Selection Indices for Yield Improvement in Bread Wheat (Triticum aestivum L.)

Fifty diverse genotypes of bread wheat were evaluated in a randomized block design with three replication for the study of selection indices during Rabi, 2013. Thirtyone selection indices involving grain yield per plant (X1) and four yield components viz., biological yield per plant (X2), number of grains per main spike (X3), 1000-grain weight (X4) and grain weight per main spike (X5) were constructed using the discriminant function analysis. Discriminant function analysis indicated that selection efficiency of the function was improved by increasing the number of characters in the index. Among the single character indices, number of grains per main spike exhibited higher genetic advance and relative efficiency over straight selection for grain yield per plant. The index based on five characters viz., grain yield per plant, biological yield per plant, number of grains per main spike, 1000-grain weight and grain weight per main spike recorded the highest genetic advance as well as relative efficiency and selection efficiency. These characters could be advantageously exploited in the bread wheat breeding programmes.

Wheat (Triticum aestivum L.) is the staple food for a large part of the world population including India.At present world population is about seven billion and is growing at the rate of one billion every fourteen years.India is second most populous country with more than one billion people.Food grains requirement of India is estimated to be 250 million tones by 2025.Understanding of interrelationship between component characters helps in determining which character to select when improvement of the related complex character is desired.Yield in crop is a quantitative traits and has a complex genetic control mechanism and hence, direct selection it not much effective on it.The plant breeder has certain desired plant characteristics in his mind while selecting for particular genotypes and for this he applies various weight to different traits for arriving on decisions.This suggests the use of selection index which gives proper weight to each of the two or more character to be considered.Hazel and Lush (1943) showed that the selection based on such an index is more efficient than selecting individually for the various characters.An application of discriminant function developed by Smith (1936) helps to identify important combination of yield components useful for selection by formulating suitable selection indices.Therefore, the objective of the present study was to construct and assess the efficiency of selection indices in wheat

MATERIALS AND METHODS
Fifty genotypes of bread wheat were sown in a Randomized Block Design (RBD) with three replications during rabi 2013.Each genotype was accommodated in a single row of 2.5 m length with a spacing of 22.5 cm between rows.The experiment was surrounded by two guard rows to avoid damage and border effects.The fertilizers in the experimental area was applied at the rate of 120 kg N 2 ha -1 , 60 kg P 2 O 5 ha -1 and 40 kg K 2 O ha -1 .Other recommended agronomical practices in vogue were followed for reaping good crop.Data were recorded on randomly selected five plants from each genotype and average value was used for the statistical analysis for 14 characters viz., days to 50% flowering, grain filling period, days to maturity, plant height, number of effective tillers per plant, length of main spike, peduncle length of main spike, number of spikelet per main spike, number of grains per main spike, grain weight per main spike, 1000 grain weight, grain yield per plant, biological yield per plant, harvest index.Discriminant function analysis described by Dabholkar (1992) was used to construct the selection indices involving six characters, seed yield per plant (X 1 ), number of primary branches per plant (X 2 ), 100-seed weight (X 3 ), biological yield per plant (X 4 ), harvest index (X 5 ) and days to maturity (X 6 ).For computing selection indices, seed yield per plant was considered as the dependent variable with the relative efficiency of 100 per cent.The model suggested by Robinson et al. (1951) was used for the construction of genetic advance as well as selection indices and development of a required discriminant function using six characters along with seed yield per plant.

RESULTS AND DISCUSSION
Selection indices for grain yield and other characters were constructed and examined to identify their relative efficiency in the selection of superior genotypes.The results on selection indices, discriminant function, expected genetic gain and relative efficiency are presented in Table 1.The basis for the development of the selection indices has been provided by Smith (1936), Hazel (1943) and Robinson et al. (1951).Hazel and Lush (1943) 1) based on five characters constructed in all possible combinations revealed that the selection efficiency was high over straight selection when selection was based on individual components.Number of grain per mains spike showed a genetic advance of 17.60 %, which was higher than those calculated for other characters including grain yield per plant suggested that number of grains per main spike proved to be better selection index based on one character.
The highest genetic gain (Table 1) of 21.91% was obtained when selection was simultaneously based on discriminant function of two characters, e.g.biological yield per plant (X 2 ) and number of grains per main spike (X 3 ).When three characters, e.g. grain yield per plant (X 1 ), biological yield per plant (X 2 ) and number of grains per main spike (X 3 ) were taken together, the genetic advance increased to 23.98%.Index based on combination of four characters, i.e. grain yield per plant (X 1 ), biological yield per plant (X 2 ), number of grains per main spike (X 3 ) and 1000-grain weight (X 4 ) recorded high genetic gain of 26.44%.The maximum gain of 27.27% was achieved by taking five characters at a time, i.e. grain yield per plant (X 1 ) and four yield components viz., biological yield per plant (X 2 ), number of grains per main spike (X 3 ), 1000-grain weight (X 4 ) and grain weight per main spike (X 5 ).Thus, there was an increase in the genetic gain as well as relative efficiency with an increase in the character combinations.
Thus, the current study revealed that the index which includes more than one character, gave high genetic advance, suggesting the utility of constructing of selection indices for effecting simultaneous improvement in several characters.Hazel and Lush (1943) stated that the superiority of selection based on index increases with an increase in the number of characters under selection.Smith (1936), Rao (1974), Ferdous et al. (2010) and Kemelew (2011) were also with the same opinion that an increase in characters resulted in an increase in genetic gain and that the selection indices improve the efficiency than the straight selection for grain yield alone.
Therefore, due weightage should be given to days to maturity while formulating selection index of wheat crop.Overall, selection index consisting of four traits viz., grain yield per plant, biological yield per plant, number of grains per main spike,  that when relative efficiency of single character index was measured over straight selection for grain yield per plant, the efficiency was declined to less than 100 per cent.This observation indicated that the indirect selection through individual traits over straight selection for pod yield per plant alone would not be effective.
It is interesting to note that selection efficiency (Table 2) improved with an increase in number of characters in combination with yield.For example, average selection efficiency of 266.3%, when one character was included in selection function.Similarly, the selection efficiency was 454.32% for two characters, 602.57% for three characters, 729.71% for four characters and 846.31% for five characters selection indices improve the selection efficiency than the straight selection for yield alone with an increase in the number of characters under selection.Some of the selection indices with high relative efficiency listed in Table 1 indicated that the highest efficiency was observed with a combination of five characters (846.39%).Selection indices with five characters, i.e. grain yield per plant (X 1 ) and four yield components viz., biological yield per plant (X 2 ), number of grains per main spike (X 3 ), 1000-grain weight (X 4 ) and grain weight per main spike (X 5 ), therefore, appear to be more useful.It can be seen that grain yield per plant (X 1 ), biological yield per plant (X 2 ) and number of grains per main spike were the characters being commonly involved in more number of the combinations, 1000grain weight (X 4 ) and grain weight per main spike (X 5 )the next being and in order (Table 3).
Keeping in view, the basic idea of saving time and labour in a selection programme, it would be desirable to base the selection of few characters.In the present study, selection index based on five characters gave maximum genetic gain and high efficiency over straight selection, but practically it is more cumbersome to use in the selection exercise.However, in practice, the plant breeder might be interested in maximum gain with minimum number of characters.In the present study, selection index based on three characters (Grain yield per plant + Biological yield per plant + Number of grains per main spike) showing genetic gain (23.97%) and selection efficiency (743.97%)comparable to some extent of those based on four or more characters, which is more desirable and practically possible to use breeder than the index that includes more number of characters.Therefore, from this investigation, it is concluded that improvement of grain yield in bread wheat could be achieved by selecting the parents with these three characters; Grain yield per plant + Biological yield per plant + Number of grains per main spike.
stated that the superiority of selection based on index increases with an increase in the number of characters under selection and Mc Vetty and Evans (1980) and Esheghi et al. (2011) also suggested that the selection index to be superior to direct selection in wheat.A total of thirty one selection indices (Table

Table 1 .
Selection index, discriminant function, expected genetic advance in yield and relative efficiency from the use of different selection indices in bread wheat

Table 3 .
Highest selection efficiency with characters combination in bread wheat