Stability in cotton production is necessary for the stability of Pakistan's cotton based economy. Induction of heat resistance in Pak-Upland cottons can help in achieving stable cotton production, since heat stress has been recognised as one of the most important yield limiting factors in cotton.
The objectives of the research project were to:
1. understand morphological, physiological, and genetic bases of variation for heat resistance in Pak-Upland cotton;
2. investigate variation in phenotypic expression of different morphological and physiological plant traits in Pak-upland cottons under heat stressed and non-stressed regimes;
3. understand pattern of inheritance and type of genetic variability associated with various morphological and physiological parameters under heat-stressed and non-stressed regimes, and
4. gain information on the status of various upland cotton cultivars for their utility in future breeding program for improving heat resistance.
To achieve these objectives, experiments were carried out under controlled (greenhouse) and natural (field) conditions. The primary treatment was the provision of heat-stressed and non-stressed regimes. These regimes were provided by growing experimental material in two different chambers maintained at 46/30 ± 2°C and 35/21 ± 2°C in the greenhouse, and by sowing in early April and early June in the field. Maximum and minimum temperatures in the field were found significantly higher under April regime as compared to those in the June regimes particularly at reproductive stage. Field experiments were repeated for two years. Experimental materials comprised two sets: eight upland cotton cultivars and their 15 F1, crosses obtained in line x tester model; and 3F1, crosses with their parental, F2 and back cross generations to both parents. Data were recorded for membrane thermostability, stomatal conductance, leaf temperature, leaf area, leaf water potential, epicuticular wax, yield components, fibre quality parameters, and seed physical traits including seed number per boll, seed weight, volume, density and surface area. Physiological traits were recorded at reproductive stage. Data were statistically treated to obtain information on combining ability variations, heritability, genetic and epistatic effects, and phenotypic and genotypic correlations.
The experimental results indicated that temperature both in the greenhouse and field was a significant source of variation in almost all the plant traits evaluated in this study. Genotypes (cultivars as well as hybrids) reacted with the temperature regimes to modify not only their relative phenotypic expression but also their combining ability effects Genotypes x years interaction in the field experiments was relatively less important as compared to genotypes x temperature regimes interaction for plant traits other than those related to morphology and yield. Repeating experiments over sowing dates provided more variable environments than repeating experiments over years.
Because of the significantly high magnitude of genotype x environment interaction, heritability (broadsense) estimates were moderate, 58 and 40 percent for heat tolerance index and cellular membrane thermostability, respectively, while for rest of the plant traits evaluated, it ranged from virtually zero to 28 percent. Heritability (broadsense) estimated under individual environments were, however, moderate to high (>70%) and statistically significant (P<0.01), except that for lint index and stomatal conductance, which were non-significant (P>0.05).
In general, heat stress provided sharper distinction among cultivars as well as among hybrids. Heat stress favoured the expression of non-additive type of genetic variability for most of the traits. Additive proportion of genetic variability was essentially small for all the traits, nevertheless, its magnitude increased in the presence of heat stress for most of the traits. GCA effects expressed. by cultivars did not necessarily reflect their phenotypic expression per se. Similarly, a good specific combination was not necessarily the product of two good general combiners.
Regarding cellular membrane thermostability (CMT), results were very encouraging and it came out to be a good measure of tissue tolerance of heat stress in upland cotton. It also carried positive relationship with seed cotton yield under heat stress, hence could be useful as screening method for short listing heat tolerant cotton genotypes from amongst the germplasm or segregating populations. Cultivars MNH-552, CIM-448, CRIS-19 and NIAB-Karishma appeared to be good general combiners thus potential donor parents for high CMT.
Regarding morphology and yield traits, presence of heat stress exposed higher genetic variability and favoured variation due to GCA under field conditions. Although variation due to general combining ability was significant for seed cotton yield, boll weight, boll number and earliness index, variation associated with all the morphological and yield traits was predominantly of non-additive type.
The results suggested that the June (non-stressed) regime favoured the expression of non-additive genetic variability associated with the expression of MSH, TNN and HNR under field conditions, whereas, April (heat stressed) regime favoured the expression of additive variability. Cultivars CRIS-19 and MNH-552 were identified as better combiners for high seed cotton yield under heat stress. In addition, FH-634 and CIM-443 also expressed good GCA for high yield in the field.
Supraoptimum temperature regime in the greenhouse suppressed variation for all seed traits, excepting number of seeds per boll, for which variation was available under supraoptimum temperature regime. Variation in seed traits was predominantly due to specific combining ability. Number of seeds per boll under supraoptimum temperatures in the greenhouse could be used to select gametes superior in heat tolerance. Cultivars FH-634, MNH-552 and HR109-RT expressed themselves as good general combiners for higher number of seeds per boll under supraoptimum temperature regime. Number of seeds per boll also showed significant and positive relationship with seed cotton yield both at phenotypic and genotypic levels.
Greenhouse regimes could not prove appropriate for the proper development of fibre traits. On over all basis fibre quality was better under June (noon-stressed) regime. Loss of fibre quality under heat-stressed field regime varied among cultivars. FH-900 expressed the highest reduction in staple length and strength, while MNH-552 expressed the lowest reduction in staple length and fibre uniformity. Heat stressed regime favoured the expression of both general and specific combining ability variations for 50% span length, fineness and lint index. In the absence of heat stress, specific combining ability (non-additive) variation was predominant in the expression of all the fibre traits. FH-634 was a good general combiner for longer staple length, higher strength and lint index under heat stressed regime. Seed cotton yield was found significantly associated with fibre strength at genotypic level, offering an opportunity of simultaneous selection of the two traits in this material.
On over all bases, stomatal conductance and leaf area were reduced and leaf temperature increased substantially in the presence of heat stress. Genotypic differences among cotton genotypes were, however, evident. Cultivar CRIS-19 showed the lowest, and FH-634 and HR109-RT the highest depression in stomatal conductance under supraoptimum temperature regime as compared to optimum one. Reduction in leaf area under supraoptimum temperatures was lowest in NIAB-Karishma and the highest in FH-634. Both general and specific combining ability variations were significant for leaf area under greenhouse regimes, whereas, stomatal conductance in the greenhouse was predominantly controlled by specific combining ability. General combining ability variation was important for the expression of leaf area under both field regimes and stomatal conductance under non-stressed field regime. Cultivars FH-634, CIM-443 and CRIS-19 were good general combiner for higher stomatal conductance and FH900, MNH-552, CIM-443 and NIAB-Karishama for smaller leaf area under heat stressed regimes. The relationship between the traits was more pronounced under heat-stressed field and greenhouse regime. Variation in stomatal conductance carried large environmental influence. Negative association between seed cotton yield and leaf temperature under heat stressed field regime was due to higher stomatal conductance, which caused transpirational cooling of leaf temperature.
Critical analysis of the data revealed four traits to be the most important from the pragmatic viewpoint in breeding for heats resistance, viz., membrane thermostability, earliness index under heat stress, heat tolerance index, and number of seeds per boll. Breeding strategy based on the accumulation of general and specific combining ability, like intermating and recurrent selection procedures was proposed for almost all the traits evaluated.