Introduction
Using heterosis (hybrid vigour) to raise yield and fibre quality of cotton has long been an objective for researchers and producers (Meredith and Brown, 1998). Heterosis in cotton was reported as early as in 1894 by Mell (cited indirectly from Randhawa and Singh, 1994), and the foundation of the modern concept of heterosis was laid in 1908 by Shull (Randhawa and Singh, 1994). Since then, interspecific, intraspecific as well as intervarietal heterosis in cotton has been reported by a number of researchers who depicted mid-parent heterosis (MPH) from 0.19% to 122.7% and better-parent heterosis from 0.02% to 93.83% in lint yield (Turner, 1953, Davis 1978, Patil and Chopde, 1985, Gupta and Singh 1987). Many further reports have described heterosis in fibre quality, vegetative and reproductive growth, and photosynthetic production in cotton (Bhatt and Rao, 1981; Whisler et al., 1986; Khan, 2002; Chen et al., 2005). Heterosis studies have driven the release of many advanced cotton hybrids and large-scale heterosis utilization, particularly in India and China (Basu and Paroda, 1995; Wu et al., 2004).

It is reported that the genetic basis for yield heterosis in cotton is mainly due to additive and dominance effects (White and Kohel, 1964; White, 1966; Marani, 1968), while the physiological basis of heterosis for lint yield in cotton is probably attributed to the enhanced photosynthetic capacity, increased dry weight accumulation and more partitioning of assimilates to reproductive sinks (Whisler et al., 1986; Wells et al., 1988; Li and Jiang, 1992). The photosynthetic rates of cotton leaves under a given environmental conditions is a function of the various biophysical and biochemical processes involved during the diffusion of CO₂ from atmosphere to chloroplast and the subsequent enzymatic reactions. The leaf transpiration and stomatal resistance are directly related to number of stomatal present per unit leaf area (Van de Roovart and Fuller, 1935). It has been suggested that a reduced stomatal frequency would be expected to reduce stomatal conductance (Penman and Schofield, 1951) which in turn reduce rate of water loss and increase the ratio of photosynthesis to transpiration in wheat (Jones, 1977). Similary Austin (1977) reported that low stomatal frequency increased stomatal resistance and decreased the transpiration in barley which inturn increased the yield due to increased water use efficiency under rainfed conditions.

Shimshi and Ephrat (1975) were of the opinion that wheat cultivars with a wide stomatal aperture produce higher yields without consuming more water. However, they also stated that permeability was significantly correlated with short term transpiration, short term photosynthesis and yield.

Wong et al. (1979) reported that stomatal conductance was correlated with photosynthetic rate and stomatal aperture is determined by the capacity of mesophyll to fix carbon. Further, Hutmacher and Kreig (1983) noticed that photosynthetic rate of leaves had a curvilinear relationship with leaf conductance.

Gopinath and Madalageri (1985) reported heterosis for stomatal frequency and leaf area over mid parent values in F₁ hybrids of egg plant. In a similar observation Hazra et al. (1989) noticed marked heterosis for length, breadth and number of stomata on upper and lower surface in F1s from ten genotypes of vigna unguiculata.

Dhopte et al. (1988) reported that boll number and transpiration rate had direct positive effect on yield, while stomatal conductance had a direct negative effect.

Heterosis is defined as the increased vigour of the F₁ generation over the mean of the parents or over the better parent (Hayes et al., 1955). Shull (1914) first coined the term heterosis. Heterosis has been observed for yield and other characters in cotton by many workers. Commercial exploitation of hybrid vigour in cotton has been successful in India with release of Hybrid 4 in 1969.

Heterosis produced by the joint effects of all the loci as the sum of their separate contributions can be represented by the formula (Falconer, 1981).

HF1 = dy²

Where,
d = Magnitude of dominance
y = Allelic frequency differences at a locus in the parental populations

The genetic causes involved in the expression of heterosis are dominance and non-allelic interactions (Hayes and Foster, 1976). The magnitude of heterosis can be maximized if the parents are genetically diverse from each other. Parents should differ for maximum number of yield influencing loci so that F₁ exhibits the dominance effect at as many of the yield influencing loci as possible.

Heterosis works as a basic tool for improvement of crops in form of F₁ and F₂ populations, and economic heterosis (over standard cultivar). It also contributes to choose genotypes with desired genetic variance, vigor and maternal effects. Therefore, it is essential to have detailed information about desirable parental combiners in any breeding program, which can reflect a high degree heterotic response. In intra- and inter-specific heterosis, yield increase over better parent or greater than best commercial cultivar (useful heterosis) has been documented (Baloch et al., 1993b; Galanopoulou- Sendouca and Roupakias, 1999; Wei et al., 2002; Yuan et al., 2001 & 2002; Khan et al., 2007; Khan, 2011). Both positive and negative heterotic values have been detected, demonstrating potential of hybrid combinations for traits improvement in breeding programs (Hassan et al., 1999; Khan et al., 2009). F₁ hybrids with high heterosis were also associated with higher inbreeding depression; therefore, moderate type of heterosis has some stability in segregating populations (Tang et al., 1993; Soomro, 2000; Soomro and Kalhoro, 2000). Therefore, heterotic studies can provide basis for exploitation of valuable hybrid combinations in future breeding program.

The main objective of this study to study the heterosis and mean per se performance of seed cotton yield and physiological parameters in inter specific crosses.

1 Results and Discussion
Heterosis is the superiority of F₁ over the mean of the parents or over the better parent or over the standard check (Hays et al., 1956), with respect to agriculturally useful traits. The primary objective of heterosis breeding is to achieve a quantum jump in yield and quality of crop plants.

Cotton improvement programmes primarily lay emphasis on development of hybrids, which have contributed in improving the productivity of cotton. Hybridization is the most potent technique for breaking yield barriers. Selection of parents on the basis of phenotypic performance alone is not a sound procedure, since phenotypically superior lines may yield poor combinations. It is therefore essential that parents should be chosen on the basis of their combining ability. Combining ability analysis is the most widely used biometrical tool for identifying prospective parents and for formulating breeding procedures most likely to succeed.

The two barbadense lines DB 533 and DB 534 were selected for creating recombinational variability for combining ability. Twenty eight F₄ lines derived from this cross were utilized to assess recombinational variability for combining ability by crossing them with 4 hirsutum testers. These results are presented below.

1.1 Analysis of variance (RBD)
The preliminary RBD analysis was carried out for four characters under study for all genotypes involved in the present investigation viz., 112 crosses (Line x Tester), 28 lines, 4 testers , two commercial checks (MRC 6918 Bt check and DCH 32 non Bt check) and eight bench mark crosses. Mean sum of squares for four characters are presented in Table 1. ‘F’ test indicated highly significant variation among the genotypes for all the characters.

Table 1 Analysis of variance for seed cotton yield and physiological parameters

 
1.2 Mean per se performance and estimation of heterosis
Mean per se performance of four hirsutum females and 28 barbadense males (Table 2) and derived F₁ crosses and commercial checks (Table 3). Further, results of heterosis values over mid parent and commercial checks for various characters were studied to assess the variability for combining ability were given in Table 4.

Table 2 Per se performance of 28 barbadense lines and 4 hirsutum testers for seed cotton yield and physiological parameters

Table 3 Per se performance of derived F1 crosses for seed cotton yield and physiological parameters

Table 4 Heterosis over mid parent and commercial checks for lint index, seed cotton yield, photosynthesis rate, stomatal conductance and transpiration rate in derived F1 crosses

 
1.3 Seed cotton yield (kg·ha^-1)
Seed cotton yield values ranged from 1368.15 [DB 533 x DB 534 F₅ IPS 49] to 441.81 [DB 533 x DB 534 F₅ IPS 33] among males/ lines, 2503.93 [DH 98-27] to 1870.91 [178-24] among females/testers and 2884.26 [DH 98-27 X (DB 533 x DB 534 F₄ IPS 49)] to 1146.20 [178-24 X (DB 533 x DB 534 F₄ IPS 33)] among derived F₁ crosses.

Per cent heterosis of F₁ crosses over their respective mid parental values ranged from 108.16 [DH 98-27 X (DB 533 x DB 534 F₄ IPS 49)] to -38.94 [178-24 X (DB 533 x DB 534 F₄ IPS 33)]. Thirty crosses showed significant positive heterosis and only one cross showed significant negative heterosis over their mid parent. Majority of workers viz., Tuteja et al. (1996), Doss and Kadambavanasundaram (1997), Siruguppa and Parameswarappa (1998), Neelima (2002) and Potdukhe (2002) also reported heterosis over mid parent. The cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 49) (39.97) recorded highest significant positive heterosis over commercial check MRC 6918 and the cross 178-24 X (DB 533 x DB 534 F₄ IPS 33) (-44.37) exhibited lowest significant negative heterosis over MRC 6918. Two crosses showed significant heterosis in positive direction and two crosses showed significant heterosis in negative direction over MRC 6918. In case of DCH 32 non Bt check, the cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 49) (44.31) showed highest significant positive heterosis over this check, but the cross 178-24 X (DB 533 x DB 534 F₄ IPS 33) (-42.65) recorded lowest significant negative heterosis value over DCH 32 commercial check. Two crosses exhibited significant positive heterosis over DCH 32 commercial check and only one cross showed significant negative heterosis. Majority of workers viz., Tuteja et al. (1996), Doss and Kadambavanasundaram (1997), Siruguppa and Parameswarappa (1998), Neelima (2002) and Potdukhe (2002) also reported heterosis over mid parent.

1.4 Photosynthetic rate (µmol CO₂ m^-2·s^-1)
The photosynthetic rates of cotton leaves under a given environmental conditions is a function of the various biophysical and biochemical processes involved during the diffusion of CO2 from atmosphere to chloroplast and the subsequent enzymatic reactions. The values of photosynthetic rate ranged from 27.15 [DB 533 x DB 534 F₅ IPS 13] to 16.49 [DB 533 x DB 534 F₅ IPS 8] among males/ lines, 29.62 [178-24] to 21.20 [DH18-31] among females/testers and 34.25 [ZCH 8 X (DB 533 x DB 534 F₄ IPS 52)] to 3.15 [ZCH 8 X (DB 533 x DB 534 F₄ IPS 33)] among derived F₁ crosses.

Per cent heterosis of F1s over their respective mid parent values ranged from 74.56 [ZCH 8 X (DB 533 x DB 534 F₄ IPS 52)] to -84.91 [ZCH 8 X (DB 533 x DB 534 F₄ IPS 33)]. Fourty four crosses showed significant positive heterosis and twenty crosses showed significant negative heterosis over their mid parent. The cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 52) (18.97) recorded highest significant positive heterosis over commercial check MRC 6918 and the cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 33) (-89.07) exhibited lowest significant negative heterosis over MRC 6918. Thirty nine crosses showed significant heterosis in negative direction and only one cross showed significant heterosis in positive direction over MRC 6918. In case of DCH 32 check, the cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 52) (41.83) showed highest significant positive heterosis over this check, but the cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 33) (-86.97) recorded lowest significant negative heterosis value over DCH 32 commercial check. Thirty crosses exhibited significant positive heterosis over DCH 32 commercial check and eighteen crosses showed significant negative heterosis.

1.5 Stomatal conductance (µmol·m^-2·s^-1)
The rate of stomatal conductance, or its inverse, stomatal resistance, is directly related to the boundary layer resistance of the leaf and the absolute concentration gradient of water vapor from the leaf to the atmosphere. Stomatal conductance values ranged from 2.38 [DB 533 x DB 534 F₅ IPS 55] to 0.55 [DB 533 x DB 534 F₅ IPS 15] among males/ lines, 1.22 [178-24] to 0.78 [DH18-31] among females/testers and 1.17 [ZCH 8 X (DB 533 x DB 534 F₄ IPS 52)] to 0.09 [178-24 X (DB 533 x DB 534 F₄ IPS 15)] among derived F₁ crosses.

Per cent heterosis of F₁ crosses over their respective mid parental values ranged from 75.94 [ZCH 8 X (DB 533 x DB 534 F₄ IPS 52)] to -90.67 [178-24 X (DB 533 x DB 534 F₄ IPS 15)]. Sixty three crosses showed significant negative heterosis and eleven crosses showed significant positive heterosis over their mid parent. The cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 52) (53.95) recorded highest significant positive heterosis over commercial check MRC 6918 and the cross 178-24 X (DB 533 x DB 534 F₄ IPS 15) (-88.16) exhibited lowest significant negative heterosis over MRC 6918. Thirty eight crosses showed significant heterosis in negative direction and nine crosses showed significant heterosis in positive direction over MRC 6918. In case of DCH 32 check, the cross ZCH 8 X (DB 533 x DB 534 F₄ IPS 52) (234.29) showed highest significant positive heterosis over this check, but the cross 178-24 X (DB 533 x DB 534 F₄ IPS 15) (-74.29) recorded lowest significant negative heterosis value over DCH 32 commercial check. Seventy four crosses exhibited significant positive heterosis over DCH 32 commercial check and six crosses showed significant negative heterosis. The majority of crosses showing significant negative heterosis over mid parent and MRC 6918 for stomatal conductance, while showed significant positive heterosis over DCH 32 check. Gopinath and Madalageri (1985) reported heterosis for stomatal conductance over mid-parent values. Majority of hybrids possessed higher stomatal conductance as compared to parents. This may be attributed to higher stomatal frequency, length and breadth of stomata observed in hybrids. Austin (1977) and Jing and Ma (1990) reported that reduced stomatal frequency would reduce stomatal conductance and increase the yield due to increased water use efficiency under water limiting conditions.

1.6 Transpiration rate (mmol H₂O m^-2·s^-1)
The leaf transpiration and stomatal resistance are directly related to number of stomatal present per unit leaf area (Vande Roovart and Fuller, 1935). Transpiration rate values ranged from 16.66 [DB 533 x DB 534 F₅ IPS 105] to 7.92 [DB 533 x DB 534 F₅ IPS 14] among males/ lines, 11.77 [178-24] to 10.09 [DH 98-27] among females/testers and 10.04 [DH 98-27 X (DB 533 x DB 534 F₄ IPS 1)] to 2.07 [178-24 X (DB 533 x DB 534 F₄ IPS 32)] among derived F₁ crosses.

Per cent heterosis of F1s over their respective mid parent values ranged from -5.85 [178-24 X (DB 533 x DB 534 F₄ IPS 38)] to -79.10 [178-24 X (DB 533 x DB 534 F₄ IPS 32)]. Ninety seven crosses showed significant negative heterosis over their mid parent. The cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 1) (55.26) recorded highest significant positive heterosis over commercial check MRC 6918 and the cross 178-24 X (DB 533 x DB 534 F₄ IPS 32) (-67.93) exhibited lowest significant negative heterosis over MRC 6918. Thirteen crosses showed significant heterosis in negative direction and only one cross showed significant heterosis in positive direction over MRC 6918 Bt check. In case of DCH 32 check, the cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 1) (93.92) showed highest significant positive heterosis over this check, but the cross 178-24 X (DB 533 x DB 534 F₄ IPS 32) (-59.94) recorded lowest significant negative heterosis value over DCH 32 commercial check. Two crosses exhibited significant negative heterosis over DCH 32 commercial check and seventeen crosses showed significant positive heterosis.

2 Conclusion
In this part of the study nature and magnitude of variability for combining ability was assessed against four hirsutum tester included in the heterotic box.

The derived F₁ crosses (28 barbadense lines x 4 hirsutum testers) were compared with the bench mark crosses (two barbadense lines x 4 hirsutum testers) of the heterotic box, best Bt check hybrid (MRC 6918) and non Bt check (DCH 32). Many derived F₁ crosses were found to be more productive than non Bt check DCH 32 (48 hybrids) and the Bt check MRC 6918 (35 hybrids). The potential crosses like DH 98-27 X (DB 533 x DB 534 F₄ IPS 49), DH 98-27 X (DB 534 x DB 533 F₄ IPS 22) and DH 98-27 X (DB 533 x DB 534 F₄ IPS 52) recorded highest per se performance for seed cotton yield. These potential crosses recorded highly significant heterosis over mid parent for seed cotton yield. They also recorded significant heterosis for other physiological parameters.

Apart from showing high productivity the potential cross DH 98-27 X (DB 533 x DB 534 F₄ IPS 49 showed higher value of photosynthetic rate and stomatal conductance. This potential cross is example for blending of yield characters and physiological parameters.

3 Materials and Method
To create recombinational variability for combining ability, the elite barbadense lines DB 533 and DB 534 were crossed during 2007~2008. During two seasons 2008~2009 and 2009~2010 these barbadense crosses were advanced to F₂ and F₃ generations, respectively. The F₃ lines were evaluated for productivity and fiber quality parameters realizing the emphasis laid on developing ELS (Extra Long Stable) cotton hybrids out of 171 F3 lines, only those F₃ lines with acceptable fiber strength were utilized in the study on recombinational variability of combining ability. During 2010~2011 those twenty eight F₄ lines of barbadense cross DB 533 × DB 534 depending on the higher value of fiber tenacity, were crossed with the selected four hirsutum testers viz., DH 98-27 (T₁), ZCH 8 (T₂), 178-24 (T₃) and DH 18-31 (T₄) selected based on earlier study. Each barbadense F₄ line was involved in a set of crosses (112 crosses refer to as derived F₁ crosses) were subjected to Line x Tester analysis.

The physiological observations have taken by using portable photosynthesis system of Infra Red Gas Analyzer (IRGA), which measures gas exchange parameters apart from environmental parameters There are several methods of measuring CO₂ fixation or exchange in plants but, the modern techniques of determining CO₂ fixation using infra red gas analysis (IRGA) of CO₂ is most widely employed owing to the precision of detecting very small changes in CO₂ concentrations. This method is very sensitive for CO₂ uptake by small leaves or even segments of leaves. The IRGA records the change in CO₂ concentration in the system and the rate of change with time gives an estimate of the CO₂ or water exchange rate. The main advantage of this method is that it can be used at a wide range of CO₂ concentrations, light, relative humidity and temperature and for studying the effects of these environmental factors that parameters influencing photosynthesis or gas exchange parameters.

3.1 Photosynthetic rate (µmol CO₂ m^-2·s^-1)
Photosynthesis, the conversion of light energy to chemical energy and the utilization of the chemical energy. The rate of photosynthesis is affected by a number of factors including light levels, temperature, availability of water, and availability of nutrients. If the conditions that the plant needs are improved the rate of photosynthesis increases.

3.2 Stomatal conductance (µmol·m^-2·s^-1)
Stomatal conductance, measured in µmol m^-2 s^-1 is the measure of the rate of passage of carbon dioxide CO₂ entering, or water vapor exiting through the stomata of a leaf. Stomata are small pores on the top and bottom of a leaf that are responsible for taking in and expelling CO₂ and moisture from and to the outside air. The rate of stomatal conductance, or its inverse, stomatal resistance, is directly related to the boundary layer resistance of the leaf and the absolute concentration gradient of water vapor from the leaf to the atmosphere.

3.3 Transpiration rate (mmol H₂O m^-2·s^-1)
Loss of water in the form of water vapour from the internal living tissue of the leaf through the aerial parts such as leaf, green stem, etc., under the influence of sunlight is called as transpiration. The excess amount is transpired through the aerial parts of the plants. Thus, only 5 per cent of the absorbed water is retained in the plants and remaining 95 per cent is lost through aerial parts the leaves are most important for transpiration.

Heterosis of F₁ over mid parent (MP) and commercial check (MRC 6918 and DCH 32) were calculated by methods of Turner (1953) and Hayes et al. (1955) as given below.

Per cent heterosis in F₁ over mid parent (MP) = (F₁-P)/MP × 100
Where, Mid parent (MP) = (P₁+P₂)/2

Per cent heterosis in F₁ over commercial check (CC)= (F₁-CC)/CC × 100
Where, MP = Mid parent, CC = Commercial check

Mean sum of squares due to error from RBD analysis was considered to compute standard error (S.E.) of estimated heterosis as follows.

S.E. for heterosis over mid parent

S.E. (Hmp) = [(3/2 × EMS)/r]^0.5

S.E. for heterosis over commercial check

S.E. (Hcc) = (2 × EMS / r)^0.5

Where, EMS = Error mean sum of squares

The critical difference values in each case were worked out by multiplying their corresponding S.E. values with table ‘t’ value at error degree of freedom at 5 and 1 per cent levels of significance.

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