Root Hair Morphology, Pattern and Viability of Lycopersicon Esculentum Mill. and Root Epidermal Patterning of Echium Plantagineum L. and Triticum Aestivum L.
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. In the majority of angiosperm species, all epidermal cells have the potential to produce a root hair. These species are currently placed in a single group (Type I), which is believed to have no specific pattern of root hair distribution, separate from species which have alternating long and short cells with root hairs generated from the short cells (Type II), and from those with hairs in files of short cells, separated by one or more files of hairless longer cells (Type III). This thesis questions the validity of these grouping and urges reconsideration. Work on two Type I species, tomato (Lycopersicon esculentun Mill. cv Beefsteak Improved) and Echium plantagineum L. clearly shows that pattern in root hair distribution is present. It takes the form of earliness of root hair initiation associated with longer epidermal cells and with specific files in both species, although it is much more marked in E. plantagineum. It has also been shown that cells that initiate hairs earlier subsequently produce longer hairs in E. plantagineum. Thus, E. plantagineum shares marked differences in hair performance by file with Type III species but differs from them in that, in Type III, hairs are associated with short-celled files and the long-celled files are hairless, whereas in E. plantagineum the long-celled files produce hairs earlier and result in long-hair files alternating with one or two files characterized by late initiation and short hairs. On these grounds, there is strong justification for placing E. plantagineum in a different, new Type. E. plantagineum is in the Boraginaceae s.l. which has representatives in both the current Type I and Type III groups. To this extent it could be seen as a transitional grouping but E. plantagineum does not fit comfortably in either the current Type I or Type III group. Further problems have been identified with a Type II species, wheat (Triticum aestivum L. cv Kotuku), which has clusters of hair cells in a file (a Type I characteristic) in some locations, but in others it has long (hairless) and short (with hairs) cell alternation (Type II characteristics). These findings indicate that the boundaries between these three groups are blurred. Moreover, this has implications for the relationships between Types I, II, and III, and highlights the complexity of epidermal cell patterning, the need to reconsider the way in which classification of root epidermal pattern is used, and the need for greater understanding of the means of generating these patterns. Epidermal cells of tomato roots have a wide range of cell and root hair lengths. The greater length and higher density of root hairs in the first-formed 3 mm of the root (Initial Region) are associated with shorter epidermal cells and slower elongation of the root axis. Two distinct populations of root hairs occur in the Initial Region, one with hairs of mean length 2000?m (long hairs) and the other 25O?m (short hairs). The long root hairs elongate more rapidly and for longer than the short hairs. Long hairs were present in the Initial Region in seven other tomato cultivars and in other taxa of the Solanaceae. The shorter epidermal cells and longer hairs present in the Initial Region of tomato may be influenced by the different environment experienced by these cells during development. The prolific root hair mass in the Initial Region may be advantageous for establishment. Root hairs in the later-formed part of the root, more than l0 mm from the junction with the hypocotyl (General Region), are similar to those in the Initial Region, except that the long hairs are fewer, shorter (800?m), and cease elongation earlier. The bimodality of root hair length, significant differences in epidermal cell length between files, and the positive correlation between earliness of hair initiation and cell length indicate differential potential in epidermal cell vigour of expansion and root hair elongation in Type I species. Root hair viability of tomato seedlings is affected by internal factors such as hair age, hair position on the root axis, and by external factors, such as temperature. In general, hair viability is short - with a lifespan of a few days. However, hair retention can be much longer. Examination of populations of hairs along the seedling root axis over time, showed that hairs in the Initial Region retain their viability longest, and that longer hairs tend to remain viable for longer than their shorter neighbours. Temperatures of 30°C and above greatly shortened the lifespan of all hairs, although 30°C promoted elongation of the root axis. Root hair production of tomato is very responsive to the environment. A diurnal thermo/photoperiod can switch root hair production on and off giving rise to bands of cells producing hairs alternating with bands of hairless cells on the root axis of tomato seedlings grown in nutrient culture. However, a diurnal thermoperiod did not cause banding when tomato seedlings were grown in Petri dishes indicating that some external factors can take precedence over others. This thesis shows that great variation in root hair distribution pattern is present between species and even within tomato, there is enormous variation in epidermal cell length, root hair length distribution, density and viability. Variation in hair distribution and length in tomato is present throughout the seedling root under very uniform conditions, and is also greatly modified by the environment These findings strongly indicate that the way in which current grouping of root epidermal patterning is made is inadequate. A wider range of criteria should be considered. Furthermore, a broad survey should be carried out to determine the extent of variation in root hair distribution pattern in the current three Types with the view of identifying new Types and clarifying the relationships between them.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/2002