Citation: Carillo, P.; Fusco, G.M.; Del Gaudio, R.S.; De Pascale, S.; Paradiso, R. Cold Treatment Modulates Changes in Primary Metabolites and Flowering of Cut Flower Tulip Hybrids. Horticulturae 2022, 8, 371. https://doi.org/10.3390/ horticulturae8050371 Academic Editor: Genhua Niu Received: 14 March 2022 Accepted: 21 April 2022 Published: 24 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). horticulturae Article Cold Treatment Modulates Changes in Primary Metabolites and Flowering of Cut Flower Tulip Hybrids Petronia Carillo 1 , Giovanna Marta Fusco 1 , Rosaria Serena Del Gaudio 2 , Stefania De Pascale 2 and Roberta Paradiso 2, * 1 Department of Environmental, Biological and Pharmaceutical Sciences and Technologies, University of Campania Luigi Vanvitelli, 81100 Caserta, Italy; petronia.carillo@unicampania.it (P.C.); giovannamarta.fusco@unicampania.it (G.M.F.) 2 Department of Agricultural and Food Sciences, University of Naples Federico II, Portici, 80055 Naples, Italy; rosariaserena.delgaudio@unina.it (R.S.D.G.); depascal@unina.it (S.D.P.) * Correspondence: rparadis@unina.it Abstract: Tulip is one of the most important bulbous genera in the world’s floriculture. It is known that cold exposure of bulbs before planting is required to break the bulb dormancy and to promote the plant’s flowering. Preparation procedures performed by breeders differ in the duration and the thermal level, and the choice of the procedure depends on the genotype’s sensitivity to temperature; however, little is known about the metabolic responses underlying the different behaviours of the numerous commercial hybrids. We evaluated the influence of two bulb-preparation procedures, 15–18 weeks at 5 ÷ 9 ◦ C, and 9–14 weeks at 2 ÷ 5 ◦ C, in two hybrids of tulip (Tulipa gesneriana L.), ‘Royal Virgin’ and ‘Ad Rem’, grown hydroponically in a floating system. Tulip plants of the two hybrids responded differently to bulb exposure to low temperatures in terms of early flowering, as this was unaffected by the preparation procedure in ‘Royal Virgin’ (27.1 days from transplanting, on average), while it was earlier after treatment at higher temperatures compared with lower tempera- tures in ‘Ad Rem’ (24.1 vs. 26.7 days at 5 ◦ C vs. at 9 ◦ C). This different flowering earliness may be related to the diverse metabolic responses enacted by the bulbs for cold acclimation that depended on hybrid x thermal treatment. Plant leaf area and flower stem characteristics were similar in the hybrids and were unaffected by the bulb-preparation procedure. Keywords: geophytes; cold requirement; bulbs; hydroponics; metabolic profile 1. Introduction Tulip (Tulipa spp., family Liliaceae) is one of the most important genera among cut flowers and ornamental plants in the world’s floriculture [1]. Intensive hybridisation, conducted primarily by growers in the Netherlands over the centuries, has resulted in a multitude of tulip cultivars and hybrids, producing plants suitable for a wide range of cultivation conditions and a huge variety of flower shapes and colours, very appreciated as all-year-round cut flower and as a spring-flowering plant in pots, gardens, and parks. The tulip bulb has an annual replacement cycle, consisting in three phases starting from the bulb planting, as follows: the root growth, the plant growth, and the flowering. These are accompanied by the aerial part senescence, the mother bulb shrivelling, and the daughter bulb dormancy [2]. In plants in a wild environment under Mediterranean and temperate climates, this cycle starts with the floral meristem initiation and differentiation in summer, the flower bud development in winter, and the anthesis in spring. Periodical dissections of the tulip bulb during dormancy shows that an active organogenesis (differen- tiation of floral and vegetative buds within the bulb scales and of root primordia in the basal plate) takes place during this rest period [3]. In particular, Mulder and Luyten [3] described seven different distinct stages of flower bud differentiation, from phase I (vegetative apex), through phases II, P1, P2, A1, and A2, to phase G (formation of trilobed gynoecium). Horticulturae 2022, 8, 371. https://doi.org/10.3390/horticulturae8050371 https://www.mdpi.com/journal/horticulturae