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Cucurbit Genetics Cooperative Report Volume 5:48-50 (article 24) 1982

On the Silvery-Leaf Trait in Cucurbita pepo L.

O. Shifriss

Department of Horticulture and Forestry, Cook College, Rutgers University, New Brunswick, NJ 08903

Recent observations and breeding data suggest that there exists a silvery-leaf (SL) trait whose expression is subject of marked genetic and non-genetic variations, that the genetic variants include M / M for mottled leaves (2) and m / m for non-mottled leaves, and that modifier genes greatly affect the SL expression of gene M.

There are at least five true-breeding SL variants and their phenotypes, designated by symbols SL-1 and SL-5, are described in Table 1. The SL-1 phenotype is represented by 'Jersey Golden Acorn' (JGA) and some other cultivars. For example. 'Early Prolific Straightneck', m /m (2), which superficially appears to have "uniform green leaves" (2), actually exhibits the SL-1 phenotype upon closer examination.

The SL-5 phenotype is represented by NJ260 whose origin is known (4). A cross was made between JGA (SL-1) and NJ260 (SL-5). The F1 was of the SL-3 phenotype (Table 1). The F2 consisted of 686 mottled plants, which varied widely in their SL expression, and 213 non-mottled plants of the SL-1 phenotype, X2 (3:1) - 0.82, P = 0.50-0.25. Of the 686 mottled segregates only three resembled the NJ260 parent (SL-5), a frequency of about 0.3% (3/899) based on the entire F2. The backcross, F1 (SL-3) x JGA (SL-1), consisted of 204 mottled plants, which did not include a single individual of the SL-5 phenotype, and 228 non-mottled plants of the SL-1 phenotype, X2 (1:1) = 1.33, P = .75-0.50. A new true-breeding line of phenotype SL-4 (Table 1) was developed through selection in the F2 and subsequent inbred generations. The great diversity among the mottled segregates in the F2 leads me to believe that other true-breeding lines of the SL phenotypes could have been developed from this cross.

The available evidence is compatible with the hypothesis that modifier genes, acting separately or in concert, extend and/or intensify the SL expression of M. The SL-5 phenotype of NJ260 is probably conditioned by M /M and effective "extenders" and "intensifiers"; the true-breeding variant of SL-4 phenotype is probably conditioned by M /M and some "extenders" but few, if any, "intensifiers"; and the SL-3 phenotype of 'Caserta' is probably conditioned by M /M and effective "intensifiers". The genetic constitutions of SL-1 and SL-2 are somewhat less certain. The SL-1 phenotype of JGA, m /m, could be conditioned either by modifiers of M which have small silvery effects of their own or by a very low silvery expression of m. The SL-2 phenotype of 'Fordhook Zucchini' could be conditioned either by M/M and modifiers which delay and attenuate the expression of M or by weak M alleles.

Scarehuk and Lent (3) discovered that the palisade cells in the silvery areas of a mottled leaf are not in close contact wither with the epidermis or with one another, thus creating air spaces. And they believe that these air spaces are responsible for the SL expression. Giving a genetic predisposition for a breakdown in intercellular cohesion, it is evident that cells located near leaf veins are more vulnerable to this phenomenon than other cells. There are other non-genetic factors which affect the SL expression, including fluctuations in rate of plant growth.

For several years, NJ260 appeared to be free of virus infection under field conditions in New Brunswick. It occurred to me that the silvery appearance of this line may function in a way analogous to that of aluminum mulch which repels aphids and, thus, lowers the incidence of virus diseases (4). If light which contains a relatively highproporti9on of short waves repels aphids (see reference 1 for literature review), then the reflected light from silvery leaves might contain a higher proportion of short waves than the reflected light from non-silvery leaves. A preliminary test by Dr. Ron Prokopy (personal communication) confirmed this expectation.

If the correlation between high SL expression and low incidence of aphid-transmitted virus diseases is valid, it should be worthwhile to further explore the nature of this correlation and its value not only in Cucurbita but also in other cultivated genera such as Pisum and Phaseolus.

Table 1. Description, symbols and examples of true-breeding variants of the silvery leaf trait (SL) in Cucurbita pepo.

Description of silvery-leaf variants z

Symbols of phenotypes

Examples of true-breeding representatives

A. Plants bearing non-mottled leaves, m / my

 

 

1. Leaves exhibit narrow, often inconspicuous, silvery lines along both flanks of veins. This is a weak phenotype and its onset occurs early in plant development.

SL-1

'Jersey Golden Acorn' (JGA)

B. Plants bearing mottled leaves M / My

2. Leaves exhibit relatively large, but sparely-distributed, silvery patches in axils of veins. This is a weak phenotype and its onset occurs late in plant development.

SL-2

'Fordhook Zucchini'

3. Leaves exhibit relatively large and abundantly-distributed silvery patches in axils of veins. This is a strong phenotype and its onset occurs early in plant development.

SL-3

'Caserta'

4. Lightly-mottled leaves are observed occasionally, but the light silvery expression often extends over the entire leaf surface. This is a weak phenotype and its onset occurs early in development.

SL-4

A recently-developed inbred from a cross between JGA (SL-1) and NJ260 (SL-5)

5. Mottled leaves are observed occasionally, but usually the leaves are uniformly silvery. This is a strong phenotype and its onset occurs very early in plant development.

z The phenotypes of these variants are greatly affected by non-genetic fluctuations. The "strong" phenotypes are more intense in expression and more persistent during plant development than the "weak" ones.
y Tentative or incomplete genotype.

Literature Cited

  1. Kring, J.B. 1972. Flight behavior of aphids. Ann. Rev. Entomology 17:461-492
  2. Scarchuk, J. 1954. Fruit and leaf characters in summer squash. J. Hered. 45:295-297
  3. Shifriss, O. 1981. Do Cucurbita plants with silvery leaves escape virus infection? Origin and characteristics of NJ260. Cucurbit Genetics Coop. Rpt. 4:42-43.
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Page citation: Wehner, T.C., Cucurbit Genetics Cooperative;
Created by T.C. Wehner and T. Ng, 1 June 2005; design by C.T. Glenn;
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