Cucurbit Genetics Cooperative Report 7:81-83 (article 36) 1984

Further Notes on the Silvery-Leaf Trait in Cucurbita

Shifriss, O.

Department of Horticulture and Forestry, Cook College, Rutgers University, New Brunswick, NJ 09803

The silvery-leaf trait (6) raises two issues: the biological control of its complex expression and its adaptive value. The biological control is poorly understood. But there is little doubt that, under field conditions in New Brunswick, inbreds of C. pepo L. differ greatly in (i) time during plant development at which trait is first manifested, (ii) intensity of its expression, and (iii) extent of its distribution over the upper leaf surface (5). The silvery expression can be confined to veins and/or vein axils (mottled- leaves) or can be extended over the entire leaf surface.

Five different forms, SL-1 to SL-5, have been described (6). Representatives of SL-1 are 'Early Prolific Straightneck' (EPS) and 'Jersey Golden Acorn'. These cultivars are assumed to carry m/m (m for non-mottled or non-silvery leaves) as well as some modifiers of M (M for mottled or silvery leaves) which have small silvery effects of their own. Representatives of SL-2 to SL-5 are assumed to be M/M lines which differ from one another in their modifiers or controllers. The most extreme form, SL-5, is represented by NJ260. This inbred can produce uniformly silvery leaves of intense expression throughout plant life.

Several new intermediate (between SL-2 and SL-5) inbreds were developed from crosses involving NJ260. In addition, two cultivars were found to differ from previously described forms: 'Bicolor Spoon' and 'Delicata'. Our inbred of 'Bicolor Spoon' is truly green or non-silvery. Hypothetically, it carries M/m but lacks the modifiers which have small silvery effects. As to 'Delicata', Tapley et al. (8) described its leaves as "silvery green". Under our field conditions, 'Delicata' produces green leaves during spring and midsummer, and uniformly silvery leaves in late summer and fall. The silvery expression in 'Delicata' often spreads from the tips to the entire surface of the leaves rather than from the veins or vein axils. If the present hypothesis is correct, 'Delicata' carries M/M and a unique combination of controllers. But I wish I were as confident in the hypothesis as I am in the description of the above lines.

Non-genetic factors profoundly affect the expression of the silvery-leaf trait. High expressivity is obtained predictably under the following controlled conditions: 16 hr photoperiod, 11 x 10³ lu/m², 90% of which is from fluorescent tubes and 5% from incandescent bulbs, 20% at day and 15% at night. Expressivity is very low under greenhouse (shaded) conditions in midsummer when temperatures are often extremely high. However, extremely high temperatures under field conditions do not drastically reduce expressivity. These tentative observations suggest that expressivity is modified by a combination of non-genetic influences.

Unlike C. pepo, C. moschata (Duch.) Poir. is largely of tropical adaptation and most, if not all, its tropical cultivars have distinctly mottled leaves. Indeed, mottled-leaf is one of the distinguishing features of this species in several taxonomical treatises (e.g., in 9). An exception is a group of Butternut cultivars (2) which probably evolved in North America. The F₂ of an interspecific cross made between C. pepo 'Jersey Golden Acorn' (non-mottled), as seed parent, and C. moschata 'Burpee Butterbush' (non-mottled) consisted of an appreciable number of mottled as well as uniformly silvery segregates. These results indicate that the designation of a single locus for control of mottled-leaf in Cucurbita (4) is an over-simplification.

Is the silvery-leaf trait due to a neutral genotype that has been randomly fixed in our cultivars? Alternatively, does it have a selective advantage?

Initial observations suggested that the silvery-leaf trait provides an escape mechanism against aphids and aphid- transmitted virus diseases (5). Results of a subsequent field study supported this hypothesis, but showed that protection is not complete (3). Circumstantially, two other facts support this hypothesis. First, silvery leaves reflect more light than non- silvery leaves (7), and light reflectance is generally assumed to be involved in repelling aphids. Second, as pointed out above, most if not all tropical cultivars of C. moschata are mottled. Virus diseases are particularly destructive in the tropics. Therefore, leaf mottling may have a selective advantage if it provides some protection against aphids.

Results of a recent field experiment (1) showed that significantly more aphids are captured on trapping stakes placed near silvery plants than near non-silvery plants. The difference is particularly striking in the upper trapping zone, the zone of highest aphid concentration. The same experiment showed also that significantly more aphids are captured on stakes placed in bare ground than in cultivated ground occupied by plants. Since the plants of the silvery cultivar (NJ260) used in this experiment were smaller than those of the non-silvery cultivar (EPS), each silvery plot consisted of larger areas of bare ground. According to our present interpretation, this factor together with increasing evidence for the repelling action of silvery plants (the light effect) could have contributed to the higher number of aphids captured near them.

It is difficult to design and interpret field experiments of this kind. The nature of the relationship between plants and aphid behavior is still obscure, aphid infestation is unpredictable, and present silvery and non-silvery lines are not isogenic. Nevertheless, the hypothesis that the silvery-leaf trait provides and escape mechanism against aphids is potentially amenable to critical testing.

Literature Cited

1. Costa, S.D. and O. Shifriss. 1982. Trapping aphids among silvery and non-silvery lines of Cucurbita pepo L. (unpublished manuscript).
2. Coyne, D.P. 1970. Inheritance of mottle-leaf in Cucurbita moschata Poir. HortScience 5:226-227.
3. Davis, R.F. and O. Shifriss. 1983. Natural virus infection in silvery and non-silvery lines of Cucurbita pepo L. Plant Disease 67:379-380.
4. Robinson, R.W., H.M. Munger, T.W. Whitaker and G.W. Bohn. 1976. Genes of the Cucurbitaceae. HortScience 11:554-568.
5. Shifriss, O. 1981. Do Cucurbita plants with silvery leaves escape virus infection? Origin and characteristics of NJ260. Cucurbit Genetics Coop. Rpt. 4:42-43.
6. Shifriss, O. 1982. On the silvery-leaf trait in Cucurbita pepo L. Cucurbit Genetics Coop. Rpt. 5:48-50.
7. Shifriss, O. 1983. Reflected light spectra from silvery and non- silvery leaves of Cucurbita pepo. Cucurbit Genetics Coop. Rpt. 6:89-90.
8. Tapley, W.T., W.D. Enzie and G.P. Van Eseltine. 1937. The cucurbits. Vegetables of New York, Vol. I-part IV. New York Agricultural Experiment Station, Geneva, NY.
9. Whitaker, T.W. and G.N. Davis. 1962. Cucurbits. Interscience Publications, Inc., New York.