Cucurbit Genetics Cooperative Report 3:6-8 (article 4) 1980
Effects of the White Spine Allele on Skin Toughness and
Fruit Firmness in 'Wisconsin SMR 18' Cucumber
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.
