Cucurbit Genetics Cooperative Report 3:6-8 (article 4) 1980
B.F. George
Heinz, U.S.A., Tracy, CA 95376
H.M. Munger
Cornell University, Ithaca, NY 14853
The cream mature fruit color, associated with the white spine allele (b), is preferred by pickle processors over the orange mature fruit color associated with the black spine allele (B). However, many seedsmen and processors have felt that the white spine allele is also associated with less desirable tougher skin. For this reason, two near isogenic lines for sine color fro the Cornell program (69841-11,13 and 69950-7,8) were evaluated for skin toughness and fruit firmness. Comparisons were made between homozygous allelic segregant lines following five backcrosses to ‘Wisconsin SMR 18’. The original white spine source was ‘Hardin’s dwarf cucumber PG 57’. A split plot experimental design was used at Freeville, NY in 1970 with lines as main plots and alleles as split plots. Plots contained 12 plants, two per hill spaced 61 x 183 cm. The experiment was replicated three times and was repeated in Ithaca, NY in 1971 to obtain further data on skin toughness.
Fruit were graded following the PCIC standards with grades and diameters as follows: No. 2, 2.7-3.8 cm; No. 3, 3.8-5.1 cm; No.4, 5.1-5.7 cm; No. 5, 5.7-6.4 cm; and No. 6, over 6.4 cm.
Firmness was measured with a 7.9 mm diameter center punch over the locule junction using an Italian fruit pressure tester similar to the Magness midol. A minimum of ten grade 3 fruit were tested from each plot at harvest time. No significant differences were found between alleles or lines (Table 1).
Skin toughness was measured with a .08 cm plunger in a Chatillion spring puncture tester. The measurement was made as the locule junction of ten intact fruit/grade/plot at harvest time. In the 197 experiment, lines and replicates were pooled to obtain enough fruit/grade. The only significantly different skin toughness in 1970 was with mature fruit. In 1971, the fruit were tested at harvest and after 48 hrs of holding in the field. It appears that the skin toughness approximately doubled in the 48 hrs following the harvest, but the white spine (bb) skin toughness was no greater than that of black spine (BB) except in oversize grade 6 fruit (Table 1).
Pericarp samples were taken adjacent to punctures from black spine and white spine fruit and fixed in FAA. Transverse pericarp sections 48 µ thick were made with a cryostat microtome and stained with Hematoxlin and Safranin. Ten measurements of cell wall thickness and cell shape were made with a micrometer at various distances from the pericarp surface of each sample.
Cell wall thickness was found t be significantly greater in the mature pericarp of white spine fruit (Table 2). The greatest difference occurred about 70 µ from the base of the epidermal cell layer. Cell wall thickness in the younger fruit was so small that it could not be measured at 500 X. The parenchyma cells in mature bb sections 166 µ from the epidermal layer were more rounded in transverse sections while BB had flatter cells (Table 2). The cell shape pattern was not evident in grade 1 fruit. Measurements of epidermal and hypodermal cells did not reveal any differences between allelic types.
A number of microchemical stains (Basic fuchsin, Sudan III, Toluidine Blue O) indicated a difference in mature fruit cuticles. No differential staining occurred with immature fruit. A considerable quantity of waxy material could be scraped from the mature white spine fruit surface without damaging the epidermis. Similar material could not be obtained from the mature black spine fruit. In addition, it was observed that after a month in storage at room temperature, the black spine mature fruit became very light weight and soft. The white spine fruit remained heavy and firm. Peeled mature white spine skin is very elastic and curls up. In contrast, the black spine skin did not stretch or curl after peeling. Thus, it appears that skin differences associated with the spine color alleles occur only in mature fruit.
Table 1. Firmness and skin toughness as related to grade and alleles. Freeville, Ithaca, 1970-1971.z
Grade |
||||||
Experiment |
2 |
3 |
4 |
5 |
6 |
Mature |
Firmness 1970 |
||||||
| BB | — | 21,100 | — | — | — | — |
| bb | — | 20,500 | — | — | — | — |
Toughness 1970 |
||||||
| BB | 71.6 | 98.0 | — | 147.8 | — | 201.9** |
| bb | 71.9 | 97.4 | — | 144.0 | — | 247.2 |
Toughness 1971 |
||||||
| BB | 35.0 | 55.9 | — | — | — | — |
| bb | 30.6 | 51.7 | — | — | — | — |
Toughness 1971 + 48 hrs |
||||||
| BB | 57.8 | 110.9 | 152.4 | 184.0 | 179.1* | — |
| bb | 58.4 | 96.7 | 168.4 | 192.8 | 218.6 | — |
z Pressure in g/cm2 required to puncture the pericarp.
*Significantly different (.05) between alleles.
**Significantly different (.01) between alleles.
Table 2. Association of cell wall thickness and cell shape ratio with spine color alleles. Ithaca, 1971.
Allele |
Grade |
Skin toughness z |
Hypodermal wall thickness (µ) |
Mesocarp cell shape ratio y |
| BB | 1 | 22.7 | 0.98 | |
| 5 | 190.3 | 1.88 | ||
| mature | 212.9 | 1.45* | 2.17** | |
| bb | 1 | 24.9 | 1.06 | |
| 5 | 185.8 | 1.64 | ||
| mature | 242.4 | 2.56 | 1.80 |
z Pressure (g/cm2) required to puncture the pericarp.
y Transverse diameter/radial diameter.
* Significant difference (.05) between alleles.
** Significant difference (.01) between alleles.