Cucurbit Genetics Cooperative Report 6:87-88 (article 44) 1983
On Spotting of Cucurbita pepo L. Fruit
A. Schaffer1 and C. D. Boyer2
Rutgers University, New Brunswick, NJ 08903
A number of C. pepo cultivars are characterized by fruits with either a "lace-like pattern" or subtle spotting (1). Even the dark-green zucchini squashes, when viewed closely, exhibit small light green specks throughout the surface of the fruit. Barber (2) correlated these spots with stomata but did not explain their cause. Paradermal sections of 'Fordhook Zucchini' (FZ) showed that the flecks or spots occurred below stomata and that the spots became recognizably large in areas where a number of stomata were aggregated. Cross sections of these areas showed that the cells in the subepidermal layers contained no chromoplasts or chloroplasts and, depending on the number of stomata in the aggregate, the colorless area extended. a few or many cell layers deep. Accordingly, white spots would then be plastidless areas that extend down to the lignified layer beneath the hypoderm while light green spots would be due to plastid-filled cells viewed through a few layers of plastidless cells. This effectively creates a window to the lower hypodermal cell layers.
Some FZ x Early Prolific F2 segregants had dark green fruit with conspicuous light green-cream spots. After 50 days past anthesis, the lighter spots turned yellow-cream while the dark green "background" remained. Cross sections of tissues of these fruit showed that the light green-cream spots were due to plastidless areas beneath stomata which served as a "window" to the lower cell layers which were filled with chloroplasts. However, when the chloroplasts of the inner cell layers converted to yellow chromoplasts the color change was observed through the "window".
The ability of different subepidermal cell layers to undergo differential plastid development is no doubt very important in determining the different pigmentation patterns of C. pepo fruit. For example, in one F2 segregant from FZ x Small Sugar the fruit developed a shiny black skin, similar to FZ. Actually, its subepidermal layer was made up of two somewhat distinct layers: a heavily pigmented upper layer with a very high chloroplast concentration and a lighter pigmented lower layer with a sparser chloroplast concentration. Portions of the dark layer degreened without converting to chromoplasts, unmasking the lighter green area below. Portions of the lighter green area did develop chromoplasts yielding a fruit with dark green, light green and orange splotches.
The genetic and physiological factors determining which cell layers undergo plastid development remain to be delineated. However, a developmental and histological approach to fruit pigmentation in C. pepo would no doubt add to our further understanding of this colorful subject.
- Barber, K. G. 1909. Comparative histology of fruits and seeds, of certain species of Cucurbitaceae. Bot. Gaz. 47:263310.
- Shifriss, O. 1949. A developmental approach to the genetics of fruit color in Cucurbita pepo L. J. Heredity 40:233241.
1Present address: The Hebrew University of Jerusalem, Rehovet 76-100, Israel.
2Present address: The Pennsylvania State University, University Park, PA 16802.