Abstract: Six wheat genotypes viz., 90-R-34, 96-R-37, Rawal 87, Rohtas 90, Chakwal 86 and Kohsar 95 were crossed in a 6x6 full diallel fashion during the crop season 1997-98 to determine the genetic architecture of some economic traits of wheat under drought condition. Genotypes differed significantly among each other for all the six traits studied. It was revealed that flag leaf area was conditioned by over-dominance type of gene action. Plant height, tillers per plant, grains per spike, 1000-grain weight and grain yield per plant were under the control of additive gene action. The genotype 90-R-34 was indicated as the best parental genotype. In case of hybrids, all the crosses of 90-R-34 with Rohtas 90 were the best hybrids showing useful characteristics viz., higher grains per spike, 1000-kernel weight and eventually high grain yield per plant. The study on the whole indicated the presence and importance of both additive and -non-additive genetic effects for the inheritance of the studied characters in the six wheat genotypes and their hybrids grown under drought condition. It would be helpful to use recurrent and pedigree selection procedures to select for well adapted drought tolerant genotypes in the subsequent generations of the crosses involving 90-R-34, 96-R-37 and Rawal 87.
Introduction
Among, the wheat growing countries of the world, Pakistan is located in the area of poor precipitation and ranks 9th in average annual production and accounts for a mere 2% of the world production. In Pakistan, the wheat crop covers an area of 7.871 million hectares annually and grain production exceeds 14.505 million tonnes which is barely sufficient to meet the domestic food requirements. An estimated increase of 3.2% in the population demands a matching increase in grain production to fulfil domestic needs.
The mechanism that helps drought resistance in wheat include early maturity i.e., ability of crop to ripe before the periods of drought, vigorous and deep root system to efficiently utilize the available moisture, a mechanism of closing stomata during drought stress to decrease water loss, and a waxy bloom on the leaf surface to decrease transpiration loss. Wheat cultivars developed for drought stress commonly have narrower leaves and lower shoot/root ratios and may have low yield potential than varieties developed for irrigated areas (Poehlman, 1987). Genetic information regarding important economic characters of wheat under drought stress is required to develop breeding strategies for drought prone areas. Such information under well irrigated conditions had been documented in the past (Chowdhry el al., 1991; Petrovic and Cermin, 1994; SaJid, 1995; Mishra et al., 1996; Mahmood and Crowdhry, 1999; Mahmood-and Chowdhry, 2000a,b. However, this information under drought conditions is scarce. Wheat improvement in the country has so far been focussed for irrigated areas, however, low production of wheat and increasing demand of food supply from the ever increasing population has compelled agricultural scientists to plan and focus their research for higher production in drought prone areas of Pakistan. The present studies were designed to determine the nature of genetic mechanisms involved in the expression of some economic traits of wheat under drought condition. The information derived from the present study can be effectively used to formulate and establish breeding programme for the development of new varieties symbolizing such characteristics as required to ensure sustainable wheat production in water deficit areas of the country.
Materials and Methods
The studies were initiated in the Department of Plant Breeding and Genetics, University of Agriculture, Faisalabad, during 1997-99. Six wheat genotypes viz., 90-R-34, 96-R-37, Rawal 87, Rohtas 90, Chakwal 86 (Chak. 86) and Kohsar 95 (Koh.95) were crossed in a 6x6 full diallel fashion during the crop season 1997-98. F1 seed along with their parents was space planted in the field using a triplicated randomized complete block design during the next crop season (1998-99). Inter-row and inter-plant spacings were kept 30 and 15 cm, respectively. Seeds were sown in holes (made with the help of dibble) at the rate of 2 seeds per site which were later thinned to single healthy seedling per site after germination. Each treatment in each replication was a single row of 5 m length comprising approximately 33 plants. After planting the experiment no surface irrigation was applied to maintain drought condition. All the other cultural operations including hoeing, weeding, fertilizers, etc. were carried out uniformly to avoid experimental error. Data for some economic plant traits viz., plant height, flag leaf area, tillers per plant, grains per spike, 1000-kernel weight and grain yield per plant were collected from randomly selected comparative plants. The data collected were subjected to analysis of variance according to Steel and Torrie (1984) to sort out significant differences among genotypes. After obtaining the significant differences, data were subjected to diallel analysis according to Hayman (1954) and Jinks (1954).
Results and Discussion
Plant height: Wheat cultivars genetically vary in plant height from short stature to medium and tall. However, phenotypic expression of any trait is the out come of the genotype x environment interaction. The six parents included in this study varied from short to medium stature. Data collected from the parents and all their hybrids revealed highly significant differences for plant height (Table 1) ranging from 82.6 to 107.5 cm. Coefficient of variability was small (1.06%) indicating a small percentage of deviation of genotype mean from the overall mean. A comparison of genotype means (Table 2) indicated that the genotype 96-R-37 produced the shortest plants (87.4 cm) among the parental genotypes followed by Rohtas 90 (90. 1 cm). The tallest plants were observed in 90-R-34 (107.5 cm) while Rawal 87, Chak.86 and Koh.95 produced plants of medium height. Plant height showed a general trend of reduction in case of hybrids. Most of the hybrids had medium plant height. Maximum plant height (105.7 cm) was recorded in Chak. 86 x 90-R-34 hybrid involving one medium and one taller parent, while the shortest plants (82.6 cm) were observed in 96-R-37 x Rawal 87 hybrid involving one short and one medium height parent. It was observed that most of, the crosses between medium height parents or between medium height and shorter height parents produced hybrids of medium plant height. Thus, a sort of additivity for plant height was evident. Graphical representation of the data (Fig. 1a) also revealed an additive type of gene action where the regression line cut the Wraxis above the origin.
Our results are in agreement with those of Chowdhry et al. (1991), IqbaI et al. (1991), Lonc and Zalewski (1991), Munir (1997), Subhani (1997) and Chowdhry et al. (1999) who also reported additive gene action with partial dominance for plant height in wheat. However, over dominance was indicated by Petrovic and Cermin (1994), Sajid (1995), Mishra et al. (1996) and Mahmood and Chowdhry (1999).
The position of the genotypes on the graph indicated that Koh.95 being closest to the origin contained the most dominant genes for plant height while the genotype Rawal 87 was located farthest from the origin and contained the least dominant genes for this trait.
Flag leaf area: Flag leaf is of utmost importance in cereals like wheat, because it provides the maximum photosynthates to be stored in the grains. A greater flag leaf area will eventually help to increase the photosynthetic efficiency by increasing the production of photosynthates which are then translocated in grains increasing their weight. Therefore, flag leaf area has a direct relationship with grain yield. Comparison of genotype means (Table 2) indicated that maximum flag leaf area among parental genotypes was recorded in 96-R-37 (31.7 CM2) which was significantly higher from all other parental genotypes. Similarly, the lowest flag leaf area (24.9 cm2) was recorded in Chak. 86. Flag leaf area of the other four genotypes was intermediary and statistically similar.
Comparison of the hybrids for flag leaf area indicated the involvement of both additive and nonadditive genetic effects. Flag leaf area of some hybrids showed increase towards the better parent and in some hybrids it decreased towards the lower parent, and showed intermediate values in some cases. The maximum flag leaf area (32.9 cm2) was recorded in Chak.86 x Rawal 87 hybrid where both parental genotypes had smaller flag leaf area but their hybrid displayed an increase over the better parent. Similar was the case in Rohtas 90 x 96-R-37, 96-R-37 x Rawal 87, 90-R34 x Rohtas 90, 90-R-34 x Chak.86, Rawal 87 x Rohtas 90, Rawal 87 x Chak.86 and Rohtas 90 x Rawal 87 hybrids. The lowest flag leaf area (21.0 cm2) was recorded in the Chak.86 x 90-R-34 hybrid which showed over dominance towards its lower parent. This type of behaviour was found in crosses like 90-R-34 x Rawal 87,90-R-34 x Koh.95, 96-R37 x 90-R-34,90-R-34 x Koh.95, Chak.86 x 90-R34, Koh.95 x 90-R-34, Koh.95 x 96-R-37 etc. Crosses like 96-R-37 x Rohtas 90 and 96-R-37 x Chak. 8 6 showed flag leaf area which was near to mid-parent value. Graphical representation of the data (Fig. 1b) showed an over dominant type of gene action where the intercept of the regression line was negative. Over dominance gene action for flag leaf area was also reported by Iqbal et al. (1991), Sajid (1995), Munir (1997) and Chowdhry et al. (1999), however, additive gene action with partial dominance was indicated by Subham (1997), Mahmood and Chowdhry (1999) and Mahmood and Chowdhry (2000a).
| Table 1: | Analysis of variance of some economic traits of wheat under drought condition (means squares) |
| * P≤0.05, **P ≤0.01 | |
| Table 2: | Mean values of parental genotypes, their crosses and statistical significance for plant height, flag leaf area and tillers per plant in a 6 x 6 diallel cross of wheat under drought condition |
| Means sharing common letters do not differ using DMR test at 5% P. | |
On the basis of location of genotypes on the graph and distance from the origin, Koh.95 was found to have maximum dominant genes followed by 90-R-34 while the genotype 96-R-37 being farthest from the origin contained the most recessive genes. Gene constitution in Rawal 87, Chak.86 and Rolitas 90 was intermediate.
Tillers per plant: Tiller number per plant is a vital yield related trait. It is directly related with number of spikes per plant. Greater number of tilers per plant will, thus, ensure higher grain yield. Under drought conditions the development of tillers is greatly affected due to reduced availability of moisture.
| Table 3: | Mean values of parental genotypes, their crosses and statistical significance for grains per spike, 1000-kernel weight and grain yield per plant in a 6 x 6 diallel cross of wheat under drought condition |
| Means sharinc, common letters do not differ using DMR test at 5% P. | |
The plant strives to complete its life cycle in short time by reducing its vegetative growth to enter into reproductive stage (a process called rapid penological development). Thus, tillerinc, is greatly reduced under drought. Under such circumstances, the selection of genotypes that have tillering stability will help to increase yield under drought conditions.
Analysis of data for tiller number per plant indicated that this trait showed a relatively higher coefficient of variability (8.46%) and Tgenotypes differed highly significantly for the trait (Table 1). A study of mean values (Table 2) of genotypes revealed that tillers per plant ranged from 5.3 to 7.7 among, the genotypes.
| Fig. 1: | Er/Vr graphs for a) plant height, b) flag leaf area, c) tillers per plant, d) grains per spike, e) 1000-grain weight and f) grain yield per plant |
The parental genotype Koh. 95 produced the maximum number (7.7) of tillers per plant closely followed by 96-R-37 which produced statistically similar number (7.6) of tillers per plant. Rest of the 4 genotypes produced lower number of tillers (5.3 to 5.7) per plant and were statistically similar to each other. Performance of hybrids indicated that maximum number (7.7) of tillers per plant were produced by direct and reciprocal crosses of 96-R-37 and Koh.95 followed by Koh.95 x 90-R-34 and Chak-l-.86 x Koh. 95 hybrids. These all hybrids involve Koh. 95; the best parent for tiller number per plant and were statistically at par to each other. The lowest number (5.2) of tillers per plant was recorded in Chak. 86 x 90-R-3 4 hybrid. All hybrids producing tillers in the range of 5.2 to 6.2 were shown to be statistically similar. Further study of hybrids indicated the involvement of additive genetic effects with partial dominance. This was inferred due to the fact that means of most of the hybrids showed increase towards better parent. This increase, however, was less in some hybrids and greater in others.
Graphical representation of the data (Fig. 1c) indicated an additive type of gene action for the inheritance of tillers per plant. Additive type of gene action for tillers per plant in wheat was also reported by Malik et al. (1989), Chowdhry et al. (1991), Mahmood and Chowdhry (1999) and Chowdhry et al. (1999) while Mishra et al. (1996) indicated complete dominance and Lonc and Zalewski (1991), Senapati et al. (1994), Munir (1997) and Subhani (1997) indicated over dominance gene action for this trait.. Location of array points on the graph threw light on the gene constitution in the genotypes. It was shown that maximum dominant genes were contained in Rohtas 90 followed by Rawal 87. The genotypes 96-R-37 being distant from the origin contained the least dominant genes.
Grains per spike: Number of grains per spike is one of the most important components of grain yield in wheat. Genotypes showing stability for this trait are often better tolerant to drought conditions. However, weight of grains should also be given due importance while making selection for this trait Significant differences prevailed for number of grains per spike among the 36 genotypes compared Table 1). However, its coefficient of variation was smaller (1.09%).
Statistical comparison of genotype means (Table 3) revealed that maximum number of grains per spike (64.3) was recorded in 90-R-34 followed by Chak.86 (63.1). The lowest number (57.6) was recorded in Rawal 87. Number of grains per spike obtained in Koh.95, 96-R-37 and Rohtas 90 was 60.1, 61.3 and 59.7, respectively. Comparison of hybrids indicated that hybrids between 90-R-34 and 96-R-37 and other hybrids involving these genotypes produced relatively more number of grains per spike as compared to hybrids synthesized from other parental genotypes with a few exceptions. Thus, maximum number of grains per spike was recorded in Chak.86 x 90-R-34 (63.7) followed by 90-R-34 x 96-R-37 (6-3.2), Chak.86 x 96-R-37 (63.2), 90-R-34 x Chak. 86 (63.1) and 90-R-34 x Koh.95 (62.9) hybrids. Most of the hybrids synthesized with the genotype Rawal 87 (lowest parent) were having lower number of grains per spike e.g., Koh.95 x Rawal 87 (58.4), Rohtas 90 x Rawal 87 (58.7), Rawal 87 x Rohtas 90 (58.9), etc.
Graphical representation of the data Fig. 1d displayed and additive action for grains per spike. Present findings are in accordance with Petrovic and Cermia (1994), Mishra et al. (1996), Munir (1997), Mahmood and Chowdhry (2000b) and Sener et al. (2000) who also indicated an additive gene action for grains per spike. However contradictory results have also been reported by Chowdhry et al. (1991), Lonc and Zalewski (1991), Senapati et al. (1994) and Subhani (1997) who indicated over dominance gene action for grains per spike.
Distribution of array points indicated that 96-R-37 being nearest to the origin contained the most dominant genes followed by Rawal 87. Koh. 95 being farthest from the origin hold the least dominant genes. Gene constitution in 90-R34, Rohtas 90 and Chak.86 was intermediary.
1000-kernel weight: 1000-kernel weight is a vital yield component and is more or less stable character of wheat cultivars. However, under drought this trait may be affected to a greater extent and genotypes showing high 1000-kernel weight under irrigated conditions may not be able to produce grains of similar weight under drought. This is possible due to the shortage of moisture which forces plant to complete its grain formation in relatively lesser time.
After obtaining significant differences for 1000-kernel weight, comparison among genotypes (Table 3) revealed that maximum 1000-kernel weight (43.5 and 43.3 g) was produced by Rawal 87 and 90- R-314, respectively. These two genotypes differed significantly from the other four parents but not from each other. Comparison of hybrids indicated that maximum 1000-kernel weight (44.8 g) was recorded in Rawal 87 x 90-R-34 hybrid. It was also noted that all hybrids involving Rawal 87 or 90-R-34 produced higher 1000-kernel weight. Similarly, it was also observed that all hybrids of 96-R-37 produced lower 1000-kernel weight and the lowest weight (39.7 g) was recorded in Koh.95 x 96-R-37 hybrid.
Graphic presentation (Fig. 1e) is played an additive type of gene action for the inheritance of 1000 kernel weight. Additive type of gene action for 1000-kernel weight was also reported by Chowdhry el al. (1991), Petrovic and Cermin (1994), Subhani (1997) and Chowdhry et al. (1999) while Malik et al. (1989) and Lonc and Zalewski (1991) indicated complete dominance and Chowdhry et al. (1989) and Lonc and Zalewski (1989), Sajid (1995) and Mishra et al. (1996) indicated over dominance for 1000-kernel weight. Location of array points displayed that Chak, 86 contained the most dominant genes for 1000-kernel weight followed by 90-R-34. The genotype Rohtas 90 was located farthest from the origin and contained the least dominant genes. Gene constitution in Rawal 87, 96-R-37 and Koh.95 was found to be intermediary.
Grain yield per plant: Statistical analysis of the data regarding grain yield per plant revealed significant differences among genotypes (Table 1) with 3.48% coefficient of variation. It was further revealed (Table 3) that the genotype 90-R-34 produced the highest (23.9 g) grain yield per plant while Rawal 87, Chak 96-R-37 produced the lowest grain yield per plant (20.0, 20.0 and 20.4 g, respectively). In case of hybrids maximum grain yield per plant (24.5 g) was obtained in the cross Rolitas 90 x 90-R-34 followed by 90-R-34 x Rohtas 90 (24.3 g), 90-R-34 x 96-R-37 (24.2 g), Chak.86 x 90-R-314 (23.8 g), Koh.95 x 90-R-34 (23.7 g) and Rawal 87 x 90-R-34 (23.7 g). It was noted that all crosses producing high grain yield per plant involved 90-R-34 as one of the parents. This fact indicated the usefulness of this genotype for synthesizing high yielding genotypes under drought.
Graphic representation of the data (Fig. 1f) displayed an additive action of genes for grain yield per plant. Additive gene action controlling grain yield per plant in wheat was also reported by Malik et al. (1989), Subhani (l997), Mahmood and Chowdhry (1999) and Chowdhry et al. (1999) while Chowdhry et al. (1991) and Sener et al. (2000) found complete dominance and Chowdhry et al. (1989) Sajid (1995) and Munir (1997) found over dominance for grain yield per plant.
The distribution of array points in the graph indicated that the genotype 90-R-34 was located nearest to the origin and contained the most dominant genes followed by Rohtas 90. Genotypes Chak. 86 and 96-R-37 were located at the distant position and contained the least dominant genes.
It was observed that, among parental genotypes best performance was shown by 90-R-34 which had maximum grains per spike and grain yield per plant. It also showed high values for flag leaf area and 1000- grain weight. In case of hybrids, all the crosses of 90-R-34 with Rohtas 90 were the best hybrids showing useful characteristics viz., higher grains per spike, 1000-kernel weight and eventually high grain yield per plant. Thus, these crosses could be the potential breeding material for further selection and may yield transgressive segregates that could be selected for drought conditions. Other crosses that may prove useful in this respect include 90-R-34 x 96-R-37, Rawal 87 x 90-R-34, Chak.86 x 90-R-34 and Koh-95 x 90-R-34.
Thus, this study on the whole indicated the presence and importance of both additive and nonadditive genetic effects for the inheritance of the studied characters in the six wheat genotypes and their hybrids grown under drought conditions. It would be helpful to use recurrent and pedigree selection procedures to select for well adapted drought tolerant genotypes in the subsequent of the crosses involving 90-R-34, 96-R-37 and Rawal 87.