Our study was novel in cutting-edge photosynthesis research. With this research, recommendations for agricultural success and greater crop yields can be made. This research contributed to the understanding of plant health in relationship to environmental conditions. More intense light was correlated with leaves under stress, and therefore, it could be determined that crops grown in shaded environments would be healthier, according to Barber and Andersson (1992). ΦII levels described the efficiency of photosystem II in the chloroplast membrane of the leaves of the trees. Linear electron flow (LEF) described the conversion of light energy to chemical energy in the leaves. PQ SPAD measured the chlorophyll content of the leaves. As plants were exposed to more intense light, their photosynthetic processes experienced stress and yielded lower ΦII and higher LEF. Seemann et al. (1987) found that plants have been observed to acclimate their photosynthetic apparatus to varying light intensity. We studied the differences in leaf ΦII, LEF, and PQ SPAD between interior and exterior canopies of three Acer rubrum trees. Our study was connected to a prior research project from 2015, so we could compare photosynthesis in the interior and exterior canopies of the same trees studied in two sequential fall semesters.
The research hypotheses were that the leaves in the interior canopy would exhibit differences in ΦII and LEF, and equivalent mean PQ SPAD compared to exterior leaves. Higher ΦII was expected in the shaded interior leaves because with less light intensity, photosystem II was less stressed and more efficient. Lower LEF was expected in the interior leaves since there was less light energy available to be converted into chemical energy. PQ SPAD was not believed to change between canopy locations because chlorophyll content was not affected by light intensity. We hypothesized that there would be no difference in photosynthesis between 2015 and 2015 because light intensity would be relatively similar at this time of year compared to last year. The research conducted in our study advanced other research in this area by determining where agriculture might be more successful, producing higher crop yields, in addition to adding to a longitudinal study of these Acer trees.
Three Acer rubrum trees in the Benefactor's Plaza just south of the Old Horticulture building on Michigan State University's campus were studied. The trees were biological sample replicates and all planted in 2007 and identified by tree codes 2007024002, 2007024003, and 20070241*06. Data was collected using MultispeQ device #122, which measured light intensity, LEF, ΦII, and PQ SPAD, as described by Kuhlgert et al. (2016). Each tree was divided into northeast, southeast, southwest, and northwest quadrants. Using the second step of a step stool and starting in the northeast quadrant, five random green, dry, exterior leaves were measured before five random green, dry, interior leaves in each quadrant. The individual collecting data moved clockwise around a tree to collect 40 total technical sample replicates/tree and a total of 120 measurements/day. Data was collected on thirteen different days over five weeks. Photosynthetic trace was viewed after each piece of data was collected to determine if it was accepted or rejected. Accepted measurements were sent to the PhotosynQ website. Later, data was downloaded and analyzed by one-way ANOVA for independent samples in each location based on quadrants/tree and individual trees to pool similar data. Final comparisons between different locations within a single year and same locations between different years were done by a two-sample two-tailed independent samples t-test. Analyses were done at vassarstats.net.
Random Sampling Method
We used the second step on a step stool to reach the leaves, which were technical sample replicates, from the canopies of the trees.
There are 4 quadrants we sampled from: Northeast, Southeast, Southwest, and Northwest. We began with the Northeast quadrant and moved clockwise around the tree until the Northwest quadrant was reached.
Using MultispeQ Device #122, we began by measuring a random green, dry leaf from the exterior canopy of the starting quadrant.
After a piece of data was collected, the photosynthetic trace was analyzed to determine whether it should be accepted and sent to the Photosynq website, or rejected. If the measurement was rejected, another one was taken. This was done until 5 accepted measurements were collected.
Steps 3 and 4 were repeated in the interior canopy of the same quadrant.
Once 10 measurements were taken in a quadrant, the next quadrant in the clockwise direction was sampled. This was done until data was collected from all 4 quadrants of the tree.
Steps 3 through 6 were repeated for each tree.
Each day, forty total technical sample replicates were collected per tree. Once data for all three biological sample replicates was collected, there was a total of 120 measurements per day.
Photo of Acer Rubrum
Figure 1: Mean ΦII of Different Canopy Locations During Different Falls. In 2015, pooled data was for three biological sample replicates while 2016 was for two biological sample replicates. In 2015, the mean ΦII for interior (n=432) was 0.62 and exterior (n=413) was 0.55, while in 2016, interior (n=389) was 0.44 and exterior (n=254) was 0.44. Error bars represented +/-95% confidence intervals.
Figure 2: Mean LEF of Different Canopy Locations During Different Falls. In 2015 and 2016, pooled data was for three biological sample replicates. In 2015, the mean LEF for interior (n=432) was 9.47 μE and exterior (n=413) was 34.77 μE, while in 2016, interior (n=366) was 30.63 μE and exterior (n=477) was 59.07 μE. Error bars represented +/- 95% confidence intervals.
Figure 3: Mean PQ SPAD of Different Canopy Locations During Different Falls. In 2015, pooled data was for three biological sample replicates while 2016 was for two biological sample replicates. In 2015, the mean PQ SPAD for interior (n=459) was 34.79 and exterior (n=439) was 42.56, while in 2016, interior (n=268) was 29 and exterior (n=244) was 36.58. Error bars represented +/- 95% confidence intervals.
Table 1: t-test Comparisons of Mean ΦII, LEF, and PQ SPAD in Different Canopy Locations During Different Falls. Performed two-sample two-tailed t-test (α = 0.05) to compare leaf locations and years. For 2015 and 2016, compared interior to exterior canopy locations for each variable. Compared 2015 to 2016 for only interior canopy and also for only exterior canopy for each variable.
It was hypothesized that leaves from the interior canopy would yield higher mean ΦII, lower mean LEF, and equal mean PQ SPAD compared to exterior leaves in each year, and that there would be no difference in canopy locations between years.
As seen in Fig. 3 & Table 1, mean ΦII was greater in the interior than exterior in 2015 (t-stat=7.9, p<0.0001), but the same in 2016 (t-stat=-0.04, p=0.968). For 2015, the null statistical hypothesis was rejected, and there was a statistically significant difference between the mean ΦII in the interior versus exterior, with the interior making ~110% more. In 2016, the null statistical hypothesis failed to be rejected, which meant there was no statistically significant difference between the mean ΦII in either location. This supported the research hypothesis about canopy location for 2015, but did not support it for 2016.
The mean ΦII for comparing interior canopy between different years (t-stat=19.16, p<0.0001) and for comparing exterior canopy in different years (t-stat=9.68, p<0.0001) were both different for each canopy location (Fig. 3 & Table 1). The null statistical hypothesis was rejected, showing statistically significant differences in mean ΦII, with reductions in 2016 of interior=28% and exterior=20%. The yearly comparison research hypothesis was not supported.
As seen in Fig. 4 & Table 1, mean LEF was greater in the exterior than interior in 2015 (t-stat=17.6, p<0.0001) and in 2016 (t-stat=17.55, p<0.0001). For both years, the null statistical hypothesis was rejected, and there was a statistically significant difference between the mean LEF in the interior versus exterior, with the exterior being at least 240% greater. This supported the research hypothesis about canopy location for 2015 and 2016.
The mean LEF for comparing interior canopy between different years (t-stat=-24.38, p<0.0001) and for comparing exterior canopy between different years (t-stat=-11.94, p<0.0001) were both different for each canopy location (Fig. 4 & Table 1). The null statistical hypothesis was rejected, showing statistically significant differences in mean LEF, with increases in 2016 of interior=320% and exterior=~170%. This did not support the yearly comparison research hypothesis.
As seen in Fig. 5 & Table 1, mean PQ SPAD was greater in the exterior than interior in 2015 (t-stat=16.79, p<0.0001) and in 2016 (t-stat=28.18, p<0.0001). For both years, the null statistical hypothesis was rejected, and there was a statistically significant difference between the mean PQ SPAD in the interior versus exterior, with the exterior being ~120% greater. This did not support the research hypothesis about canopy location for 2015 and 2016.
The mean PQ SPAD for comparing interior canopy between different years (t-stat=12.51, p<0.0001) and for comparing exterior canopy between different years (t-stat=13.34, p<0.0001) were both different for each location (Fig. 5 & Table 1). The null statistical hypothesis was rejected, showing statistically significant differences in mean PQ SPAD, with reductions in 2016 of interior =~20% and exterior=16%. This did not support the yearly comparison research hypothesis.
The differences between data collected in 2015 and 2016 could have been due to climate change. Pollastrini et al. (2016) found that photosynthetic efficiency, in conjunction with solar radiation and temperature gradient, increased with latitude from southern to central Europe. This increase in photosynthetic efficiency with decreased sunlight and temperature could explain why mean ΦII and mean PQ SPAD were so much lower in 2016 than 2015.
Overall mean LEF values could have been greater in 2016 than 2015 because data may have been collected on generally sunnier days and during different times of the day with more intense sunlight.
Mean PQ SPAD values could have been greater in the exterior canopy than interior canopy in both years due to interior leaves experiencing more rapid senescence during the fall. As nights got longer in the autumn and trees received less sunlight, production of chlorophyll in leaves slowed and eventually ceased. This degradation of chlorophyll allowed leaves to show other pigments characteristic of fall color change, resulting in lower PQ SPAD readings (Gitelson and Merzlyak, 1994).
The results of this study were important to further understand successful agriculture techniques. Our research showed that changes in environmental conditions over time caused statistically significant differences in photosynthesis, which is related to the health of plants. This was consistent with findings by Andrews et al. (1995) who showed that during the early growing season, when crops were subjected to fluctuating temperatures, photoinhibition of photosynthesis was caused by chill.
It is important to continue this longitudinal study to determine the effects of climate change on photosynthesis, and we hope another group of students will next fall. Future research in this field could also examine additional photosynthetic variables, such as NPQt and ΦNPQ, to provide more detailed results. Studying the effect of the time of day, and so the overall tree location, would also be beneficial in determining the importance of light intensity in photosynthesis.
- Leaf Photosynthesis - MultispeQ Beta ONLY
Measures photosynthesis-related parameters in <15 seconds, including: Phi2, PhiNPQ, PhiNO, NPQt, qL, LEF, and SPAD. In addition, measures PAR (photosynthetically active radiation), ambient temperature and relative humidity.
Works with the MultispeQ Beta device only
- Which Acer rubrum tree code? (Multiple Choice)
- Quadrant of Tree Canopy Leaf is Located In? (Multiple Choice)
- Leaf Canopy Location (Multiple Choice)
- Random Green, Dry Leaf Count (Multiple Choice)