Evaluation of Yield Traits in Advanced Summer Squash Inbred Lines

The Cucurbitaceae family is the second largest horticultural family in terms of economic importance after Solanaceae (1). The genus Cucurbita (2x = 2n = 40), encompasses three economically important crop species such as Cucurbita pepo, Cucurbita moschata, and Cucurbita maxima, cultivated throughout temperate, sub-tropical, and tropical regions (3). Cucurbita pepo L. is the most widely grown and polymorphic of the Cucurbita species (2). Roasted summer squash seeds are used as nut in many cultures worldwide. Production of summer squash in Iran is based on local populations that are combined from several genotypes due to their cross-pollination nature. This lack of uniformity affects both the quantity and quality of squash yield and reduces the marketability of nuts. The purpose of this research was to produce various advanced inbred lines of summer squash by using local germplasm and purifying indigenous populations and also to select superior inbred lines in terms of fruit yield and other desirable agricultural traits that can be used in the production of hybrid cultivars with superior quantity and quality of fruit yield attributes.

In order to preserve internal genetic resources and produce advanced summer squash inbred lines, 83 S6 inbred lines were produced by inducing self-pollination from seven local populations. These genetic resources were obtained by planting the lines twice a year (spring planting at Golestan Agricultural and Natural Resources Research and Education Center and fall planting at Southern Kerman Agricultural and Natural Resources Research and Education Center) from 2018 to 2021. A randomized complete block design with three replications was used to evaluate fruit yield attributes of selected S6 inbred lines and their parental local populations. The following quantitative traits were considered in this study: fruits/plant, fruit weight, fruit length, fruit width, seed yield/m2, seed yield/fruit yield ratio, seed length, seed width, percentage of empty seeds and 1000-seeds weight. Also, Panel test was performed to evaluate the suitability of nuts from a consumer perspective for the following traits: separation of skin from the kernel, taste quality and desirability of seeds shape and size from the consumer's point of view, with six testers. The experimental data were statistically analyzed for variance using the R version 3.5.1. Least significant difference (LSD) test at the probability level of 1% was used to compare the means of each trait. The heat map was created by R version 3.5.1.

The results of analysis of variance at the probability level of 1% indicated that the studied genotypes had significant differences in most of the traits including seed yield/m2, mean of fruits weight, fruits/plant, fruit length, fruit width, seed yield/fruit yield ratio, seed length, seed width, percentage of empty seeds and 1000-seeds weight, taste quality and desirability of seeds shape and size. Based on LSD mean comparison of the traits in selected S6 inbred lines and their parental local populations, the highest seed yield/m2 was observed in line#34 and the highest values of other traits with significant difference were as follows: fruits/plant in line#34, both mean of fruit weight and mean of fruit length in line#248 and line#249, fruit diameter in line#84 and Radkan parental genotype, seed yield/fruit yield ratio in Noodijeh parental genotype, seed length in Noodijeh parental genotype, line#24 and line#32, seed diameter in line#222, 1000-seeds weight in line#249, taste quality in line#224, and desirability of seed shape and size from the consumer's point of view in line#222 and line#34. The results of correlation analysis also showed that the seed yield/m2 had a positive and significant correlation with fruits/plant (r=0.73), but there was no significant relationship with fruit weight. The results suggest that populations with larger fruit weight and size produce fewer fruits; as a result, it reduces the seed yield. In fruits, as the length-to-width ratio increased, there was a trend toward decreased seed number, especially in the peduncular end of the fruit. It seems that breeding and selection for a greater number of small fruits is better than selecting plants with larger fruits. Also, managing agricultural operations, which increase the number of fruits, will increase seed yield/m2.

Based on the results lines #32, 34, 59, 195, 222, 224 and 252 were selected to be used in the breeding programs of cultivar production in future researches. Line#34 can be used as a promising line in compatibility and DUS projects.