Chilling sensitivity of four near-isogenic fruit-color genotypes of summer squash (Cucurbita pepo, Cucurbitaceae) and its association with tocopherol content

Highlights

• Fruit color genes B and D affected chilling sensitivity of summer squash.

• Yellow B/B fruit were more chilling sensitive than green b/b fruit.

• Gene D (enhanced pigmentation) alleviated chilling sensitivity caused by gene B.

• Exocarp tocopherol content accorded with chilling tolerance of the genotypes.

• Low-tocopherol B/B d/d fruit were prone to chilling injury.

Abstract

Accessions of Cucurbita pepo vary in the sensitivity of their young fruit (summer squash) to chilling injury (CI). The dominant gene B (Bicolor), which confers yellow fruit coloration, is known to increase susceptibility of summer squash to CI as compared with green, b/b, fruit. Another dominant gene, D (Dark stem), confers dark plant stems and somewhat enhanced coloration of the young fruit. However, the effect of this gene on chilling sensitivity of summer squash is unknown. Freshly harvested fruit of vegetable marrow-type summer squash from four near-isogenic genotypes, B/B D/D, B/B d/d, b/b D/D, and b/b d/d, were stored for 14 d at three temperature regimes, 4–5, 8–9, and 11–12 °C. Storage at 4–5 °C resulted in CI development in the fruit of all four genotypes. The yellow, B/B genotypes suffered injury also at 8–9 °C, especially the fruit of the light yellow, B/B d/d genotype that showed slight CI symptoms even at 11–12 °C. Chilling tolerance of the genotypes was in accordance with the total tocopherol contents of their exocarp, being highest in the green-skinned b/b accessions and lowest in the light yellow B/B d/d genotype. Relatively higher tocopherol content and lower CI severity were observed in the fruit of the enhanced-yellow B/B D/D genotype. The dominant D allele can partially alleviate the CI sensitivity of summer squash that is conferred by the dominant B allele.

Introduction

Summer squash are the young fruit of Cucurbita pepo L. (Cucurbitaceae), an extremely polymorphic species (Paris, 2000). C. pepo is widely grown the world over, for the production of mature fruit (pumpkins and winter squash) as well as the young fruit, and cultivars differ among one another in fruit size, shape, and color. Based on fruit shape, eight groups of cultivars, or morphotypes, are recognized (Paris, 1986) of which six are grown entirely or mostly for consumption of the young fruit. One of these six groups, the vegetable marrows, is characterized by short, tapered, cylindrically shaped fruit. Most of the vegetable marrows have light green young fruit color.

Summer squash are picked when they are immature and quite young, 2–5 d past anthesis. At this age, they are attractively glossy and tender (Paris, 2008). Fruit that are not picked on time lose their glossiness and tenderness quickly, becoming unmarketable. Summer squash have a short-term postharvest life (Lorenz, 1951), desiccating, losing weight quickly and shrivelling (Hruschka, 1977). At storage temperatures of ca. 5 °C or less, summer squash are also highly subject to chilling injury (CI), which is expressed as surface pitting and browning (Mencarelli et al., 1983; McCollum, 1990). Oxidative stress is seen as an early response to chilling (Hariyadi, Parkin, 1991), and has been implicated in lipid peroxidation in cell membranes (Wang, 1995).

There are differences among summer squash cultivars in both storability and susceptibility to CI (Mencarelli et al., 1982; Sherman et al., 1985; Nunes et al., 2003; Carvajal et al., 2011; Megias et al., 2014). Sherman et al. (1987) observed, among three pairs of near-isogenic accessions of vegetable marrow, scallop, and zucchini morphotypes, that the accessions carrying the dominant B (Bicolor) allele for yellow fruit color were more susceptible to CI than their green-fruited counterparts, of genotype b/b. There were also large differences among the three cultivar pairs. The vegetable marrow was significantly more susceptible to chilling injury than the other two. Interestingly, the vegetable marrow had light stems, genotype d/d, whilst the other two carried the dominant Dark stem allele, D (Paris, Nerson 1986). The dominant D allele also somewhat enhances the color of the young fruit, though its effect on intensity of pigmentation becomes more obvious later in fruit development, from approximately 10 d past anthesis. Near-isogenic D/D and d/d accessions have not been compared to test whether allele D can affect summer squash sensitivity to CI.

Tocopherols function as plant antioxidants involved in several physiological processes including abiotic stress tolerance (Falk, Munné-Bosch 2010; Havaux, Garcia-Plazaola, 2014). Tadmor et al. (2005) observed that the mesocarp (flesh) of mature (≥40 d post-anthesis) Cucurbita pepo fruit carrying the dominant B allele had enhanced levels of carotenoids but lacked tocopherols. To our knowledge, the exocarp (skin) of immature, young fruit of C. pepo (i.e. summer squash) has not been analyzed for tocopherol content. Therefore, it is not known if the exacerbated chilling sensitivity of summer squash attributed to gene B is associated with low tocopherol content of the exocarp.

The objective of the present investigation was to compare the response to storage at chilling and non-chilling temperatures of summer squash from four near-isogenic accessions, differing only in the allelic states of the B and D loci. Fruit of the four genotypes were compared visually for CI and were analyzed for color parameters and contents of chlorophylls a and b, total carotenoids, and tocopherols.