Cucurbit Genetics Cooperative Report 8:18-21 (Article 8) 1985
Staub, Jack
U. S. Department of Agriculture, Agricultural Research Service and Department of Horticulture, University of Wisconsin, Madison, WI 53706
Recently, usage of Cucumis sativus var. hardwickii (Royle) Kitamura germplasm (hereafter referred to as hardwickii), has increased in cucumber improvement programs because of its potential for increasing fruit yields in the cultivated cucumber, Cucumis sativus var. sativus L. (hereafter referred to as sativus) (3). Although hardwickii has the ability to set a large number of seeded fruits sequentially on each plant, its fruit are ellipsoid, bitter, and have a large seed cell (3). These negative characteristics, along with its susceptibility to most economically important North American cucumber diseases (5) provide a challenge for plant breeders interested in capitalizing on its yield potential.
Morphological differences among hardwickii accessions (7), suggest differences in their potential for transmitting useful traits (4). Hardwickii lines PI 183967 and PI 215529 differ in combining abilities for fruit number, length and diameter; lateral branch number; flowering date; sex expression and plant dry weight (4,9). Lower et al. (6) reported on type of gene action and heterosis for yield and vegetative characteristics in a cross between a sativus line (Gy 14) and an inbred hardwickii line selected from PI 183967 (LJ 90430). Variation in fruit weight per plant, lateral branch number, main stem length and fruit length and diameter can be accounted for by an additivedominance genetic model (2,6).
Flowering characteristics of hardwickii directly affect time of fruit development. Hardwickii accessions differ in photoperiodic response (4,9). Although early flowering is promoted under short days (approximately 9 to 14 hours) in hardwickii PI 183967 (LJ 90430) and LJ 91176, plants of PI 215589 will eventually flower under long days (16 hours)(l). This short day flowering response in PI 215589 appears to be controlled by a single recessive gene and is most likely allelic (1) to the delayed flowering mutant (df) reported by Shiffriss and George (8).
One objective of the USDA cucumber breeding program is to incorporate high fruit setting ability and multiple branching from hardwickii into sativus germplasm. Hardwickii germplasm from PI 183967 and PI 215589, along with several sativus processing cucumber inbreds with relatively diverse genetic backgrounds (segregating plants with high fruiting capacity), were intercrossed to form several populations. One such population originating from hardwickii x sativus matings is non-bitter, gynoecious, multiple- branching, and resistant to 6 diseases. The best of 4 F4 and F5 lines random- mated and subjected to 2 cycles of recurrent selection for fruit number under a 0.9 m between-plant spacing provided the experimental population (WI 5242). Four lines (5095, 5096, 5097 and 5098) were established by self-pollinating selected plants from the cycle 2 population. Previous studies (7) indicated that sativus x hardwickii F1 backcrossed to sativus, when tested under a close spacing (12 cm between plants and 1.5 m between rows), did not produce yields that differed significantly from those of the recurrent parent. This study was initiated to compare fruit yields bf the 4 inbred lines with the sativus inbred WI 1983 and the hybrid ‘Calypso’ at 3 plant spacings.
WI 1983 and commercial processing hybrid ‘Calypso’ were evaluated at 3 betweenplant spacings (0.23, 0.46 and 0.92 m) in randomized complete blocks with 6 replications. Supplemental irrigation and standard cultural practices were used, and plots were harvested 5 times starting when ‘Calypso’ plots had 10% oversize fruit. Rows were 1.5 m apart.
Fruit yields of hardwickii-derived lines were lower (20 to 90%) than ‘Calypso’ but nearly equal to WI 1983 in harvests 1 and 2 at all spacings. In harvests 3 through 5, fruit yields of the 4 lines were higher (10 to 200%) than both ‘Calypso’ and WI 1983 (Figure 1). Cumulative yields of WI 5095 and WI 5097 over 5 harvests under the 0.46 and 0.23 m spacings were similar to ‘Calypso’ and WI 1983. In contrast, cumulative yields of WI 5096 and WI 5098 were higher than ‘Calypso’ and WI 1983 at the 0.23 (20 and 35%), 0.46 (15 and 30%) and 0.92 (45 and 55%) meter spacings. In this study, fruit yields of 4 hardwickii derivates were competitive with ‘Calypso’ in late harvests at close spacing. However, since fruit length/diameter (L/D) ratios were short (2.3 to 2.6) and interior quality was not acceptable, F1 hybrids between these hardwickii derivates and standard processing cucumber inbreds need to be evaluated in order to determine whether yielding ability can be maintained along with improvements in L/D ratio and fruit quality.
Figure l. Comparative fruit yielts between four C. hardwickii derived inbreds and a parental C. sativus inbred and the processing cucumber hybrid ‘Calypso’ under 3 spacings. Fruit yields of harvests 1-5 are given as percent of ‘Calypso’ and harvest 6 designation represents the cumulative average over 5 harvests.
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