Abstract: Observations indicated that the species of Wiliwili trees that are native to Hawaii (Erythrina sandwicensis) are more susceptible to insect predation than that of its introduced, columnar form (Erythrina variegata). Hypotheses are tested in this research to help determine possible reasons for this occurrence.
The original motivation for comparing the species of Wiliwili that is native to Hawaii (Erythrina sandwicensis) to its introduced, columnar form (Erythrina variegata), was to determine reasons as to why the native Wiliwili show signs of heavy insect predation in its mature form, as compared to the introduced species. It was also observed that the immature, juvenile leaves of the native Erythrina sandwicensis plant showed little to no signs of insect predation, while its mature leaves showed heavy insect predation. It has not been determined for sure which insects predate on the leaves of the native Wiliwili, but caterpillars have been found on occasion on these plants.
Several hypotheses were made in an effort to determine why insect predation occurs on the mature leaves of Erythrina sandwicensis. It has been observed that the immature leaves of Erythrina sandwicensis have a heavy pubescence on its abaxial surface, possibly serving several useful functions to the juvenile leaves. This heavy pubescence helps to deter insect predation on these young leaves, and may also provide camouflage to help hide small insects or insect eggs that have been laid on the leaves. When the leaves mature and lose its pubescence, they then become more vulnerable to insect predation.
Also obseverved on both species of Wiliwili were nectaries at the base of the leaves. Another hypothesis to explain the insect predation on the native Wiliwili was that, perhaps, its nectaries don't produce any secretion that attracts ants. It has long been known that many plants have symbiotic interactions with insects, such as ants, that are beneficial to the plant for defense. Observations have been made that show the presence of ants on Erythrina variegata, but not so much on Erythrina sandwicensis. In exchange for nectar, the ants are well-known to defend their host plants from other insects that might prey on the plant. "By far, the most common sugars are sucrose, glucose, and fructose, but other sugars and short polysaccharides occur in at least trace quantities: maltose, melobiose, and raffinose. These can contain traces of amino acids and other acids, as well as numerous other organic compounds. The function of nectar is to attract pollinators. Nectaries are subdivided on the basis of their position on the plant into floral nectaries and extrafloral nectaries" (Mauseth, 1988). As seen in the images below, the nectaries of Erythrina sandwicensis do produce a secretion, but perhaps it is not beneficial to, or simply does not attract ants. This should be tested in later experiments.
Lastly, another hypothesis to explain the insect predation occurrences for these two species of Wiliwili was the presence of secretory trichomes. It was believed that the leaves of Erythrina sandwicensis did not contain secretory trichomes, while it was known that the leaves of Erythrina variegata did. However, upon observing leaves of Erythrina sandwicensis during this research, it was discovered that secretory trichomes do, in fact, exist within these leaves. This caused the hypothesis to be altered to the fact that the secretory trichomes of Erythrina variegata secrete a compound that deters insects from eating its leaves, while the secretory trichomes of Erythrina sandwicensis either do not produce a similar compound to deter insect predation, or are simply non-functioning and do not secrete anything. This hypothesis can be tested in further experiments. Dr. Fahn, an authority on seretory trichomes from Israel, confirmed the presence of secretory trichomes shown in our images of Erythrina sandwicensis leaves.
Leaf samples of Erythrina sandwicensis and Erythrina variegata were collected from the campus of the University of Hawaii at Manoa. All samples of Erythrina sandwicensis leaves were taken from the three Wiliwili trees on Maile Way, directly behind Hamilton Library. From right to left, the trees were labeled as Trees A, B, and C. All Erythrina variegata leaves were taken from the three trees located in front of Webster Hall, right next to Varney Circle. From right to left, these trees were labeled as Trees D, E, and F.
Sectioning
Sections of the leaves were placed into a fixative antifreeze solution that contained 10% Dimethylsulfoxide, 4% Formaldehyde (w/v), 1% Tween 20 and 0.05 M Sodium Cacodylate. The pH was set at 7.5. The leaf sections were first placed in a vacuum, and then stored at 5 degrees C for approximately 3 weeks. The leaf sections were then mounted on a Reichert-Jung Cryotome machine, and sectioned at 50 microns. These were stained with Toluidine Blue (0.05%). The freezing support medium (Tissue Freezing Medium, Triangle Biomedical Sciences) was diluted with water and removed by blotting prior to staining because it formed a precipitate with the stain. Unstained specimens were mounted in the freezing support medium and viewed without further treatments.
Shown below is the Reichert-Jung Cryotome machine used for cutting the leaf sections for the slides. This image was borrowed from the website of Dr. David Webb.
All microscope images of the leaf sections were taken with a Nikon Coolpix 990 Digital Camera. The leaf sections observed were of the midrib and lamina of Erythrina sandwicensis and Erythrina variegata, and were viewed and photographed at various magnifications as unstained or stained with Toluidine Blue, and viewed in both bright-field and polarized optics. All images were saved on a memory card and later downloaded onto a computer. All image editing was performed using Imaging For Windows and Microsoft Paint.
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Nikon Coolpix 990 Digital Camera
(image from Dr. David Webb) |
| To the right is a close image of an Erythrina variegata plant. |
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Shown here is a picture of a healthy, mature, whole leaf specimen of Erythrina sandwicensis. Sections were taken from this leaf and used for observation. Of the three trees located outside of Hamilton Library, this tree was located on the far right. |
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| A heavy layer of pubescence can be seen on the abaxial surface of the juvenile leaves of Erythrina sandwicensis. This xeromorphic trait also aids in the deterring of insect predation on these leaves. |
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| Close-up view of the abaxial surface of Erythrina sandwicensis. This image shows the compartmentalization features of the leaf, as well as the branched trichomes present. |
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| Similar to the last image, this picture shows the abaxial surface of Erythrina variegata. |
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| Erythrina
Variegata with nectaries at the petiole. Image borrowed from Dr. David Webb. |
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| To the right is a stem of Erythrina sandwicensis, with the leaves removed. Shown here are the nectaries present on the stem, with some secretion coming from the nectaries. What is it secreting? |
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| Here is a stem of Erythrina variegata with its nectaries shown. |
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| Secretory trichome found on the abaxial surface of Erythrina sandwicensis. Functional or non-functional? |
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| Secretory Trichome on the abaxial surface of Erythrina variegata. |
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| An image of the unstained midrib of Erythrina sandwicensis, showing the vascular bundles. |
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| An image of the midrib of Erythrina sandwicensis, viewed with cross-polarizers. The cells showing high bi-refringence indicate that they are highly organized. |
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| An image of the lower midrib of Erythrina sandwicensis, stained with Toluidine Blue. This helps to easily distinguish tissues like xylem and phloem. The waxy cuticle is another xeromorphic trait. |
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| The picture to the right shows a cross-section through the midrib of on Erythrina variegata leaf. It, too, has many of the same basic features of a typical dicot leaf. |
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| The same photo as above, but viewed with full polarization. In this photo, much more detail and organization of the midrib can be seen. |
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Through observations of this research, much was learned about the similarities and differences between Erythrina sandwicensis and Erythrina variegata. The main physical difference between these two species was the presence of pubescence on the juvenile leaves of Erythrina sandwicensis. It is hypothesized that the pubescence present helps to deter insect predation on the young leaves of Erythrina sandwicensis. This pubescence may also serve as camouflage to help hide young insects or insect eggs from other insects that might prey on them. As the leaf matures, it loses its pubescence, allowing Erythrina sandwicensis to become vulnerable to insect predation.
The physical similarities between these two different species were striking, leading to the thought that their differences may me more in their chemical compound production. Although both types of plants had many of the same physical qualities, the fact remains that Erythrina sandwicensis is much more heavily predated on by insects than Erythrina variegata. As a result of this, one hypothesis is that the nectaries on Erythrina sandwicensis do not secrete compounds that are beneficial to, or attractive to ants, like the nectaries of Erythrina variegata. Therefore, there are little to no ants on Erythrina sandwicensis that will defend the plant from other insects that might prey on its leaves. Another hypothesis regarding the insect predation is that the secretory trichomes discovered on the abaxial surfaces of Erythrina sandwicensis are not functional like those of Erythrina variegata. The secretions from the trichomes of Erythrina variegata are believed to deter insect predation. If the secretory trichomes present in Erythrina sandwicensis are non-functional, then they have no effect in deterring insects.
The observations and hypotheses formed from this research are strictly preliminary, however, and more experiments must be carried out in the future to determine whether any of these hypotheses may be disproved or not.
Mauseth, J. D. (2002). Plant Anatomy. University of Texas, Austin: The Benjamin/Cummings Publishing Company, Inc.
Webb, D. T. (2002). http://www.botany.hawaii.edu/faculty/webb/BOT410/Leaves/wiliwili.htm. Dr. David Webb's Botany 410 lecture website - University of Hawaii at Manoa. "Wiliwili - Erythrina sandwicensis".
