Penetrance and expressivity of creeping-rootedness of different kinds of progenies in alfalfa genotypes
E. Piano and L. Pecetti
Istituto Sperimentale per le Colture Foraggere, viale Piacenza 29, 26900 Lodi, Italy
Selection of deep-crowned, grazing-tolerant alfalfa varieties can rely on either rhizomatous or creeping-rooted types. Creeping-rootedness, however, is a difficult character to deal with in the breeding because of its recognized erratic expression which strongly hinders and lengthen the selection process. Information is required on evaluation procedures that could hasten the identification of genotypes with desirable expressivity and good heritability of the trait. Nine genotypes selected for creeping-rootedness were initially evaluated for penetrance (proportion of creeping plants) and expressivity (number of adventitious shoots per creeping-rooted plant) of the character in clonal progenies (cuttings) after three years of growing. In a subsequent trial, three of these genotypes were further evaluated as clonal, selfed (full-sib), and half-sib progenies for the rapidity of expression of creeping-rootedness in the year of establishment. The penetrance of the character in the nine genotypes, measured through their clonal progenies after three years, was significantly (P £ 0.01) different among them and never complete, for the proportion of creeping-rooted propagules ranged from zero to 94%, with an average value of 35% (Table 1). These results confirmed the erratic expression of creeping-rootedness, even though the progenies were grown for a period (three years) which has been considered sufficient for the character to show itself (Heinrichs, 1963). The second trial, aimed at assessing the effect of genotype and kind of progeny on the rapidity of creeping-rootedness expression, indicated that (Table 2): i) there was a significant (P £ 0.05) mean effect of progeny type, the level of the character expression following the order: clonal > half-sib >> full-sib; in particular, selfing exerted a strong detrimental effect on creeping-rootedness; and ii) the mean effect of the genotype was also significant (P £ 0.05), the C-1 genotype showing the least expression of the trait, no matter of the progeny type, and confirming in that the results of the first evaluation. Selection for creeping-rootedness cannot be apparently aided by the use of selfed progenies in the evaluation schemes. The low level of character expression in these progenies points to possible relationships with the lowering of vigor induced by inbreeding. Much remains to understand on genetics and physiology of the character and on external factors which affect its expression.
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Table 1. Proportion of creeping-rooted plants three years after planting in the clonal progenies of nine creeping-rooted genotypes |
Table 2. Proportion of creeping-rooted plants in three kinds of progenies of three creeping-rooted genotypes in the autumn (November) following establishment |
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Clone code |
% creeping-rooted plants |
Progeny |
% creeping-rooted plants |
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S-1 |
94.4 a |
S-2 |
K-1 |
C-1 |
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S-2 |
53.8 b |
Clonal |
30.5 a |
66.7 a |
2.8 a |
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|
S-3 |
0.0 c |
Full-sib |
2.8 b |
2.8 c |
0.0 a |
|
|
K-1 |
50.0 b |
Half-sib |
30.5 a |
25.0 b |
5.5 a |
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R-1 |
35.3 bc |
In each column, means followed by the same |
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R-2 |
7.7 c |
letter are not different at P £ 0.05, according to |
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C-1 |
8.3 c |
Duncan’s multiple range test |
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P-1 |
33.3 bc |
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P-2 |
30.8 bc |
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Average |
34.8 |
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Means followed by the same letter are not different at P £ 0.05, according to Duncan’s multiple range test |
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Reference
Heinrichs, D.H. (1963). Adv. Agron. 15:317-337