Aim: The aim of this project is to determine the influence of sunlight and shade on the characteristic of leaf morphology.
Background: The gradient in light received by the leaves, compel them to adapt for optimum growth in a particular environment. This adaptation takes place gradually, with changes in certain morphological features. These morphological changes that take place with time and space in which the plant grows is defined as succession and has a role to play in evolution. Certain environmental factors like shade and sunlight can present as a slightly adverse condition that affect plant growth. In order to compete with other plants that grow in a similar environment, it is essential for the oak plant to adapt certain morphological characteristic, so that it can achieve a competitive growth in a slightly adverse environment. In this paper, we look at the adaptation in leaf morphology, as a response to shade and sunlight. The leaves of the plant, help to convert radiant energy in the sunlight to chemical energy. Leaves are photosynthetically active in the radiant energy range of 400-700nm (Bohning & Burnside, 1956). As the variation in radiation take place with a season (sunlight on one extreme and shade on another extreme), the leaves adapt, so as to benefit maximum from the available sunlight (Jones & Thomas, 2007). In this study, we quantified leaf characteristic like: leaf area, prim and chlorophyll content of oak leaves exposed to different intensity of sunlight and those that grow in shade. The hypothesis of the study is: The leaves of an oak tree, adapt their morphological characteristics, deferentially in shade and light.
Result: The difference in mean leaf area, prim, chlorophyll content and area of leaves grown in sunlight and shade are shown in Fig1: A, B, and C respectively. The mean thickness of the leaves grown in sunlight was higher (236.65±13.8 µm) than those grown in shade (185.2 ± 11.2 µm). The mean prim length of leaves grown in sunlight was lower (54.10 ±16.7 cm) than those grown in shade (98.46 ± 12.7 cm). The chlorophyll content was higher in the leaves grown in sunlight (454.7 ± 27.8 mg.m2), when compared to the leaves grown in shade (331.4 ± 27.4 mg.m2). The area of leaves grown in sunlight are lower (43.8 ± 20.0) than area of leaves grown in shade (121.37 ± 18.0). The difference in the mean of the above variables, was significant at more than 95% confidence interval. Table 1, provides the values of mean, SD, df, pvalue and t value of a two tailed T-test.
Discussion: Tree observed at different levels in a forest, tend to have different canopy patterns. For example, trees that emerge above the crown of other trees, have maximum exposure to sunlight and tend to have a much broader crown, when compared to trees that are continuous and lie hidden under the shade of other trees. While some researchers suggest that it is a result of genetic difference, others claim that it’s an environmental adaptation (Ashton, Yoon, Thadani, & Berlyn, 1999). When there is more space, the canopy tends to spread out, while in crowded locations, the crown of the tree adapts, by taking a more rounded and compact form. Similarly, leaves in the same tree, also show morphological variations depending on the extent of sun exposure they receive.
The exposure to sunrays is maximum at the surface of the trees, while the leaves that are hidden in the shade, have low exposure to sun rays. Thus, leaves with maximum exposure to sunlight, are seen in the zenith of the crown. The leaves that are exposed to sunlight, have a greater photosynthetic rate and transpiration rate, when compared to leaves grown in shade. The distribution of leaves in the canopy and the shape of the tree canopy is determined by environmental factors like moisture content of the environment, temperature and exposure to the sun. (Jones & Thomas, 2007)
In shaded conditions, the heat loss is less and the larger leaves have lower resistance to transpiration, when compared to small leaves. The leaves that have maximum exposure to sunlight are at the risk of losing more water, when compared to those in the shaded region. According to the result, Fig1: A, leaves from areas exposed to sun, had more thickness. Higher thickness, offers greater storage of water and offers greater resistance to water loss.
Large leaves help to harvest more of the sun’s energy. Thus, leaves that grow in shade have larger leaves as an adaptation to lower exposure to sunlight. Even in this study, leaves in shaded area had higher area and prim (Fig1: B, D), when compared with leaves that grow in sunlight. The small size of the leaves that is grown in sunlight, helps to restrict transpiration and exposure to excess sunlight.
The leaves adapts its size, so as to achieve maximum efficiency of water use and to maximize carbon gain. The photosynthetic rate per unit area is higher in small leaves that are exposed to sun ray, when compared to large leaves that grow in shade. The results of the study, also identified higher level of chlorophyll content in leaves grown in sunlight (Fig1: C). Chlorophyll content of the leaf, is directly proportional to its photosynthesis level (Lichtenthaler, Babani, Navrátil, & Buschmann, 2013).
Conclusion: The size and chlorophyll content of the leaves of the oak tree are determined by the level of exposure to sun lights. Leaves that grow in shades have a larger size and lower chlorophyll content, when compared to leaves that grown in sunlight. A greater thickness is observed in leaves exposed to sunlight, when compared to leaves that grow in shade. A clear environmental adaptation to shade and sunlight is noticed in the leaves of the oak tree. Thus supporting the theory of succession.
References
Ashton, P., Yoon, H., Thadani, R., & Berlyn, G. (1999). Seedling Leaf Structure of New England Maples (Acer) in Relation to Light Environment. Forest Science, 45(4), 512-516.
Bohning, R. & Burnside, C. (1956). The Effect of Light Intensity on Rate of Apparent Photosynthesis in Leaves of Sun and Shade Plants. American Journal Of Botany, 43(8), 557. http://dx.doi.org/10.2307/2438868
Jones, T. & Thomas, S. (2007). Leaf-level acclimation to gap creation in mature Acer saccharum trees. Tree Physiology, 27(2), 281-290. http://dx.doi.org/10.1093/treephys/27.2.281
Lichtenthaler, H., Babani, F., Navrátil, M., & Buschmann, C. (2013). Chlorophyll fluorescence kinetics, photosynthetic activity, and pigment composition of blue-shade and half-shade leaves as compared to sun and shade leaves of different trees. Photosynth Res, 117(1-3), 355-366. http://dx.doi.org/10.1007/s11120-013-9834-1
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