Abstract

The wheat variety Kundan, along with the fast ruster variety Agra Local, were screened for seedling reaction and adult plant response for two years. Seedlings of Kundan were susceptible while adult plants showed lower susceptible response than Agra Local in the field, and lower pustule number, uredial size and longer latency period under glasshouse conditions for both the years, which indicates potentially consistent performance of the components over years, and that these measures can be used as direct parameters for identifying slow rusting genotypes / varieties. Continuous variation in the segregating populations of Kundan x Agra Local has been cited as evidence of quantitative inheritance for disease resistance. The generation mean analysis with five generations, Parents (Kundan and Agra Local), F1s, F2s and F3s indicated the presence of a higher magnitude of epistatic interactions than the additive and dominance components in the expression of different components of slow rusting and AUDPC.In this situation improvement in the character may be expected through standard selection procedure, which may first exploit the additive genetic effects and simultaneously care should be taken to see that the dominance effects are not dissipated, rather they should be enhanced.

Introduction

Leaf rust caused by Puccinia recondita Roberge ex Desmaz. f. sp. tritici Ericks. & E.Henn. is one of the most destructive and widely distributed disease in most wheat growing areas. Breeders commonly have relied on race specific leaf rust (Lr) genes for hypersensitive resistance which are very effective in reducing epidemic build up and easy to manage in breeding programmes because of their monogenic nature. However, the short-lived nature and improper use of race specific genes on a commercial scale led to their erosion within a short time. Virulence is known for many of the named genes involved in resistance to leaf rust. This has resulted in reduction of the useful genetic diversity for resistance and has created the necessity for continuously searching for more durable types of resistance.

There is a great interest in improving the durability of the intrinsically "non-durable" type of resistance by the exploitation of partial resistance. Partial resistance is characterized by a slow epidemic buildup despite a high infection type, indicating a compatible host-pathogen interaction (Parlevliet, 1975). The important components of slow rusting in leaf rust are a longer latency period, smaller pustule size, fewer pustules and lower disease severity values (area under the disease progress curve). There are reports that this type of resistance is durable and race non-specific (Kuhn et al., 1978; Dinessen, 1993; Singh et al., 1993; Aslam, 1996).

Plant breeders require more information regarding the genetics of the components of slow rusting in order to breed more effectively for this type of resistance. The present communication on slow rusting phenomenon in wheat variety Kundan against leaf rust pathogen represents work towards this objective.

Materials and Methods

The experiments were conducted from 1998-2002 at Indian Agricultural Research Institute (IARI), New Delhi, India. Kundan, the dwarf wheat (DL 153-2) bred at the Indian Agricultural Research Institute, New Delhi involving parents Tonari 71 and NP 890, embodies a good degree of genetic homeostasis enabling it to register its presence among the elite cultivars over a wide range of growing conditions from rainfed to limited water and nutrient inputs, as well as from normal to late planting. Its larger grains, mostly lacking in dwarf wheats, are an attractive feature (Sawhney, 1993). Kundan is resistant to important rust diseases and has constantly maintained its level of leaf rust resistance for over two decades, during which many rust pathotypes evolved.

Kundan was selected on the basis of historical observations of slow rusting to wheat leaf rust. It was evaluated for components of partial resistance to leaf rust both in the glasshouse and field conditions. The most widely virulent pathotype 77-5 (121R63-1) of Puccinia recondita f. sp. tritici was used for these experiments. The avirulence/ virulence formula for this pathotype is as follows:
Lr9, Lr18, Lr19, Lr22a, Lr24, Lr 25, Lr28, Lr29, Lr30, Lr32, Lr 34, Lr48, Lr49 / Lr1, Lr2a, Lr3, Lr10, Lr11, Lr12, Lr13, Lr14a, Lr15, Lr16, Lr17, Lr20, Lr23, Lr26, Lr27+31, Lr33.

Glasshouse studies: Kundan along with fast ruster Agra Local, was tested in glasshouse at seedling stage following the procedure proposed by Joshi et al. (1986). Seedlings of the parents were raised in 10 cm pots and 10 seeds were sown in each pot. Infection types were recorded 12 days after inoculation following the scale proposed by Stakman et al. (1962).

Kundan and Agra Local were also sown in 30 cm pots with 5 plants per pot. Thinning was carried out to achieve one tiller per plant. Plants at flag leaf stage were sprayed with a suspension of freshly harvested urediospores where Tween-20 was used as surfactant. The suspension had 20 urediospores per microscopic field (10x X 10x) on an average. Each flag leaf was sprayed uniformly with urediospores. The inoculated plants were then transferred to a moist chamber for 48 hours after which the pots were transferred to open glasshouse benches. Experiment was repeated for a second year.

The infected flag leaves were individually evaluated for latency period, pustule size and pustule number. Latency period (days) was calculated by using the formula given by Das et al. (1993). Pustule size was calculated by using the formula, pi/4 x length x width (mm²) (Kochman & Brown, 1975). The length and breadth of the uredia were measured by the micrometer. From each flag leaf 10 uredinia were recorded and where less than 10 uredinia were present, the size of all the uredinia were measured. Pustule number was counted as the number of uredinia per unit area.

Field studies: Kundan along with Agra Local were also planted in the field for two consecutive crop seasons to compare the glasshouse observations with the field reaction where the disease increase is multicyclical. Plots consisted of pairs of 3 m rows seeded 10 cm apart with a distance of 18 cm between rows and 36 cm between the plots. Spreader rows composed of Agra Local, Kharchia Local and Lal Bahadur were planted all around the experimental block and between the beds. The spreader rows were inoculated using a hypodermic syringe with pathotype 77-5 after 55-60 days of sowing, so that the proper disease spread could occur. When the spreader plants were 50% infected, the genotypes were scored three times for rust severity and response using the "Modified Cobb's" scale (Peterson et al., 1948) at weekly intervals. The area under the disease progress curve (AUDPC) was calculated from these disease scores using the computer programme developed at CIMMYT.

Genetic analysis: Kundan was crossed with Agra Local and the seeds from both the parents and the crossed seeds were harvested separately. The F1s were advanced in the National Phytotron Facility, IARI, New Delhi during the off-season and the F2 seeds of each plant were harvested separately. Some of the F2 seeds were planted in the main crop season to obtain F3 seeds. The parents and F1s (in four replications), 200 F2 seeds and 100 F3 families with 20 plants per family were evaluated for components of slow rusting in the glasshouse and parents, F1, F2 (200 plants) and F3 (100 families with 40 plants per family) were evaluated for AUDPC in the field. Inoculations and recording of data were carried out as described earlier.

Statistical Analysis: For each family the plot mean values in each generation were averaged over replications to obtain generation means. These generation means formed the basis for calculation of various genetic parameters. Scaling tests proposed by Mather (1949) and Hayman & Mather (1955) and the joint scaling test devised by Cavalli (1952) were used for testing the adequacy of additive dominance model. Further analysis of data was performed following a five parameter model (Hayman, 1958), as five generations were present for estimation of five parameters (m), (d), (h), (i) and (l).

Results and Discussion

The seedlings of Kundan showed a high infection type of "3+" whereas the fast ruster, Agra Local showed "4" based on the "0-4" scale described by Stakman et al. (1962). Under the glasshouse conditions, Kundan showed long latency period (20.89, 21.70), smaller pustule size (0.166, 0.134) and less number of pustules (10.75, 10.25) whereas the fast ruster, Agra Local had the short latency period (9.85, 11.60), larger pustule size (0.274, 0.396) and higher uredinial number (36.40, 34.10) in both the seasons (1998- 99, 1999- 2000), respectively. Under field conditions, Agra Local showed the highly susceptible response with higher AUDPC values (1455.0, 1300.0) whereas Kundan showed the lowest AUDPC values (218.5, 217.0) for both the seasons. The components of slow rusting and AUDPC are consistent in Kundan over both years and they could be used as reliable direct parameters for measuring slow rusting resistance. Ohm & Shaner (1976), Kulkarni & Chopra (1980), Andres & Wilcoxson (1986), Singh et al. (1991) and Prabhu et al. (1993) also reported a similar trend in the case of AUDPC and for slow rusting components.

It can be observed that selection for the longer latency period, lower pustule size and pustule number would result in genotypes with low AUDPC. As there are many components involved in slow rusting, each perhaps with its own system of genetic control, the evolution of aggressive races would expected to be slow and therefore slow rusting offers a durable form of host resistance. The slow rusting exhibited by Kundan seems to be the result of the cumulative effects of all these components. Although these studies showed a strong association between the components, we do not know whether the same or different genes, either linked or present on different chromosomes, govern these components. Linkage between genic loci, or pleiotropism by one gene locus, could result in the phenotypic associations noted above since the expression over locations was consistent for longer latency period, lower uredial number, smaller uredial size and lower AUDPC values. The results can be better explained if it is assumed that there are no separate genes controlling the components of partial resistance, but only genes for partial resistance per se (Parlevliet, 1986), which results in a pleiotropic basis for the components of partial resistance.

Generation means formed the basis for calculation of various genetic parameters (Table 1). Normal distributions were not evident in the F2 progeny. This lack of a normal distribution may be due to the presence of dominance, epistasis, or linkage between the leaf rust resistance genes. Continuous variation, i.e., no discrete classes, in the segregating populations of crosses involving cultivars with slow rusting resistance has been reported for wheat leaf rust pathogen interactions (Kuhn et al., 1978; Bjarko & Line, 1988) and has in some instances been cited as evidence of quantitative inheritance for disease resistance.

D scaling test (Mather, 1949; Hayman & Mather, 1955) was applied to judge the adequacy of the model and to asses the genetic components of mean for AUDPC and other components of slow rusting to leaf rust, and it showed the non significant values for the characters but scaling test is applicable to individual test involving different types of family available, whereas the joint scaling test effectively combines the whole set of scaling test into one and thus offers a more general, adoptable and informative approach (Mather & Jinks, 1977) (Table 2).

Cavalli (1952) joint scaling tests showed the presence of epistatic effects for all the characters as the chi-square values were significant (Table 2). So all the characters were extended to the five parameter model for their practical implication. In these traits, besides additive and dominance, epistatic effects were also found to play an important role in the expression. It indicates that as the inheritance of quantitative characters becomes more complex, the contribution of epistatic gene effect to their inheritance becomes greater.

The five parameter model give reliable estimates of major genetic components as well as epistatic gene effects. The estimation of the five parameters revealed the significant contribution of both additive and dominance gene effect for all the traits (Table 2). All the characters showed the predominance of additive effect over the dominance effect. [h] and [l] components of the five parameter model of all the characters showed the values in same direction as it indicates the existence of complementary epistasis and it also indicates the role of this type of epistasis in selection. Epistatic interaction was observed in the expression of different components of slow rusting and AUDPC by different workers (Luthra et al. (1996) for latency period; Gautam (1980) for pustule number; Das et al. (1993) for pustule size; Luthra et al. (1996), Singh et al. (1991), Phogat et al. (1995), Dalal & Singh, (1994) and Das et al. (1992) for AUDPC).

In the present study, it was found that all the characters showed all three types of gene action, additive, dominance and epistasis. In such a situation improvement in the character may be expected through standard selection procedure, which may first exploit the additive gene effects and simultaneously care should be taken to see that the dominance effects are not dissipated, rather they should be enhanced. Under such circumstances, reciprocal recurrent selection breeding procedure would be the best available method to meet the requirement, as it will utilise simultaneously all three types of genetic effects.

Acknowledgements

First author of this paper gratefully acknowledges the financial support from Council for Scientific and Industrial Research and Indian Agricultural Research Institute, given as a scholarship during Ph.D. programme. The author is also very grateful to Dr. J.B. Sharma for his valuable guidance during the research programme.

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