| Plant height is a quantitative character affected considerably by aneuploid
conditions which have made it cumbersome to determine the number of dwarfing
genes or the type of gene action. There are three major recessive semidwarfing
genes Rht 1 and Rht 2 (ALLAN et al. 1968, FICK & QUALSET
1973). These genes were identified to be located on chromosome 4A and 4D
(GALE et al. 1975). On the other hand a variety 'Tom Thumb' has a
semidwarf gene Rht 3 located on 4A (MORRIS et al. 1975). A
dominant gene of dwarfism designated as 'Al-bian 1' is reported to
be located on chromosome 4D (IZUMI et al. 1981). In the course of
present investigation, a number of dwarf plants with height range of 74
to 107 cms. were obtained in NP 880. Cytological observations of these dwarf
mutant lines in M4 did not indicate any visible chromosomal abnormality.
It is possible that the major dwarfing gene (GALE et al. 1975) located
on chromosome 4A and 4D were altered due to chemical treatments, along with
some of the other minor genes or modifiers, thereby giving expression to
variable type of dwarfing mutants. It is interesting that wherever, the
height was reduced upto 100 cms, the 7th internode (basal) was conspicious
by its absence. In this category, evident reduction in all the internodes
was recorded, but the first internode showed expression to its full extent.
When the height was further reduced, the reduction was noticeable in all
the internodes. It is possible that separate genes or minor genes governing
the length of each internode are affected. This assumption finds further
support from our experimental data obtained in tetraploid wheat HD 4502
with the same set of treatments, where tall mutants showed increase in length,
primarily confined to first internode (unpublished). NP 880, the tall wheat variety has bold grains with lusture, so the main objective in the mutagenic studies was to induce dwarfing which may be lodging resistant with same lusture and chapati making, quality. Although the lodging resistance and culm length reduction are not always associated but the selected mutant lines have shown that reduction in height which was primarily linked with change in first and second internode length showed lodging resistance in different yield trials under field conditions. Short coleoptile in dwarf and semidwarf varieties are primarily associated with poor seedling emergence under dryland farming, hence high yielding dwarf wheat varieties have not been properly exploited in the rainfed areas (SHARMA et al. 1982). Present studies pertaining to different dwarf mutants have indicated not only the wide range of variability for culm length, but also considerable range of variability in coleoptile length (1.5 - 6.8 cms), thereby demonstrating the scope for further selection to combine short culm with somewhat long coleoptile. |
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