Fruit set and development are dependent on auxin, gibberellin, and cytokinin, which cause parthenocarpic development in many species when applied ectopically. Commercial sprays containing these hormones are used to improve apple fruit set, size, and shape, but have been implicated negatively in other aspects of fruit quality. We applied gibberellic acid (GA₃), synthetic auxin (NAA), and the auxin-transport inhibitor NPA to ‘Honeycrisp’ apple flowers. Fruit retention and size were quantified throughout development, and seed number and fruit quality parameters were measured at maturity. GA₃ alone caused the development of seedless parthenocarpic apples. At maturity, GA₃-treated apples were narrower due to reduced ovary width, indicating that GA₃ induced normal growth of the hypanthium, but not the ovary. GA₃-treated fruits were also less acidic than hand-pollinated controls, but had similar firmness, starch, and sugar content. To further understand the regulation of parthenocarpy, we performed tissue-specific transcriptome analysis on GA₃-treated, NAA-treated, and control fruits, at 18 days after treatment and again at maturity. Overall, transcriptome analysis showed GA₃-treated and hand-pollinated fruits were highly similar in RNA expression profiles. Early expression differences in putative cell division, cytokinin degradation, and cell wall modification genes in GA₃-treated ovaries correlated with the observed shape differences, while early expression differences in the acidity gene Ma1 may be responsible for the changes in pH. Taken together, our results indicate that GA₃ triggers the development of parthenocarpic apple fruit with morphological deviations that correlate with a number of candidate gene expression differences.

Dormancy in fruit trees is a crucial process that involves many molecular and physiological mechanisms to protect plant organs from abiotic stresses throughout the winter months. This process has become an increasingly important focus of study in light of a changing and more volatile climate. Two plant hormones, abscisic acid (ABA) and ethylene, have been shown to play a role in the regulation of dormancy. Both ABA and ethylene-related compounds have been applied previously to fruit trees to alter the onset and duration of endodormancy, which is the phase of dormancy during which cold hardiness is greatest and buds are protected until a specific number of cold hours are met. We aimed to investigate pear ( Pyrus communis cv. d’Anjou) endodormancy responses to the application of both compounds simultaneously using S-ABA and 1-aminocyclopropane-1-carboxylic acid (ACC), an ethylene precursor. Treatment initially resulted in earlier leaf abscission relative to untreated trees. Using freeze tests, we found that cold hardiness was generally higher for treated vegetative shoots in the weeks and months after application following an immediate temporary decrease in cold hardiness. Gene expression of an ABA catabolism gene related to CYP707A genes was significantly lowered in reproductive buds at two timepoints corresponding to higher cold tolerance of treated vegetative shoots. However, expression of other dormancy-related genes tested was not significantly different between buds of treated and untreated trees on these dates. Overall, this research indicates that pear dormancy and cold hardiness can be at least partially altered by hormone plant growth regulator application.

Quince ‘Eline’ (Cydonia oblonga L.) is a cold-hardy rootstock for pear (Pyrus communis L.) that could potentially be adopted by the U.S. pear industry if it can be shown to perform under conditions that have prevented prior quince rootstock adoption. These conditions include colder winters and hotter, drier summers than the European regions where quince ‘Eline’ is currently used. Preplant soaking, the practice of submerging bare-root tree roots in water to rehydrate them following cold storage, will potentially be a critical component of preventing transplant shock if pear orchards grafted to quince ‘Eline’ are to be established in hot or dry areas. This study tested five soaking treatments for ‘Bartlett’ pear grafted on quince ‘Eline’ with a ‘Beurre Hardy’ pear interstem, and measured multiple factors related to tree establishment success throughout the first growing season. The treatments included a 0 h control as well as 24, 48, 72, and 96 h soaking times. The tree weight decreased when not soaked and increased when soaked for 48 or 72 h. Tree health remained similar among the soaking treatments, with all trees surviving, though the health assessment was limited by the use of an ordinal scale. The growth varied, with the 24, 48, and 72 h treatments showing the greatest trunk cross-sectional area and leader length increase. Stomatal-conductance-to-water-vapor measurements, used as a proxy for tree water stress, showed that the 96 h treatment resulted in more highly stressed trees. Altogether, this study identifies the benefit of preplant soaking for ‘Bartlett’ grafted to quince ‘Eline’, and highlights the 24 to 72 h range as a best practice for establishment for U.S. pear growers.

Induction and development of healthy roots is necessary for both micropropagation and the successful rescue of in vitro genetically transformed or edited plants. Pear ( Pyrus ) genotypes are known to be recalcitrant to rooting treatments, and responses are highly genotype-dependent. Floral foam has been used successfully in some species to improve rooting, or de novo root regeneration (DNRR). In this study, the use of floral foam was assessed for effects on DNRR efficiency and root qualities, for five genetically diverse Pyrus genotypes. DNRR efficiency was compared between floral foam, agar, and covered agar containing the same rooting hormone composition to assess the role of darkening of the root zone during rooting. The effect of floral foam was genotype-dependent, and increased DNRR efficiencies in most genotypes. In the case of P. communis 'Tsarigradka', the delivery method of auxin (dipping in a high concentration solution, rather than receiving a low concentration dissolved in the medium) in combination with floral foam was necessary to increase DNRR efficiency. Rooting in agar with the root zone covered to impose darkness did not show improvements over uncovered agar, suggesting that darkening of the root zone is not sufficient to explain improvements from using floral foam. Together, these results suggest that use of floral foam can be an effective strategy for rooting plants produced from in vitro methods.