Development of S 1 inbred lines and estimation of inbreeding depression in two maize ( Zea mays L . ) populations

To know the effect of inbreeding depression, this study was conducted in the Agricultural Experimentation Block of the University of Swabi, during the two cropping seasons of 20132014. The S1 inbred lines were developed from two of the maize populations viz. popcorn (‘PC’) and sweet corn (‘SC’). Plant height (PH), ear height (EH), ear length (EL), leaf area (LA), number of leaves (NL) and number of tassel branches (TB) were investigated. Highly significant differences (P≥0.01) were observed for PH and LA in S1 lines however; this effect was nonsignificant within the parental population. While the rest of the traits did not show any significant variation in the first generation of inbreeding (S1). Of the developed 20 S1 inbred lines, mean data revealed that maximum reduction in PH (169.55 cm) was in the set of inbred lines obtained from ‘PC’ as compared to that of ‘SC’ (159.65 cm). Similarly, reduction of 44.92 cm was noticed in LA of ‘PC-S1-4’ as compared to the lines developed from ‘SC’. The results indicated that except PH and LA, other traits like EH, EL, NL and TB are closely linked and hence in first generation of selfing there was not a significant variation in the S1 developed from ‘PC’ and ‘SC’. However, the potential inbred lines need to be re-evaluated for diversity, GCA and SCA when homozygosity is complete in later generations.


Introduction
Botanically, cultivated maize is known as (Zea mays L.) and it belongs to the grass family (Poaceae).It is medium to tall annual plant with an extensive fibrous root system [1].Maize is one of the widely cultivated crops of the world that is being harvested around the globe each month of the year [2].It is the crop of interest for breeders, consumers and farmers with diverse usage in our daily life [3].Besides the cultivated maize, different sub-species of maize have also been reported [4].It is the world's leading cereal crop with global production of 695 million tones and per unit area yield of 4815 kg ha -1 [5].In Pakistan during the year 2011-12, maize was planted on 1083 thousand hectares with total production of 4271 thousand tons, having an average yield of 3893 kg ha -1 [6].Inbred lines are the key input required for the development of high quality hybrids.Most of the developed countries are using the three lines technology exploiting male sterility for the development of hybrids.In Pakistan and other developing countries, the inbreds are randomly developed for this purpose.Though the maize can be grown on all types of soils ranging from sandy to clay; however, medium textured soil of pH 6.5 to 7.5 is more suitable for it [7].Similarly, due to its protandrous nature, pollen shedding normally begins 1-3 days before the silks have emerged in the same plant and usually continues for a period of several days after the silks are ready to be pollinated, thus making it hard for the breeder to develop the inbred lines.An open-pollinated maize variety consists of a range of heterogeneous genotypes from which homozygous and homogeneous (inbred lines) could be developed [8,9].The use of single and double crossed hybrids obtained from inbred lines has transformed the corn industry [10].This study was undertaken to know the effect of inbreeding on the development of S1 inbred lines in two of land races or indigenous populations of popcorn and sweet corn on major agronomic and morphophysiological traits.

Materials and Methods
The experiment was conducted at the Agricultural Experimentation Block of The University of Swabi during the year 2013-2014.Two of the experimental population i.e. sweet corn (SC) and popcorn (PC) seeds were obtained from the Cereals Crops Research Institute (CCRI) Pirsabak Nowshera.The seeds were sown in twenty rows with plant to plant and row to row distance of 25 and 75 cm respectively.Two seeds were used per hill at one-inch depth in soil followed by recommended agronomic practices.Table 1 shows the list of S1 inbred lines that were developed from 'PC' and 'SC' population respectively as per the procedure previously reported [11].Data Recording Data were recorded on the following agronomic and yield parameters by randomly selecting five plants per line.

Plant height (PH) cm
Data on plant height was recorded for five randomly selected plants using measuring scale rod.The height was measured from soil level to the upper most node in centimeter (cm).

Ear height (EH) cm
Data on ear height was recorded for five randomly selected plants using measuring rod.The ear height was considered from ground level to the node bearing the cob/ear and recorded in cm.

Ear length (EL) cm
Ear length was recorded using a scale meter from the tip to the lowermost grain/base.Leaf area (LA) cm 2 It was recorded on randomly selected five plants through a scale meter.Measurement was taken on tip, middle, base of leaf no. 4 and 5.The mean of recorded values was then used for calculating the leaf area.

Number of leaves (NL)
Total number of photo-synthetically active leaves was counted and data recorded on five randomly selected plants.

Number of tassel branches (TB)
Number of tassel branches was counted on five randomly selected plants for calculating the mean.

Procedure for Inbred Lines Development
The female part (ear/cob) was covered using silk bag prior to silk emergence.Care was taken to avoid any cross-pollination or contamination of pollen during pollination.The tassel was covered in morning and evening.Pollination was started at 11:00 am and continued till 12:30 pm.The silk bag was replaced with tassel bag containing pollen in a way that no foreign pollens could enter the shoot bag or touch the silk of the cob.

Statistical Analysis
Obtained data were subjected to statistical analysis (ANOVA) in RCBD.Statistix TM (version 8.1) statistical software was used for analysis at the appropriate level of significance.

Analysis of variance of the investigated traits of the parental populations
Data recorded for different parameters on the parental populations were statistically analyzed and have been presented in Table 2.It can be seen in Table 2 that all of the morphological traits of the selected parental populations were non-significant (P≥0.01).

Mean values for morpho-physiological traits within the parental populations
Means of some of the morpho-physiological traits within the two-parental populations have been given in Table 3. Maximum height of 165.14 cm was attained by the parental population 'PC' as compared to (160.63 cm) of 'SC'.Maximum value for cob height (57.09 cm), cob length (16.60 cm) and leaf area (296.35cm 3 ) was recorded for the parental population 'SC'.Variation had been previously observed in traits such as plant height, cob height, leaves and leaf areas in a study conducted on twenty different inbred lines [12].The parents or base populations selected here had narrow range of variation which was non-significant and hence confirms the findings of other researchers [13].

ANOVA results, mean squares for morpho-physiological traits within the S1 inbred lines derived from the parental populations ('PC' and 'SC')
ANOVA results of the S1 inbred lines derived from 'PC' and 'SC' have been presented in Table 4 for most of the important traits.Since, these inbred lines were derived from the parental populations with narrow genetic base, therefore, most of traits were found to be non-significant except PH and LA which were highly significant.

Cob Height (CH) cm
Mean data for CH and the deviation from the parental lines ('PC' and 'SC') have been given in Table 5. Maximum and minimum values for CH among the S1 lines derived from 'PC' were recorded for 'PC-S1-9' (58 cm) and 'PC-S1-4' (54.2 cm).Similarly, the maximum and minimum cob height of 58.7 and 54 cm were recorded respectively for S1 lines 'SC-S1-1' and 'SC-S1-3' derived from 'SC'.Maximum and minimum reduction in CH among the inbred lines derived from PC were recorded for 'PC-S1-8' (+2.05 cm) and 'PC-S1-4' (-0.45 cm).Similarly, among the inbred lines derived from 'SC', the highest and least reduction was recorded for 'SC-S1-3' (+3.09 cm) and (-1.11 cm) was noted for 'SC-S1-9'.The effect of inbreeding depression was slightly higher in 'SC' rather than 'PC'.Heterosis effects in maize F1 hybrids for days to maturity, plant height, ear height, kernels ear -1 , 1000-grain weight and grain yield kg ha -1 were found to be depressed severely during inbred lines development process.However, theses parameters boosted effectively in recombination process [17].Average values of inbreeding depression for plant height and ear-height were recorded as 21.50 cm and 4.87 cm, respectively [16].
Inbred line with maximum NL would be of benefit for the best combination in hybrid development [18].Loss of 20-30% has been reported for NL in local and exotic maize inbred lines developed and characterized in Peshawar valley [11].

Cob length (CL) cm
Mean data for CL and deviation from the parents are presented in Table 3.5.Upper (16.65 cm) and lower (12.9 cm) limit of CL was observed respectively in inbred lines 'PC-S1-6' and 'PC-S1-4' both developed from 'PC'.Similarly in S1 inbred lines obtained from SC, the highest and lowest CL was recorded for 'SC-S1-3' (16.65 cm) and 'SC-S1-9' (12.9 cm).Regarding inbreeding depression the maximum loss was recorded for 'PC-S1-4' (+3.59 cm) and the minimum for 'PC-S1-2 (-0.01 cm)'.For the 'SC', highest reduction was recorded for 'SC-S1-9' (3.7 cm) while the least CL difference of -0.05 cm was obtained for SC-S1-3.As stated above this trait was also highly influenced by the inbreeding.Previously, positive correlation with the cycle of generation of inbreeding; and, decrease to 12 cm in CL has been reported in maize [19,20].Tassel branches (TB) No. Maximum TB, as is evident from Table 6, were observed among S1 inbred line obtained from 'PC' which was 15.3 for 'PC-S1-5', while fewer tassel branches (12.2) were recorded for 'PC-S1-8'.Among the 'SC' maximum number of TB were counted for 'SC-S1-8'(15.8)and the fewest number of TB were recorded for 'SC-S1-1'as 12.6.Surprisingly the reduction or depression was high in 'PC' than that of 'SC' for this important trait.The environment also played an important role in the expression of traits like tassel branches [13].Loss of vigour in inbred line development has been found to is associated with the pedigree of the plants [21].

Table 3 . Mean values for PH, CH, NL, CL, TB and LA of the two populations ('PC' and 'SC')
Pop corn,SC: Sweet corn, PH: Plant Height, CH: Cob Height, NL: Number of leaves, CL: Cob length, TB: Number of tassel Branches, LA: Leaf area

Table 4 . Analysis of variance results of the S1 inbred lines in-terms of investigated traits in the year 2013-14
No. of Leaves, CL: Cob Length, TB: Tassel Branches, LA: Leaf Area NS: Non-significant, Df: Degree of freedom, PH: Plant Height, CH: Cob Height, NL:

Table 5 . Mean values and deviation of S1 inbred lines
('PC' and 'SC') for the PH, CH and NL.