Anatomy and secondary thickening pattern of the stem in Tithonia diversifolia (Hemsl) a gray.
The Mexican sunflower, Tithonia diversifolia (Asteraceae) is native to North America but was introduced into West Africa for ornamental purposes (Akobundu and Agyakwa 1987). It was also reportedly cultivated for the same reason in India (Dutta, 1981), but has how become a problem weed of fiedcrops, wastelands and roadsides especially in South Western Nigeria (Smith and Anisu, 1997). In Nigeria, two morphological variants of T. diversifolia are often recognized on the basis of colour of the petiole i.e. the green and the purple varieties. Our preliminary observations on the populations of this weed in Ogbomoso ecosystem however suggested the distinction to be a result of responses to different levels of shade and moisture.
The Nigeria T. diversifolia is a fast-growing, water-loving annual weed that becomes woody at maturity. The fast growing habit of this weed puts it at an adavantage over most other weeds which are eventually overpowered by T. diversifolia when growing together. Because of this property, the weed is being considered potentially useful for fallow land management (Liasu, and Atayese 1999). Also, the wood of older stems may have a potential application in wood-based industries for making pulp and paper, match splints and tooth picks while the bast could be found useful in rope making and carpet weaving.
Studies on the biology of weeds are important for a design of an effective and lasting weed management programme. Also, anatomical and biochemical data do reveal their economic potentials and systematic affinity. Flowers and leaves of T. diversifolia cultivated in Egypt were found to contain a high percentage of sesquiterpene lactones, which possess a ganglionic stimulant property, and induces lowering of blood sugar level in animals (Sayed et al., 1980). The Indian T. diversifolia also contains these active ingredients in all the above-ground parts (Baruah et al 1979).
A review of the literature shows a dearth of information on T. diversifolia in Nigeria. In fact no publication could be traced in connection with its anatomy. The present study therefore sets to elucidate the anatomy and secondary thickening pattern in the stem of T. diversifolia in Nigeria with a view to contributing some information that may be of systematic and economic value as well as provide explanation for its competitiveness as a weed in both rich and poor soils.
Materials and methods
Fresh but mature Tithonia diversifolia stands, about 1m tall with basal diameter of 3-4 cm were uprooted from the wild population within the campus of Ladoke Akintola University of Technology, Ogbomoso in July, 2003. A total of eight stands, two from four different Ecological locations (i.e. under the shade, in bright light, on moderate moisture and in waterlogged soil) were harvested.
Tissue sectioning and observation
A serial transverse sectioning of the stem was undertaken from about 1mm below the soot apex to the stem base by means of a freezing stage microtome available at the University of Ibadan, Nigeria. Up to 10 sections were obtained from each plant specimen at a distance of about 10cm. the sections were stained in 1% alcoholic safranin (in 50% ethanol) and mounted in dilute glycerine.
Observations were made on binocular microscope on all the slides prepared. Particular attention was paid to the initials, the primary tissues (epidermis, ground tissue, the vascular tissue) as well as the activities of the primary tissues leading to secondary thickening of the stem. Photomicrographs were also taken using a microscope with camera attachment, available at the University of Ilorin, teaching hospital, Ilorin, Nigeria.
Results and discussions
Origin of the primary vascular tissues
About 1mm below the shoot apex, transverse sections (T.S) reveal a complete ring (or cylinder) of procambial strands. T.S. cut from a few mm below this show the differentiation of primary vascular tissues from the procambial cylinder. The first tissue to originate are the 4-8 radial rows of protoxylem vessels, then the phloem cells, metaxylem vessels and the fibrous bundle cap in the order stated (Figs. 1A and B).
Anatomy of the primary stem
The mature primary stem of T. diversifolia has a peripheral ring of 24-36 collateraL vascular bundles each with a conspicuous crescent-shaped bundle cap and a strip of densely stained cambial cells between the primary phloem and the primary xylem (Figs. 1A and B). Large bundles more or less alternate with thin bundles both types embedded in thin-walled cells that makes up the fundamental tissue (Figs. 1B and C). Such thin bundles have been found in many members of Asteraceae (Metcalfe and Chalk, 1972) and certain other families (Nicolas 1913). They were said to represent branches of the peripheral ring of strands which sometimes serve as aid in the identification of the species (Metcalfe and Chalk, 1972). In Asteraceae, thin vascular bundles among the large ones were regarded as medullary bundles or vestigial structures to be interpreted as relics of the scattered vascular bundle system which is believed to have existed in ancestral forms (Wordsel, 1919). This view is supported by our observations that the circumsference of the thin bundles is slightly smaller than that of the larger bundles (fig. 1B).
[FIGURE 1 OMITTED]
On the outside of the vascular cylinder lies 5 to 8 layers of angular collenchyma which forms the main mechanical tissue in the primary stem. The outermost layer of tissue is formed by uniseriate, oval to rectangular shaped epidermal cells which may bear simple trichomes.
Secondary thickening process in the stem of T. diversifolia
The secondary thickening pattern in the stem of T. diversifolia is not particularly consistent with most reported works on woody dicotyledons (Esau, 1945, 1965, 1977, Dutta 1981) (figs 1C-F). The secondary tissue formation begins with radial elongation of the vascular bundles which is predicated on a series of periclinal cell division of the intrafascicular cambium (Fig 1C). In this and other processes that follow, the medullary (or thinner) vascular bundles are the more active. The parenchyma cells on either sides of these medulary vascular bundles and at the position corresponding to the intrafasicular cambium dedifferentiate. The resultant cambial cells within and around the vascular bundles differentiate into an inner strip of secondary xylem (wood) elements and an outer strip of secondary phloem (bast) elements.
These activities widen the more peripheral part of each of the medullary vascular bundles with the secondary tissues to impart a wedge shape, the narrow end of which is occupied by primary xylem elements (Fig. 1D). This aggregate of primary and secondary tissues is herein referred to as Type I wedge shaped tissue. The wood in Type I wedge consists of vessels of different sizes and of no particular arrangement but with much fibres and little or no parenchyma. Its bast consists of sieve tube elements, some fibres and parenchyma (Fig 1D). These observations confirm that both tangential and radial cell divisions precede the actual tissue differentiation.
Within the thicker vascular bundles the cambium is less active. Only periclinal or tangential divisions are observable. This is immediately followed by differentiation into an inner strip of secondary xylem and an outer strip of secondary phloem tissues. Both secondary tissue types occupy a place near the narrow peripheral part of the resulting wedge-shaped tissue. This is the Type II wedge. In the Type II wedge, the young wood consists only of radial rows of cells (xylem vessels, much fibres and little or no parenchyma) while the bast consists of sieve tube elements, some fibres and parenchyma (Fig. 1D). The anatomy of wood in this case confirms that only tangential cell division could have preceeded the formation of these tissues.
Wood and bast elements continue to differentiate as inner and outer strips of tissues respectively and in the manner earlier described. However, much more secondary tissues (wood and bast) are produced by the Type I wedge with its cambium layer exhibiting two different types of cell division. Again, much more of the wood is produced than the bast in both Type I and Type II wedges. The activities of both the more active Type I and the less active Type II wedges gradually fill the inter-fascicular bundle areas with secondary tissues. They also lead to an increase in girth of the stem as a result of an inner continuous cylinder of thick wood and an outer cylinder of bast which is relatively thin (Figs.1E and F). Wood elements do not extend to the centre of the stem which remains occupied with thin-walled cells. Moreover, these thin-walled cells also occur frequently within the secondary xylem areas as long rays.
In the stem of T. diversifolia, secondary thickening is essentially due more to the activity of strips of meristems within the vascular bundles than those within the interfascicular areas. However, the integrity of the epidermis with the overlying cuticle is not maintained at the end of the secondary thickening process. This is evidenced by the localized periderm formations with lenticels in those locations where the epidermis is ruptured (Fig 1F).
Further distinctive features of the anatomy of the secondarily thickened stem of T. diversifolia include the occurrence of a nodulated ring of procambium from which vascular bundles develop, and the maintenance of discrete wedge-shaped tissues, an aggregate of primary and secondary vascular tissues at the early part of secondary growth. The wedge-shaped vascular bundles in the family Asteraceae according to Greulach (1977) are actually those discrete wedge-shaped Types I and II tissues observed by us. The stems pith, medullary rays and cortex are very extensively parenchymatous. In order to compensate for the few lignified tissue, additional support by turgid parenchyma cells to T diversifolia. This may account for the usual collapse of stem during the dry seasons.
Finally, no complete ring of vascular cambium was observed. The complete cylinder of peripheral wood and bast is formed by a progressive lateral differentiation of the dedifferentiated parenchyma in the interfascicular areas. The uniqueness of the vascular tissue of T. diversifolia when compared with other dicot plants is probably one of the reasons for its strong dominance when competing with other weeds when water is not limiting especially in poor soils. Muoghalu and Chuba, (2005) recently observed similar survival strategies in seed germination of Tithonia diversifolia and T. rotundifolia. Apparently, lower amount of mineral nutrients and photosynthates will be required to build size and volume and with the advantage in size and volume attained, other competing plants are virtually crowded out or overtopped by the closed canopy of the Tithonia community (Liasu and Atayese, 1999).
Tithonia diversifolia like most members of the family asteraceae (compositae) exhibits a unique stem anatomy and secondary growth pattern which has the potential of minimizing sink demand for photosynthates while providing maximal skeletal support in spite of the scanty wood through turgid parenchyma tissues. This implies advantage in competition with crops and other weeds for domination in poor soils especially when water is not limiting.
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Muoghalu, J.I. and D.K. Chuba, 2005 Seed germination and reproductive strategies of Tithonia diversifolia (hemsl.) Gray and Tithonia rotundifolia (P.M.) Blake. Applied Ecology and Environmental Research, 3(1): 39-46.
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Smith, M.A.K. and O.O. Anisu, 1997. Some aspects of the biology of Mexican sunflower (Tithonia diversifolia). Nigerian Journal of Weed Science., 10: 1-4
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M. O. Liasu, Department of Pure and Applied Biology, Ladoke Akintola University of
Technology, PMB 4000. Ogbomoso. Oyo State. Nigeria.
Mobile Phone: 08036669838
M.O. Liasu and A.T.J. Ogunkunle
Department of Pure and Applied Biology, Ladoke Akintola University of Technology, Ogbomoso, Oyo State,
M.O. Liasu and A.T.J. Ogunkunle,: Anatomy and Secondary Thickening Pattern of the Stem in
Tithonia diversifolia (Hemsl) a Gray, Adv. in Nat. Appl. Sci., 1(1): 21-25, 2007
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|Title Annotation:||Original Article|
|Author:||Liasu, M.O.; Ogunkunle, A.T.J.|
|Publication:||Advances in Natural and Applied Sciences|
|Date:||Sep 1, 2007|
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