The grain yield in cereals can be defined in terms of the rate of dry matter accumulation in grain per unit time and the length of grain formation period. Hence,breeding of early maturing varieties is an important objective in durum wheat research in view of the increasing importance of intensive crop sequences. Short duration genotypes can be adjusted easily in double cropping and are also suitable for conditions of short winter. Existing wheat varieties are differentiated in length of period from flowering to maturity, the differences are considerable, and ranged from 30 to 70 days. In general, varieties having a long period of seed development, flower earlier and this conditioned trait are undoubtedly genetic and even have ecological significance. This trait plays a different role in various regions. Hence, when this trait will be taken into account, the length of period from flowering to maturity can be used as a marker for selection. Results of various experiments (Sterzycki 1978; Sinha et al. 1979) clearly indicated that among varieties of wheat considerable variation according to earliness and length of grain formation period is observed and the length of the period from flowering to maturity determines the quantum of yield. Nasyrov (1978) led emphasis on physiological components for increasing the yield potential of wheat. Further breakthrough in yield may be obtained by exploiting the genetic information for important physiological trait related to yield (Austin et al. 1980). Thus, there is a need for exploitation of such important character for further amelioration of wheat production under diverse environments. Early grain filling period is an index of the general efficiency of a genotype in terms of giving maximum returns per unit time. The number and the weight of the grains determine the final yield of grain per spike of wheat, it contains. The weight attained by any grain depends on the duration of the grain filling period; rate of supply of assimilates to grain and the rate of incorporation of these into its structure from anthesis onwards (Singh et al. 1977; Prabhu and Sharma 1984; Dasgupta and Mondal 1989; Sood and Dawa 1999). The yield of late sown durum wheat is reduced mainly due to reduced grain filling period and not due to reduced rate of supply of assimilates. Experimentally it is evident that seed number and seed weight are two most important yield components in durum wheat. Weight of any grain depends on duration of the grain filling period. Grain filling is a very critical physiological phase in source-sink relationship. It is the period when photosynthetic assimilates are supplied and incorporated into the seed. There are scanty references for genetic studies of grain filling period in durum wheat. Information on genetics of parents and mode of gene action governing duration of grain filling would be useful in devising suitable breeding procedures for developing early genotypes suitable for varied environmental conditions.

Very limited studies are reported on the inheritance of grain filling period particularly in durum wheat. The present research work was, therefore, undertaken through generation mean analysis in twelve generations of the three crosses of durum wheat to provide information on: (a) the relative significance of different types of gene action viz., main effofects, digenic and trigenic epistatic interactions and their components for grain filling period in normal and late sown environments. (b) Role of heterosis and inbreeding depression in the grain filling period in normal and late sown environments and the gene actions involved in manifestation of these phenomena. Based upon the results achieved, an attempt was made to suggest breeding methodology to be followed in durum wheats for further improvement of grain filling period for further amelioration of grain yield.

Materials and methods

Materials, field design and standard statistical procedures used in this study are the same as in the previous one (Sharma and Sain 2003). The number of days taken from the date of heading to the date of physiological maturity (turning yellow) of the selected plant was recorded as a grain filling period.