Evaluation of the cucumber germplasm collection for fruit yield and quality

Abstract

Breeding for improved yield in cucumber (Cucumis sativus L.) has been one of the important objectives of cucumber breeding. Although the average yield of cucumber has increased by 100% over the last four decades, yield progress has remained flat in the past two decades. Thus, it is necessary to improve yield in cucumber by finding new sources of yield genes in the germplasm collection. Discovery of new sources may be difficult, however, because measurement of yield in a diverse array of cucumber cultigens is confounded by the different sex types found in cucumber.

Ethylene is a plant growth regulator known to alter sex expression in cucumber. Preliminary studies were conducted on a set of pickling and slicing cucumber cultigens differing in sex expression. Cultigens were subjected to contrasting rates of ethephon and two harvest stages under field conditions in North Carolina. Our objectives were to determine whether ethephon had an effect on fruit yield and quality in a diverse set of cucumber cultigens, and to determine whether harvesting at the 10 or 50% oversized fruit stage had an effect on the fruit yield and quality. No differences for yield traits (total, marketable, percentage of culls) or fruit quality rating were observed between the harvest dates for both pickling and slicing cucumbers. Ethephon treatments had a significant effect on yield and fruit quality rating. Yield traits were found to differ for the different ethephon treatments based on the sex expression (monoecious, gynoecious) of the cultigens.

In the above experiment, we also measured the effects of ethephon on several vegetative and floral traits. Ethephon had an effect on most traits evaluated: days to first flower, days to 50% flower, days to first fruit set (pickles only), days to 50% fruit set (pickles only), and fruit quality rating. However, ethephon had no significant effect on days to vine tip-over stage (five nodes), or days to first fruit set or days to 50% fruit set in slicing cucumber cultigens.

To study the effects of ethylene on the sex expression further, ethephon was applied to inbred lines of cucumber and interactions with genes for sex expression were evaluated. Inbreds used were isogenic pairs that differed for the F gene conditioning monoecious or gynoecious sex expression. Ethephon played an important role in improving most of the traits such as gynoecious rating, and fruit number and weight compared to the untreated control. Ethephon was effective in increasing pistillate flower number in monoecious inbreds, but had little effect on gynoecious inbreds.

Information derived from the previous studies was used to evaluate all cucumber plant introduction (PI) accessions from the USDA cucumber germplasm collection for fruit yield, earliness, and quality. All cultigens were sprayed with ethephon to convert them to gynoecious sex expression in order to make the evaluation more uniform for all cultigens regardless of their genes for sex expression. A total of 817 cultigens were tested using recommended cultural practices. Significant differences were observed among cultigens for all traits evaluated. High yielding cultigens were identified in each of the four cucumber types. In some cases, high yielding cultigens also had high early and marketable yield, and acceptable fruit quality and days to harvest.

Regression equations were developed for predicting marketable, early, and cull fruit weight in the different cucumber types. Results from our analysis showed that the fruit weight of each grade was best estimated using the fruit number of that grade (early, marketable, and cull) along with the total fruit weight and total fruit number.