uL/L
2. μL CO2 Produced= μL *(CO2dark(uL/L)-CO2init(uL/L)) =0.018* 952.41 uL/L=17.14
3. Respiration Rate (μL CO2 cm -2 h-1) = μL CO2 Produced/leaf area(cm2)/0.5=17.14/10.23(cm2)/0.5hr=3.35(μL CO2 cm -2 h-1)
4. CO2light (uL/L) – CO2dark (uL/L) = 590.05 (uL/L)- 1868.09 (uL/L) = -1278.04uL/L
5. uL CO2Consumed= μL *(CO2light (uL/L) – CO2dark (uL/L)) = 0.018* -1278.04uL/L =-23.00
6. Photosynthetic rate (μL CO2 cm -2 h-1) = )= μL CO2 Consumed /leaf area(cm2)/0.25=-23.00/10.23(cm2)/0.25hr=-9.00(μL CO2 cm -2 h-1)
7. LMA (g m-2) = (Dry mass(mg)/1000)/(Leaf area(cm2)/10,000)=( 97.83mg/1000)/(10.23 m2 /10,000)= 95.65(g m-2)

Table 1 Averages for three colors leaves

Leaf color

CO2init (uL/L)

CO2dark (uL/L)

CO2light (uL/L)

Leaf area (cm2)

Fresh mass (mg)

Dry mass (mg)

CCI (unitless)

Time in dark (min)

Time in light (min)

Green

915.68

1868.09

590.05

10.23

252.67

97.83

19.28

30.04

79.83

Yellow

638.05

1183.25

1248.66

9.87

134.08

31.58

1.18

30.04

36.04

Yellow Green

432.60

1807.29

819.36

10.51

191.50

59.88

3.45

30.08

63.37

Table 2 Photosynthetic and Respiration Rates.

Leaf color

CO2dark(uL/L)-CO2init(uL/L)

μL CO2 Produced (Dark)

Respiration Rate (μL CO2 cm -2 h-1)

CO2light (uL/L) – CO2dark (uL/L)

uL CO2Consumed (Dark)

Photosynthetic rate (μL CO2 cm -2 h-1)

LMA (g m-2)

Green

952.41

17.14

3.35

-1278.04

-23.00

-9.00

95.65

Yellow

545.20

9.81

1.99

65.41

1.18

0.48

32.00

Yellow Green

1374.69

24.74

4.71

-987.93

-17.78

-6.77

56.98

Figure 1 CCI

Figure 2 Photosynthetic rate

Figure 3 respiration rate

Figure 4 LMA

#REF! Green

Yellow

Yellow Green

3.3523469257132712 1.9887268310455852 4.7096446232921281 3.3523469257132712 1.9887268310455852 4.7096446232921281 leaf color

μL CO2 cm -2 h-1)

Green

Yellow

Yellow Green

95.648762440516435 32.001580658548619 56.980486265204711 95.648762440516435 32.001580658548619 56.980486265204711 Leaf Colour

gm-2

#REF! GREEN

YELLOW

Yellow Green

19.279166666666665 1.175 3.4458333333333333 19.279166666666665 1.175 3.4458333333333333 leaf color

Green

Yellow

Yellow Green

-8.9970807041215544 0.47718865691730261 -6.7692303788072232 leaf color

μL CO2 cm -2 h-1)

#REF! Green

Yellow

Yellow Green

3.3523469257132712 1.9887268310455852 4.7096446232921281 3.3523469257132712 1.9887268310455852 4.7096446232921281 leaf color

μL CO2 cm -2 h-1)

Green

Yellow

Yellow Green

95.648762440516435 32.001580658548619 56.980486265204711 95.648762440516435 32.001580658548619 56.980486265204711 Leaf Colour

gm-2

#REF! GREEN

YELLOW

Yellow Green

19.279166666666665 1.175 3.4458333333333333 19.279166666666665 1.175 3.4458333333333333 leaf color

Green

Yellow

Yellow Green

-8.9970807041215544 0.47718865691730261 -6.7692303788072232 leaf color

μL CO2 cm -2 h-1)