Effects of gamma rays on some wheat characters

A.S. LARIK

Department of Botany and Plant Breeding, Sind University Agriculture College Campus, Tandojam, Pakistan.

The present investigations are aimed at providing reliable and scientific information on the effect of ionizing radiations during second mutation generation (M₂) on four hexaploid wheat varieties namely Wisconsin supremo, No. 43, Kenya plume and Kenya hunter with respect to leaf length/Breadth ratio and chaff or glume colour.

Leaf Length/Breadth ratio

The character leaf length/breadth ratio, which affects the Carbon-Nitrogen ratio and ultimately the productivity, in present studies displayed an erroneous trend suggesting that it is not very much affected by radiation in any regular fashion (Table 1). Changes have been noticed which, though positive, were irregular. I feel inclined to accept the views of other workers like GREGORY (1956), OKA et al. (1958) and SCRASCIA (1965) that in characters, polygenic in nature a near equal number of beneficial and deterimental mutation is caused.

The observation is reflective of the compensating tendency by these biotypes to moderate the damage incurred to them through gamma-rays enabling them to evade the threatened extinction. Leaf lengths were elongated or shortened while leaf breadths were narrowed or broadened reciprocally. It may be assumed that histological observation might have revealed almost an equal number of stomata etc., in the irradiated and control population, the factors essential for the execution of vital plant metabolic activities.

In case of cereal crops where tillering is one of the yield components and it covers nearly the whole of the field, for the small distance kept between rows and plants, the lesser the vegetative (leafy) growth the better, it is, for the photosynthetic activity. It has been seen that fields with very dense populations, though are promising for yields, lodge very badly due to the weakness of stem. This weakness is ascribable to the etiolation effect resulting into and enlarged development of the internodes. The etiolation in its turn is the consequence of the lack of sunlight penetration, so very neccessary for the photosynthetic activity by the mesophyll cells of the leaf which are present throughout the length of culm. Reduction therefore, in the area of the leaf or rather reduction in vegetative growth would, though reduce the area of photosynthetic activity per leaf, ultimately increases the number of leaves involved in the manufacture of carbohydrates, enlarging the photosynthetic activity of the plant which would restore the vigour of the culms and would reduce the chances of lodging.

Chaff or glume colour

Genetic studies have indicated the dominance of red or brown glume colour over white. (Both mono-genic and digenic factors have been proposed by genetic workers).

Recessive mutations are generally known to servive and hence easily detectable as against the dominant ones which are deleterious. The fact that white chaff of variety Kenya Hunter was transformed into red (a dominant character mutant) conveys that the chaff character is controlled through the inter-action of genes. The variety Kenya hunter during the present studies displayed a mutation from white chaff to red chaff appearance in M₂. This is change from recessive character to a dominant one and can only happen when the original genotype of the parent is dominant associated with an inhabitory factor preventing the expression of the dominant trait. In the present case, it seems that the variety Kenya hunter carries the genes for redness 'RR' but fails to express it because of inhibitory gene 'II' and hence its expression is white chaff 'IIRR.' The radiated population with white chaff character seems to have undergone deletion, loosing the inhibitory factor or the inhibitory gene might have got dissolved with the result that the mutated progeney is left with only the genes for redness 'R' that found their expression in the absence of the epistatic gene 'II'. Glume colour alterations have been reported by ARNASON et al. (1952) and SWAMINATHAN et al. (1958).

Abnormalities in the development of glume, as observed in the present study, have often been reported by other workers like MACKEY (1954), KIHARA and TSUNEWAKI (1963). This change would be expected in lieu of the pleiotropic gene 'Q' which probably gets disturbed in speltoid compactoid expressions. Other segmental changes can not be ruled out.

It seems that the microspores of the same spike or floret arise from diverse cells during micro-sporogenesis. This assumption is derived from the fact the PMC of the same anther bore hetermorphic aberrant chromosomes. GAUL (1964) also reported this experience.

(Received Jan. 4, 1977)