Phenotypic plasticity, or differing phenotypes from one genotype in different environmental conditions, is a way for sessile organisms to adapt to changing environmental conditions (Valladares et al., 2007). Plasticity was expected to be abundant, however, it did not occur as often in nature due to resource limitations and environmental stress (Valladares et al., 2007).
An experiment by Matos tested the phenotypic plasticity to light availability in shade and sun leaves of coffee trees (Matos et al., 2009). Their research indicated that “compared [to] sun leaves, shade leaves had a lower stomatal density, a thinner palisade mesophyll, a higher specific leaf area, and improved light capture…” (Matos et al., 2009). The sun leaves were described as “generally thicker with an enhanced quantity of palisade mesophyll” (Matos et al., 2009).
Our objective was the presence of phenotypic plasticity in avocado trees based on the differences in the morphology of shade and sun leaves. In our study, we asked whether there is a difference in surface area, length-to-width ratio, mass, specific leaf mass, and color between shade leaves and sun leaves in avocado trees. We hypothesized that there would be no significant differences in surface area, length-to-width ratio, mass, specific leaf mass, and color between shade leaves and sun leaves. METHODS:
We collected our seventy samples of avocado tree (Persea americana) leaves, in equal amounts of sun and shade leaves, at an avocado tree grove located north of Building 3 and University Drive at Cal Poly Pomona on Thursday, October 24, 2013 at 9:00 am. They were randomly and interspersedly collected throughout the grove. We split the grove into five areas, split into five teams of two, and was assigned to one of the five areas. Each team picked a number for the trees in their region and a random number was selected from a random number table to select a tree corresponding to that number. A random number table was used to pick the corresponding quadrant, branch, and leaf. This process was done twice on each tree in the understory for shade leaves, and in the canopy for sun leaves.
Each leaf was measured for its surface area, length-to-width ratio, mass, specific leaf mass, and color. Surface area was measured by a leaf area meter in squared centimeters. Length-to-width ratio was measured by measuring the length (vertically along the bridge of the leaf) and the width (horizontally on the widest part of the leaf) with a ruler in centimeters, and dividing the length by the width. Mass was calculated by a balance in grams. Specific leaf mass (thickness) was measured by dividing the mass by its surface area in grams per squared centimeter. Color was measured by having three reference leaves provided by the instructor, indicating light (L), medium (M), and dark (D) leaves and compared our collected leaves.
After recording all of the data, these data were then input into a statistical program called StatCat to determine normality through a normality test. The data for surface area, length-to-width ratio, mass, and specific leaf mass for sun and shade leaves were both normal, therefore, we chose a paired sample t-test for all of them. A normality test was not needed for color for sun and shade leaves due to it being a nominal scale data. The number of light, medium, and dark shade leaves were tallied up according to color, and the same was done for the sun leaves. A contingency table was made in Excel, and used in StatCat to test our hypothesis. The paired sample t-tests were also done through StatCat, which then gave us the appropriate results to test our hypotheses.
Shade leaves had a significantly larger surface area than sun leaves (t = -3.7313, P = 0.00069; Table 1). Shade leaves had a significantly larger length-to-width ratio than sun leaves (t = -2.7162, P = 0.01031). Shade leaves had no significant difference in mass than sun leaves (t = -1.4871, P = 0.1462). Shade leaves had a significantly smaller specific leaf mass than sun leaves (t = 5.82093, P = 1.5×10-6). Shade leaves were significantly darker than sun leaves (X2 = 18.417, P = 0.0001).