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In the cut flower market, traditional breeding is still the best way to achieve new rose cultivars. The geographical delocalization of cultivar constitution (generally made in Europe and North America) and plant cultivation (large areas in Africa and South America) represents a limit point for crossing and selection. Rose breeders often need to overcome geographical distances, resulting in asynchrony in flowering among crossing parents, by storing and sending pollen. Hence, a key aspect in breeding programs is linked to pollen availability and conservation, jointly with the identification of parameters related to pollen fertility. In this study we present the results of three different trials. In the first, pollen diameter and pollen viability were chosen as fertility predictors of 10 Rosa hybrida commercial cultivars. In the second trial, aliquots of dried pollen grains of six R. hybrida cultivar were stored under two different temperatures (freezer at T = −20 °C and deep freezer at T = −80 °C) and after a wide range of conservation period, their viability was measured. In the third trial, the effective fertilization capacity of frozen pollen of 19 pollen donor cultivars was evaluated during 2015 crossing breeding plan, performing 44 hybridizations and correlating the number of seeds and the ratio seeds/crossing, obtained by each cultivar, with in vitro pollen germination ability.

Fertility of hybrid tea roses is often reduced due to their interspecific origin but also to intensive inbreeding. New genotypes used as pollen donors represent an economic risk for a breeding programme, as their influence on seed production is unknown. In this study 11 cut rose genotypes were selected from a company database as high fertile or low fertile male parents, according to the number of seeds per hybridisation. Pollen morphology and in vitro germination of the selected genotypes were characterised. Pollen was either small (mean diameter 30 μm), elliptical and crossed by furrows (normal). High correlations were found between the number of seeds produced per hybridisation and the pollen diameter (r = 0.94) or the percentage of normal pollen (r = 0.96). In order to evaluate the predictive power of the models, we conducted regression analyses and performed a validation experiment on genotypes not present in the database and without background information on fertility. Pollen diameter and percentage of normal pollen were characterised and fitted in the regression models for seed set predictions. Validation with an independent dataset gave a good prediction for 83.3% of the data. This indicates that using either the mean pollen diameter or the percentage of normal pollen resulted in effective fertility prediction. This tool could enhance the genetic variability in crossings between hybrid tea roses, thus creating possibilities for less economically risky exploitation of new tetraploid genotypes as male parents.

Concentrations of endogenous abscisic acid (ABA) and indole-3-acetic acid (IAA) in Rosa hybrida seed coat and embryos were determined at 28, 35, 42, and 49 days after pollination (DAP), a period encompassing the early-middle stages of seed development. No studies on rose have ever documented simultaneous change in ABA and IAA during these developmental phases in both seed coat and embryo. Plant growth regulators were extracted and then quantified by using high performance liquid chromatography based on solid phase extraction purification. In both the seed coat and embryo, ABA content decreased from 28 DAP (4.39 and 1.36 pmol mg−1, respectively) and onward. Endogenous IAA in seed coat followed the same trend. In contrast, IAA in embryo began to increase at 28 DAP (2.06 pmol mg−1), peaked at 42 DAP (5.06 pmol mg−1), and then declined dramatically at 49 DAP (1.17 pmol mg−1). In embryo, the IAA/ABA ratio was always >1.0 and showed a tendency to increase from 28 DAP to the maximum significant rate at 42 DAP (9.20). The ABA decrease associated with increased IAA levels in embryo could be a result of crosstalk between these two phytohormones. Such a change in the IAA/ABA ratio may signal the end of endodormancy caused by ABA at the pre-cotyledonary stage and the start of increased embryo cell division during the cotyledonary stage, which also results in increased hip weight.

Breeding is a long process in the rose industry. Premature death of developing embryos due to incompatibility and delayed germination, caused by seed dormancy, are the main causes. Immature seed rescue may offer a valuable way to increase hybridisation efficiency, to introduce genotypes with interesting ornamental traits, and to shorten time needed to obtain seedlings. In this study, in vitro immature seed rescue was carried out in Rosa hybrida L. Seeds from 7 to 28 days after pollination (DAP) were sterilized and placed onto two germination media: Murashige and Skoog medium (full strength macro and micro nutrients and vitamins, MS0) and MS0 with 6-benzylaminopurine (BAP; 2.5 mg/L) and gibberellic acid 3 (GA3; 0.5 mg/L). Four different conditions of light and temperature were applied. At 15 and 30 DAP, the abscisic acid (ABA) concentration was quantified in fully formed embryos of two different genotypes of hybrid roses. Preliminary data showed that only seeds harvested at 21 DAP germinated in both media, this after 2 months of in vitro culture. This result is supported by the observation that basal levels of ABA were found in embryos at 30 DAP. The general role of ABA on preventing precocious germination and vivipary suggests an early and transient endodormancy in embryos of hybrid roses. The integration of these two approaches with the aim of defining protocols for precocious in vitro germination and investigating the pathway of dormancy in R. hybrida is highlighted.